diff --git a/fluxer_app_proxy/src/config.rs b/fluxer_app_proxy/src/config.rs
index bae9894c2..f69c67675 100644
--- a/fluxer_app_proxy/src/config.rs
+++ b/fluxer_app_proxy/src/config.rs
@@ -2,10 +2,340 @@
 
 use fluxer_common::config::{self as cfg, GeoipS3Config, GeoipSourceConfig};
 use fluxer_svc::config::{DatabaseBackend, normalize_host, parse_hosts};
+use reqwest::Url;
 use std::env;
+use std::fmt;
 
 const DEFAULT_DISCOVERY_UPSTREAM_URL: &str = "http://localhost:8088/api/.well-known/fluxer";
 
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub enum InvalidAppProxyEnvironmentError {
+    InvalidValue {
+        name: &'static str,
+        value: String,
+        expected: &'static str,
+    },
+}
+
+impl InvalidAppProxyEnvironmentError {
+    fn new(name: &'static str, value: &str, expected: &'static str) -> Self {
+        Self::InvalidValue {
+            name,
+            value: value.to_owned(),
+            expected,
+        }
+    }
+}
+
+impl fmt::Display for InvalidAppProxyEnvironmentError {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::InvalidValue {
+                name,
+                value,
+                expected,
+            } => write!(formatter, "{name} must be {expected}, got {value:?}"),
+        }
+    }
+}
+
+impl std::error::Error for InvalidAppProxyEnvironmentError {}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct HttpUrl(Url);
+
+impl HttpUrl {
+    pub fn parse(name: &'static str, value: &str) -> Result<Self, InvalidAppProxyEnvironmentError> {
+        let url = Url::parse(value.trim()).map_err(|_| {
+            InvalidAppProxyEnvironmentError::new(name, value, "a valid HTTP or HTTPS URL")
+        })?;
+        if !matches!(url.scheme(), "http" | "https")
+            || url.host_str().is_none()
+            || !url.username().is_empty()
+            || url.password().is_some()
+            || url.fragment().is_some()
+        {
+            return Err(InvalidAppProxyEnvironmentError::new(
+                name,
+                value,
+                "an HTTP or HTTPS URL with a host and no credentials or fragment",
+            ));
+        }
+        Ok(Self(url))
+    }
+
+    pub fn as_url(&self) -> &Url {
+        &self.0
+    }
+}
+
+impl fmt::Display for HttpUrl {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0.fmt(formatter)
+    }
+}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct HttpEndpoint {
+    url: Url,
+    csp_origin: String,
+}
+
+impl HttpEndpoint {
+    pub fn parse(name: &'static str, value: &str) -> Result<Self, InvalidAppProxyEnvironmentError> {
+        let mut url = HttpUrl::parse(name, value)?.0;
+        if url.query().is_some() {
+            return Err(InvalidAppProxyEnvironmentError::new(
+                name,
+                value,
+                "an HTTP or HTTPS endpoint without a query or fragment",
+            ));
+        }
+        if !url.path().ends_with('/') {
+            let mut path = url.path().to_owned();
+            path.push('/');
+            url.set_path(&path);
+        }
+        let csp_origin = url.origin().ascii_serialization();
+        if csp_origin == "null" {
+            return Err(InvalidAppProxyEnvironmentError::new(
+                name,
+                value,
+                "an HTTP or HTTPS endpoint with a tuple origin",
+            ));
+        }
+        Ok(Self { url, csp_origin })
+    }
+
+    pub fn with_host_prefix(
+        &self,
+        name: &'static str,
+        prefix: &str,
+    ) -> Result<Self, InvalidAppProxyEnvironmentError> {
+        if !is_dns_bucket_name(prefix) {
+            return Err(InvalidAppProxyEnvironmentError::new(
+                name,
+                prefix,
+                "a DNS-compatible bucket name",
+            ));
+        }
+        let host = self
+            .url
+            .host_str()
+            .expect("validated HTTP endpoint must have a host");
+        let prefixed_host = if host.starts_with(&format!("{prefix}.")) {
+            host.to_owned()
+        } else {
+            format!("{prefix}.{host}")
+        };
+        let mut url = self.url.clone();
+        url.set_host(Some(&prefixed_host)).map_err(|_| {
+            InvalidAppProxyEnvironmentError::new(name, prefix, "a DNS-compatible bucket name")
+        })?;
+        let csp_origin = url.origin().ascii_serialization();
+        Ok(Self { url, csp_origin })
+    }
+
+    pub fn as_url(&self) -> &Url {
+        &self.url
+    }
+
+    pub fn as_str(&self) -> &str {
+        self.url.as_str().trim_end_matches('/')
+    }
+
+    pub fn csp_origin(&self) -> &str {
+        &self.csp_origin
+    }
+}
+
+fn is_dns_bucket_name(value: &str) -> bool {
+    if value.is_empty() || value.len() > 253 {
+        return false;
+    }
+    value.split('.').all(|label| {
+        if label.is_empty() || label.len() > 63 {
+            return false;
+        }
+        let bytes = label.as_bytes();
+        if !bytes[0].is_ascii_lowercase() && !bytes[0].is_ascii_digit() {
+            return false;
+        }
+        if !bytes[bytes.len() - 1].is_ascii_lowercase() && !bytes[bytes.len() - 1].is_ascii_digit()
+        {
+            return false;
+        }
+        bytes
+            .iter()
+            .all(|byte| byte.is_ascii_lowercase() || byte.is_ascii_digit() || *byte == b'-')
+    })
+}
+
+impl fmt::Display for HttpEndpoint {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        formatter.write_str(self.as_str())
+    }
+}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct CspSource(String);
+
+impl CspSource {
+    pub fn parse(name: &'static str, value: &str) -> Result<Self, InvalidAppProxyEnvironmentError> {
+        if value
+            .bytes()
+            .any(|byte| byte.is_ascii_whitespace() || matches!(byte, b';' | b','))
+        {
+            return Err(InvalidAppProxyEnvironmentError::new(
+                name,
+                value,
+                "one CSP source without whitespace or policy delimiters",
+            ));
+        }
+        if value == "*" {
+            return Ok(Self(value.to_owned()));
+        }
+        if is_csp_keyword_source(value) || is_csp_nonce_or_hash_source(value) {
+            return Ok(Self(value.to_owned()));
+        }
+        if matches!(
+            value,
+            "http:" | "https:" | "ws:" | "wss:" | "data:" | "blob:"
+        ) {
+            return Ok(Self(value.to_owned()));
+        }
+        if let Some(source) = parse_csp_network_source(value) {
+            return Ok(Self(source));
+        }
+        Err(InvalidAppProxyEnvironmentError::new(
+            name,
+            value,
+            "a supported CSP keyword, scheme, wildcard, nonce, hash, or HTTP(S)/WS(S) source",
+        ))
+    }
+
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+}
+
+fn is_csp_keyword_source(value: &str) -> bool {
+    matches!(
+        value,
+        "'self'"
+            | "'unsafe-inline'"
+            | "'unsafe-eval'"
+            | "'wasm-unsafe-eval'"
+            | "'strict-dynamic'"
+            | "'report-sample'"
+    )
+}
+
+fn is_csp_nonce_or_hash_source(value: &str) -> bool {
+    let Some(inner) = value
+        .strip_prefix('\'')
+        .and_then(|value| value.strip_suffix('\''))
+    else {
+        return false;
+    };
+    let Some((algorithm, encoded)) = inner.split_once('-') else {
+        return false;
+    };
+    if !matches!(algorithm, "nonce" | "sha256" | "sha384" | "sha512") || encoded.is_empty() {
+        return false;
+    }
+    encoded.bytes().all(|byte| {
+        byte.is_ascii_alphanumeric() || matches!(byte, b'+' | b'/' | b'_' | b'-' | b'=')
+    })
+}
+
+fn parse_csp_network_source(value: &str) -> Option<String> {
+    let (scheme, authority_and_path) = value.split_once("://")?;
+    if !matches!(scheme, "http" | "https" | "ws" | "wss") {
+        return None;
+    }
+    let wildcard = authority_and_path.starts_with("*.");
+    let parse_value = if wildcard {
+        format!(
+            "{scheme}://csp-wildcard.invalid.{}",
+            &authority_and_path[2..]
+        )
+    } else {
+        value.to_owned()
+    };
+    let url = Url::parse(&parse_value).ok()?;
+    if url.host_str().is_none()
+        || !url.username().is_empty()
+        || url.password().is_some()
+        || url.query().is_some()
+        || url.fragment().is_some()
+    {
+        return None;
+    }
+    let mut source = url.origin().ascii_serialization();
+    if source == "null" {
+        return None;
+    }
+    if wildcard {
+        source = source.replacen("csp-wildcard.invalid.", "*.", 1);
+    }
+    if url.path() != "/" {
+        source.push_str(url.path());
+    }
+    Some(source)
+}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct CspReportUri(HttpUrl);
+
+impl CspReportUri {
+    pub fn parse(name: &'static str, value: &str) -> Result<Self, InvalidAppProxyEnvironmentError> {
+        if value
+            .bytes()
+            .any(|byte| byte.is_ascii_whitespace() || matches!(byte, b';' | b','))
+        {
+            return Err(InvalidAppProxyEnvironmentError::new(
+                name,
+                value,
+                "one HTTP or HTTPS report URI without whitespace or policy delimiters",
+            ));
+        }
+        Ok(Self(HttpUrl::parse(name, value)?))
+    }
+}
+
+impl fmt::Display for CspReportUri {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0.fmt(formatter)
+    }
+}
+
+fn warn_invalid(error: InvalidAppProxyEnvironmentError) {
+    tracing::warn!(%error, "ignoring invalid app proxy environment value");
+}
+
+fn parse_optional_http_url(name: &'static str, value: Option<String>) -> Option<HttpUrl> {
+    let value = value?;
+    match HttpUrl::parse(name, &value) {
+        Ok(url) => Some(url),
+        Err(error) => {
+            warn_invalid(error);
+            None
+        }
+    }
+}
+
+fn parse_optional_http_endpoint(name: &'static str, value: Option<String>) -> Option<HttpEndpoint> {
+    let value = value?;
+    match HttpEndpoint::parse(name, &value) {
+        Ok(endpoint) => Some(endpoint),
+        Err(error) => {
+            warn_invalid(error);
+            None
+        }
+    }
+}
+
 fn parse_env_or_warn<T: std::str::FromStr>(name: &str, raw: &str, default: T) -> T {
     raw.parse::<T>().unwrap_or_else(|_| {
         tracing::warn!(
@@ -22,10 +352,10 @@ pub struct AppProxyConfig {
     pub host: String,
     pub port: u16,
     pub static_dir: String,
-    pub index_upstream_url: Option<String>,
-    pub static_cdn_endpoint: Option<String>,
-    pub s3_public_endpoint: Option<String>,
-    pub s3_uploads_bucket: String,
+    pub index_upstream_url: Option<HttpUrl>,
+    pub static_cdn_endpoint: Option<HttpEndpoint>,
+    pub s3_public_endpoint: Option<HttpEndpoint>,
+    pub s3_uploads_endpoint: Option<HttpEndpoint>,
     pub discovery_upstream_url: String,
     pub discovery_refresh_interval_ms: u64,
     pub release_channel: ReleaseChannel,
@@ -88,17 +418,17 @@ impl ReleaseChannel {
 
 #[derive(Clone, Debug, Default)]
 pub struct CspConfig {
-    pub extra_default_src: Option<Vec<String>>,
-    pub extra_connect_src: Option<Vec<String>>,
-    pub extra_img_src: Option<Vec<String>>,
-    pub extra_media_src: Option<Vec<String>>,
-    pub extra_font_src: Option<Vec<String>>,
-    pub extra_script_src: Option<Vec<String>>,
-    pub extra_style_src: Option<Vec<String>>,
-    pub extra_frame_src: Option<Vec<String>>,
-    pub extra_worker_src: Option<Vec<String>>,
-    pub extra_manifest_src: Option<Vec<String>>,
-    pub report_uri: Option<String>,
+    pub extra_default_src: Vec<CspSource>,
+    pub extra_connect_src: Vec<CspSource>,
+    pub extra_img_src: Vec<CspSource>,
+    pub extra_media_src: Vec<CspSource>,
+    pub extra_font_src: Vec<CspSource>,
+    pub extra_script_src: Vec<CspSource>,
+    pub extra_style_src: Vec<CspSource>,
+    pub extra_frame_src: Vec<CspSource>,
+    pub extra_worker_src: Vec<CspSource>,
+    pub extra_manifest_src: Vec<CspSource>,
+    pub report_uri: Option<CspReportUri>,
 }
 
 impl CspConfig {
@@ -114,23 +444,34 @@ impl CspConfig {
             extra_frame_src: read_csp_sources("FLUXER_CSP_EXTRA_FRAME_SRC"),
             extra_worker_src: read_csp_sources("FLUXER_CSP_EXTRA_WORKER_SRC"),
             extra_manifest_src: read_csp_sources("FLUXER_CSP_EXTRA_MANIFEST_SRC"),
-            report_uri: cfg::non_empty_env("FLUXER_CSP_REPORT_URI"),
+            report_uri: read_csp_report_uri("FLUXER_CSP_REPORT_URI"),
         }
     }
 }
 
-fn read_csp_sources(name: &str) -> Option<Vec<String>> {
-    let sources: Vec<String> = cfg::read_env(name, "")
+fn read_csp_sources(name: &'static str) -> Vec<CspSource> {
+    cfg::read_env(name, "")
         .split([',', ' ', '\t', '\n'])
         .map(str::trim)
         .filter(|source| !source.is_empty())
-        .map(str::to_owned)
-        .collect();
+        .filter_map(|source| match CspSource::parse(name, source) {
+            Ok(source) => Some(source),
+            Err(error) => {
+                warn_invalid(error);
+                None
+            }
+        })
+        .collect()
+}
 
-    if sources.is_empty() {
-        None
-    } else {
-        Some(sources)
+fn read_csp_report_uri(name: &'static str) -> Option<CspReportUri> {
+    let value = cfg::non_empty_env(name)?;
+    match CspReportUri::parse(name, &value) {
+        Ok(report_uri) => Some(report_uri),
+        Err(error) => {
+            warn_invalid(error);
+            None
+        }
     }
 }
 
@@ -175,6 +516,21 @@ impl AppProxyConfig {
         )
         .max(1);
 
+        let s3_public_endpoint = parse_optional_http_endpoint(
+            "FLUXER_S3_PUBLIC_ENDPOINT",
+            cfg::non_empty_env("FLUXER_S3_PUBLIC_ENDPOINT"),
+        );
+        let s3_uploads_bucket = cfg::read_env("FLUXER_S3_BUCKET_UPLOADS", "fluxer-uploads");
+        let s3_uploads_endpoint = s3_public_endpoint.as_ref().and_then(|endpoint| {
+            match endpoint.with_host_prefix("FLUXER_S3_BUCKET_UPLOADS", s3_uploads_bucket.trim()) {
+                Ok(endpoint) => Some(endpoint),
+                Err(error) => {
+                    warn_invalid(error);
+                    None
+                }
+            }
+        });
+
         Self {
             host: cfg::read_env("FLUXER_APP_PROXY_HOST", "0.0.0.0"),
             port: parse_env_or_warn(
@@ -183,10 +539,16 @@ impl AppProxyConfig {
                 8080u16,
             ),
             static_dir: cfg::read_env("FLUXER_STATIC_DIR", "./static"),
-            index_upstream_url: cfg::non_empty_env("FLUXER_APP_PROXY_INDEX_UPSTREAM_URL"),
-            static_cdn_endpoint: cfg::non_empty_env("FLUXER_STATIC_CDN_ENDPOINT"),
-            s3_public_endpoint: cfg::non_empty_env("FLUXER_S3_PUBLIC_ENDPOINT"),
-            s3_uploads_bucket: cfg::read_env("FLUXER_S3_BUCKET_UPLOADS", "fluxer-uploads"),
+            index_upstream_url: parse_optional_http_url(
+                "FLUXER_APP_PROXY_INDEX_UPSTREAM_URL",
+                cfg::non_empty_env("FLUXER_APP_PROXY_INDEX_UPSTREAM_URL"),
+            ),
+            static_cdn_endpoint: parse_optional_http_endpoint(
+                "FLUXER_STATIC_CDN_ENDPOINT",
+                cfg::non_empty_env("FLUXER_STATIC_CDN_ENDPOINT"),
+            ),
+            s3_public_endpoint: s3_public_endpoint.clone(),
+            s3_uploads_endpoint,
             discovery_upstream_url: resolve_discovery_upstream_url_from_env(),
             discovery_refresh_interval_ms: parse_env_or_warn(
                 "DISCOVERY_REFRESH_INTERVAL_MS",
@@ -458,7 +820,9 @@ mod tests {
     fn csp_config_default_has_no_extra_sources() {
         let c = CspConfig::default();
         assert!(
-            c.extra_default_src.is_none() && c.extra_script_src.is_none() && c.report_uri.is_none()
+            c.extra_default_src.is_empty()
+                && c.extra_script_src.is_empty()
+                && c.report_uri.is_none()
         );
     }
 
diff --git a/fluxer_app_proxy/src/csp.rs b/fluxer_app_proxy/src/csp.rs
index c011265cd..db011142c 100644
--- a/fluxer_app_proxy/src/csp.rs
+++ b/fluxer_app_proxy/src/csp.rs
@@ -1,16 +1,25 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
-use crate::config::CspConfig;
+use crate::config::{AppProxyConfig, CspConfig, CspSource, HttpEndpoint};
+use axum::http::HeaderValue;
+use axum::http::header::InvalidHeaderValue;
 use rand::RngExt;
-use reqwest::Url;
+
+const CSP_NONCE_HEX_DIGITS: usize = 32;
+const CSP_VALIDATION_NONCE: &str = "00000000000000000000000000000000";
+
+const _: () = assert!(
+    CSP_VALIDATION_NONCE.len() == CSP_NONCE_HEX_DIGITS,
+    "the nonce a policy is validated with must be shaped like the nonce a request carries"
+);
 
 #[derive(Clone, Debug, Default)]
 pub struct RuntimeCspSources {
-    pub static_cdn_endpoint: Option<String>,
-    pub media_endpoint: Option<String>,
-    pub s3_public_endpoint: Option<String>,
-    pub s3_uploads_bucket: Option<String>,
-    pub branding_image_origins: Vec<String>,
+    pub static_cdn_endpoint: Option<HttpEndpoint>,
+    pub media_endpoint: Option<HttpEndpoint>,
+    pub s3_public_endpoint: Option<HttpEndpoint>,
+    pub s3_uploads_endpoint: Option<HttpEndpoint>,
+    pub branding_image_origins: Vec<HttpEndpoint>,
 }
 
 const FRAME_SOURCES: &[&str] = &[
@@ -74,16 +83,99 @@ const WORKER_SOURCES: &[&str] = &["https://*.fluxer.app", "blob:"];
 
 const MANIFEST_SOURCES: &[&str] = &["https://*.fluxer.app"];
 
+#[derive(Debug)]
+pub enum CspCompileError {
+    InvalidAssetPolicy(InvalidHeaderValue),
+    InvalidSpaPolicy(InvalidHeaderValue),
+}
+
+impl std::fmt::Display for CspCompileError {
+    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::InvalidAssetPolicy(_) => {
+                formatter.write_str("the asset content security policy is not a valid header value")
+            }
+            Self::InvalidSpaPolicy(_) => {
+                formatter.write_str("the SPA content security policy is not a valid header value")
+            }
+        }
+    }
+}
+
+impl std::error::Error for CspCompileError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            Self::InvalidAssetPolicy(source) | Self::InvalidSpaPolicy(source) => Some(source),
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct CompiledCspPolicy {
+    config: CspConfig,
+    asset: HeaderValue,
+}
+
+impl CompiledCspPolicy {
+    pub fn from_config(config: &AppProxyConfig) -> Result<Self, CspCompileError> {
+        Self::compile(
+            config.csp.clone(),
+            &RuntimeCspSources {
+                static_cdn_endpoint: config.static_cdn_endpoint.clone(),
+                media_endpoint: None,
+                s3_public_endpoint: config.s3_public_endpoint.clone(),
+                s3_uploads_endpoint: config.s3_uploads_endpoint.clone(),
+                branding_image_origins: Vec::new(),
+            },
+        )
+    }
+
+    pub fn compile(
+        config: CspConfig,
+        configured_sources: &RuntimeCspSources,
+    ) -> Result<Self, CspCompileError> {
+        let asset_sources = RuntimeCspSources {
+            static_cdn_endpoint: configured_sources.static_cdn_endpoint.clone(),
+            ..RuntimeCspSources::default()
+        };
+        let asset = HeaderValue::from_str(&build_asset_csp(&config, &asset_sources))
+            .map_err(CspCompileError::InvalidAssetPolicy)?;
+        HeaderValue::from_str(&build_csp(
+            &config,
+            CSP_VALIDATION_NONCE,
+            configured_sources,
+        ))
+        .map_err(CspCompileError::InvalidSpaPolicy)?;
+        Ok(Self { config, asset })
+    }
+
+    pub fn asset_header(&self) -> HeaderValue {
+        self.asset.clone()
+    }
+
+    pub fn spa_header(&self, nonce: &str, runtime_sources: &RuntimeCspSources) -> HeaderValue {
+        assert!(
+            nonce.len() == CSP_NONCE_HEX_DIGITS
+                && nonce.bytes().all(|byte| byte.is_ascii_hexdigit()),
+            "a CSP nonce must be a 128-bit hexadecimal value"
+        );
+        HeaderValue::from_str(&build_csp(&self.config, nonce, runtime_sources)).expect(
+            "every CSP source is a validated keyword, scheme, or ASCII origin, so a policy built \
+             from them is always a valid header value",
+        )
+    }
+}
+
 pub fn generate_nonce() -> String {
     let bytes: [u8; 16] = rand::rng().random();
     hex::encode(bytes)
 }
 
-pub fn build_csp(config: &CspConfig, nonce: &str, runtime_sources: &RuntimeCspSources) -> String {
+fn build_csp(config: &CspConfig, nonce: &str, runtime_sources: &RuntimeCspSources) -> String {
     build_csp_directives(config, Some(nonce), runtime_sources).join("; ")
 }
 
-pub fn build_asset_csp(config: &CspConfig, runtime_sources: &RuntimeCspSources) -> String {
+fn build_asset_csp(config: &CspConfig, runtime_sources: &RuntimeCspSources) -> String {
     build_csp_directives(config, None, runtime_sources).join("; ")
 }
 
@@ -95,7 +187,7 @@ fn build_csp_directives(
     let mut directives = Vec::with_capacity(14);
 
     let mut default = vec!["'self'".to_owned()];
-    extend_from(&mut default, config.extra_default_src.as_deref(), &[]);
+    extend_from(&mut default, &config.extra_default_src, &[]);
     directives.push(format!("default-src {}", default.join(" ")));
 
     let mut script = vec![
@@ -106,21 +198,17 @@ fn build_csp_directives(
     if let Some(n) = nonce {
         script.insert(1, format!("'nonce-{n}'"));
     }
-    extend_from(
-        &mut script,
-        config.extra_script_src.as_deref(),
-        SCRIPT_SOURCES,
-    );
+    extend_from(&mut script, &config.extra_script_src, SCRIPT_SOURCES);
     extend_runtime_sources(&mut script, runtime_sources, true, false);
     directives.push(format!("script-src {}", script.join(" ")));
 
     let mut style = vec!["'self'".to_owned(), "'unsafe-inline'".to_owned()];
-    extend_from(&mut style, config.extra_style_src.as_deref(), STYLE_SOURCES);
+    extend_from(&mut style, &config.extra_style_src, STYLE_SOURCES);
     extend_runtime_sources(&mut style, runtime_sources, true, true);
     directives.push(format!("style-src {}", style.join(" ")));
 
     let mut img = vec!["'self'".to_owned(), "blob:".to_owned(), "data:".to_owned()];
-    extend_from(&mut img, config.extra_img_src.as_deref(), IMAGE_SOURCES);
+    extend_from(&mut img, &config.extra_img_src, IMAGE_SOURCES);
     extend_runtime_sources(&mut img, runtime_sources, true, true);
     for origin in &runtime_sources.branding_image_origins {
         push_endpoint_source(&mut img, Some(origin));
@@ -128,44 +216,32 @@ fn build_csp_directives(
     directives.push(format!("img-src {}", img.join(" ")));
 
     let mut media = vec!["'self'".to_owned(), "blob:".to_owned()];
-    extend_from(&mut media, config.extra_media_src.as_deref(), MEDIA_SOURCES);
+    extend_from(&mut media, &config.extra_media_src, MEDIA_SOURCES);
     extend_runtime_sources(&mut media, runtime_sources, true, true);
     directives.push(format!("media-src {}", media.join(" ")));
 
     let mut font = vec!["'self'".to_owned(), "data:".to_owned()];
-    extend_from(&mut font, config.extra_font_src.as_deref(), FONT_SOURCES);
+    extend_from(&mut font, &config.extra_font_src, FONT_SOURCES);
     extend_runtime_sources(&mut font, runtime_sources, true, true);
     directives.push(format!("font-src {}", font.join(" ")));
 
     let mut connect = vec!["'self'".to_owned(), "data:".to_owned()];
-    extend_from(
-        &mut connect,
-        config.extra_connect_src.as_deref(),
-        CONNECT_SOURCES,
-    );
+    extend_from(&mut connect, &config.extra_connect_src, CONNECT_SOURCES);
     extend_runtime_sources(&mut connect, runtime_sources, true, true);
     extend_runtime_s3_sources(&mut connect, runtime_sources);
     directives.push(format!("connect-src {}", connect.join(" ")));
 
     let mut frame = vec!["'self'".to_owned()];
-    extend_from(&mut frame, config.extra_frame_src.as_deref(), FRAME_SOURCES);
+    extend_from(&mut frame, &config.extra_frame_src, FRAME_SOURCES);
     directives.push(format!("frame-src {}", frame.join(" ")));
 
     let mut worker = vec!["'self'".to_owned(), "blob:".to_owned()];
-    extend_from(
-        &mut worker,
-        config.extra_worker_src.as_deref(),
-        WORKER_SOURCES,
-    );
+    extend_from(&mut worker, &config.extra_worker_src, WORKER_SOURCES);
     extend_runtime_sources(&mut worker, runtime_sources, true, false);
     directives.push(format!("worker-src {}", worker.join(" ")));
 
     let mut manifest = vec!["'self'".to_owned()];
-    extend_from(
-        &mut manifest,
-        config.extra_manifest_src.as_deref(),
-        MANIFEST_SOURCES,
-    );
+    extend_from(&mut manifest, &config.extra_manifest_src, MANIFEST_SOURCES);
     extend_runtime_sources(&mut manifest, runtime_sources, true, false);
     directives.push(format!("manifest-src {}", manifest.join(" ")));
 
@@ -187,78 +263,40 @@ fn extend_runtime_sources(
     include_media: bool,
 ) {
     if include_static {
-        push_endpoint_source(target, runtime_sources.static_cdn_endpoint.as_deref());
+        push_endpoint_source(target, runtime_sources.static_cdn_endpoint.as_ref());
     }
     if include_media {
-        push_endpoint_source(target, runtime_sources.media_endpoint.as_deref());
+        push_endpoint_source(target, runtime_sources.media_endpoint.as_ref());
     }
 }
 
-pub fn http_origin(raw: &str) -> Option<String> {
-    let url = Url::parse(raw.trim()).ok()?;
-    let scheme = url.scheme();
-    if scheme != "http" && scheme != "https" {
-        return None;
-    }
-    let host = url.host_str()?;
-    let port = url
-        .port()
-        .map(|port| format!(":{port}"))
-        .unwrap_or_default();
-    Some(format!("{scheme}://{host}{port}"))
-}
-
-fn push_endpoint_source(target: &mut Vec<String>, endpoint: Option<&str>) {
+fn push_endpoint_source(target: &mut Vec<String>, endpoint: Option<&HttpEndpoint>) {
     let Some(endpoint) = endpoint else {
         return;
     };
-    let source = endpoint.trim().trim_end_matches('/');
-    if source.is_empty() || target.iter().any(|existing| existing == source) {
+    let source = endpoint.csp_origin();
+    if target.iter().any(|existing| existing == source) {
         return;
     }
     target.push(source.to_owned());
 }
 
 fn extend_runtime_s3_sources(target: &mut Vec<String>, runtime_sources: &RuntimeCspSources) {
-    push_endpoint_source(target, runtime_sources.s3_public_endpoint.as_deref());
-
-    let Some(source) = s3_uploads_bucket_origin(runtime_sources) else {
-        return;
-    };
-    push_endpoint_source(target, Some(&source));
+    push_endpoint_source(target, runtime_sources.s3_public_endpoint.as_ref());
+    push_endpoint_source(target, runtime_sources.s3_uploads_endpoint.as_ref());
 }
 
-fn s3_uploads_bucket_origin(runtime_sources: &RuntimeCspSources) -> Option<String> {
-    let bucket = runtime_sources
-        .s3_uploads_bucket
-        .as_deref()
-        .map(str::trim)
-        .filter(|value| !value.is_empty())?;
-    let endpoint = runtime_sources.s3_public_endpoint.as_deref()?.trim();
-    let url = Url::parse(endpoint).ok()?;
-    let scheme = url.scheme();
-    if scheme != "http" && scheme != "https" {
-        return None;
+fn extend_from(target: &mut Vec<String>, extra: &[CspSource], defaults: &[&str]) {
+    for source in defaults {
+        if target.iter().any(|existing| existing == source) {
+            continue;
+        }
+        target.push((*source).to_owned());
     }
-    let host = url.host_str()?;
-    let host = if host.starts_with(&format!("{bucket}.")) {
-        host.to_owned()
-    } else {
-        format!("{bucket}.{host}")
-    };
-    let port = url
-        .port()
-        .map(|port| format!(":{port}"))
-        .unwrap_or_default();
-    Some(format!("{scheme}://{host}{port}"))
-}
 
-fn extend_from(target: &mut Vec<String>, extra: Option<&[String]>, defaults: &[&str]) {
-    target.extend(defaults.iter().map(|s| (*s).to_owned()));
-
-    for source in extra.into_iter().flatten() {
-        let source = source.trim();
-        if source.is_empty() || target.iter().any(|existing| existing == source) {
+    for source in extra {
+        let source = source.as_str();
+        if target.iter().any(|existing| existing == source) {
             continue;
         }
         target.push(source.to_owned());
@@ -272,8 +310,11 @@ mod tests {
     #[test]
     fn generate_nonce_produces_32_char_hex() {
         let nonce = generate_nonce();
-        assert_eq!(nonce.len(), 32);
+        assert_eq!(nonce.len(), CSP_NONCE_HEX_DIGITS);
         assert!(nonce.chars().all(|c| c.is_ascii_hexdigit()));
+        CompiledCspPolicy::compile(default_csp_config(), &runtime_sources())
+            .unwrap()
+            .spa_header(&nonce, &runtime_sources());
     }
 
     #[test]
@@ -291,6 +332,10 @@ mod tests {
         RuntimeCspSources::default()
     }
 
+    fn endpoint(value: &str) -> HttpEndpoint {
+        HttpEndpoint::parse("TEST_ENDPOINT", value).unwrap()
+    }
+
     #[test]
     fn build_csp_includes_required_directives() {
         let config = default_csp_config();
@@ -336,7 +381,13 @@ mod tests {
     #[test]
     fn build_csp_includes_report_uri_when_configured() {
         let config = CspConfig {
-            report_uri: Some("https://example.com/csp-report".to_owned()),
+            report_uri: Some(
+                crate::config::CspReportUri::parse(
+                    "TEST_CSP_REPORT_URI",
+                    "https://example.com/csp-report",
+                )
+                .unwrap(),
+            ),
             ..Default::default()
         };
         let csp = build_csp(&config, "nonce1", &runtime_sources());
@@ -354,8 +405,8 @@ mod tests {
     fn build_csp_includes_configured_runtime_endpoints() {
         let config = default_csp_config();
         let runtime_sources = RuntimeCspSources {
-            static_cdn_endpoint: Some("https://static.example.test/".to_owned()),
-            media_endpoint: Some("https://media.example.test".to_owned()),
+            static_cdn_endpoint: Some(endpoint("https://static.example.test/")),
+            media_endpoint: Some(endpoint("https://media.example.test")),
             ..Default::default()
         };
         let csp = build_csp(&config, "nonce1", &runtime_sources);
@@ -365,12 +416,38 @@ mod tests {
         assert!(!csp.contains("https://static.example.test/ "));
     }
 
+    #[test]
+    fn a_csp_source_cannot_smuggle_a_second_directive() {
+        for injected in [
+            "https://evil.test; script-src *",
+            "https://evil.test,https://other.test",
+            "https://evil.test https://other.test",
+            "https://evil.test\nscript-src *",
+        ] {
+            assert!(
+                CspSource::parse("TEST_CSP_SOURCE", injected).is_err(),
+                "{injected:?} must not parse as a single CSP source"
+            );
+        }
+    }
+
+    #[test]
+    fn a_report_uri_cannot_smuggle_a_second_directive() {
+        assert!(
+            crate::config::CspReportUri::parse(
+                "TEST_CSP_REPORT_URI",
+                "https://evil.test/r; script-src *"
+            )
+            .is_err()
+        );
+    }
+
     #[test]
     fn build_csp_includes_s3_public_and_virtual_hosted_upload_origins() {
         let config = default_csp_config();
         let runtime_sources = RuntimeCspSources {
-            s3_public_endpoint: Some("http://localhost:3900/".to_owned()),
-            s3_uploads_bucket: Some("fluxer-uploads".to_owned()),
+            s3_public_endpoint: Some(endpoint("http://localhost:3900/")),
+            s3_uploads_endpoint: Some(endpoint("http://fluxer-uploads.localhost:3900/")),
             ..Default::default()
         };
 
@@ -380,4 +457,72 @@ mod tests {
         assert!(csp.contains("http://fluxer-uploads.localhost:3900"));
         assert!(!csp.contains("http://localhost:3900/ "));
     }
+
+    #[test]
+    fn a_compiled_asset_header_is_the_policy_every_asset_response_reuses() {
+        let sources = RuntimeCspSources {
+            static_cdn_endpoint: Some(endpoint("https://static.example.test/")),
+            media_endpoint: Some(endpoint("https://media.example.test")),
+            s3_public_endpoint: Some(endpoint("http://localhost:3900/")),
+            ..Default::default()
+        };
+        let policy = CompiledCspPolicy::compile(default_csp_config(), &sources).unwrap();
+
+        assert_eq!(policy.asset_header(), policy.asset_header());
+        let asset = policy.asset_header();
+        let asset = asset.to_str().unwrap();
+        assert!(!asset.contains("nonce-"));
+        assert!(asset.contains("https://static.example.test"));
+        assert!(
+            !asset.contains("https://media.example.test"),
+            "an asset response must not widen the policy with the endpoints only the document needs"
+        );
+        assert!(!asset.contains("http://localhost:3900"));
+    }
+
+    #[test]
+    fn a_compiled_policy_stamps_the_requests_own_nonce_and_discovery_endpoints() {
+        let policy = CompiledCspPolicy::compile(default_csp_config(), &runtime_sources()).unwrap();
+        let discovered = RuntimeCspSources {
+            static_cdn_endpoint: Some(endpoint("https://cdn.discovered.test")),
+            branding_image_origins: vec![endpoint("https://branding.discovered.test")],
+            ..Default::default()
+        };
+
+        let header = policy.spa_header("0123456789abcdef0123456789abcdef", &discovered);
+        let header = header.to_str().unwrap();
+
+        assert!(header.contains("'nonce-0123456789abcdef0123456789abcdef'"));
+        assert!(header.contains("https://cdn.discovered.test"));
+        assert!(header.contains("https://branding.discovered.test"));
+    }
+
+    #[test]
+    fn a_compiled_policy_matches_the_directives_it_was_compiled_from() {
+        let config = default_csp_config();
+        let sources = RuntimeCspSources {
+            static_cdn_endpoint: Some(endpoint("https://static.example.test/")),
+            ..Default::default()
+        };
+        let policy = CompiledCspPolicy::compile(config.clone(), &sources).unwrap();
+
+        assert_eq!(
+            policy.asset_header().to_str().unwrap(),
+            build_asset_csp(&config, &sources)
+        );
+        assert_eq!(
+            policy
+                .spa_header(CSP_VALIDATION_NONCE, &sources)
+                .to_str()
+                .unwrap(),
+            build_csp(&config, CSP_VALIDATION_NONCE, &sources)
+        );
+    }
+
+    #[test]
+    #[should_panic(expected = "a CSP nonce must be a 128-bit hexadecimal value")]
+    fn a_compiled_policy_refuses_a_nonce_it_did_not_generate() {
+        let policy = CompiledCspPolicy::compile(default_csp_config(), &runtime_sources()).unwrap();
+        policy.spa_header("not-a-nonce", &runtime_sources());
+    }
 }
diff --git a/fluxer_app_proxy/src/discovery_cache.rs b/fluxer_app_proxy/src/discovery_cache.rs
index d26a2445b..cc38785a7 100644
--- a/fluxer_app_proxy/src/discovery_cache.rs
+++ b/fluxer_app_proxy/src/discovery_cache.rs
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
+use crate::config::HttpEndpoint;
 use serde::{Deserialize, Serialize};
 use std::sync::Arc;
 use std::time::{Duration, Instant};
@@ -14,6 +15,26 @@ pub struct DiscoveryResponse {
     pub data: serde_json::Value,
 }
 
+pub fn discovery_endpoint(
+    discovery: &DiscoveryResponse,
+    key: &'static str,
+) -> Option<HttpEndpoint> {
+    let raw = discovery
+        .data
+        .get("endpoints")
+        .and_then(|endpoints| endpoints.get(key))
+        .and_then(|value| value.as_str())
+        .map(str::trim)
+        .filter(|value| !value.is_empty())?;
+    match HttpEndpoint::parse(key, raw) {
+        Ok(endpoint) => Some(endpoint),
+        Err(error) => {
+            tracing::warn!(%error, "ignoring invalid discovery endpoint");
+            None
+        }
+    }
+}
+
 pub struct DiscoveryCache {
     cached: RwLock<Option<DiscoveryResponse>>,
     cold_start_attempt: Mutex<Option<Instant>>,
diff --git a/fluxer_app_proxy/src/main.rs b/fluxer_app_proxy/src/main.rs
index 20b05281e..a743314d1 100644
--- a/fluxer_app_proxy/src/main.rs
+++ b/fluxer_app_proxy/src/main.rs
@@ -3,10 +3,13 @@
 use anyhow::Context;
 use fluxer_app_proxy::{
     config::AppProxyConfig,
+    csp::CompiledCspPolicy,
     discovery_cache::DiscoveryCache,
     geoip, invite_meta,
     routes::build_router,
-    state::{AppState, build_http_client},
+    state::{
+        AppProxyBudgets, AppState, MAX_SPA_INDEX_BYTES, build_http_client, read_bounded_text_file,
+    },
 };
 use std::sync::{Arc, OnceLock};
 use tokio::{net::TcpListener, runtime::Builder};
@@ -23,6 +26,11 @@ fn main() -> anyhow::Result<()> {
     let config = Arc::new(AppProxyConfig::from_env());
     let addr = format!("{}:{}", config.host, config.port);
 
+    let csp = Arc::new(
+        CompiledCspPolicy::from_config(&config)
+            .context("failed to compile the Fluxer app proxy content security policy")?,
+    );
+
     let geoip = Arc::new(geoip::resolver_from_app_config(&config));
 
     let runtime = Builder::new_multi_thread()
@@ -55,7 +63,7 @@ fn main() -> anyhow::Result<()> {
 
         let index_html = if config.index_upstream_url.is_none() {
             let index_path = std::path::Path::new(&config.static_dir).join("index.html");
-            match tokio::fs::read_to_string(&index_path).await {
+            match read_bounded_text_file(&index_path, MAX_SPA_INDEX_BYTES).await {
                 Ok(contents) => Some(Arc::<str>::from(contents)),
                 Err(err) => {
                     tracing::warn!(path = ?index_path, %err, "failed to preload index.html; will read per request");
@@ -68,11 +76,13 @@ fn main() -> anyhow::Result<()> {
 
         let state = AppState {
             config,
+            csp,
             http_client,
             discovery_cache,
             geoip,
             invite_meta,
             index_html,
+            budgets: AppProxyBudgets::default(),
         };
 
         let router = build_router(state);
diff --git a/fluxer_app_proxy/src/routes/assets_proxy.rs b/fluxer_app_proxy/src/routes/assets_proxy.rs
index 23906c4c7..664ed6c43 100644
--- a/fluxer_app_proxy/src/routes/assets_proxy.rs
+++ b/fluxer_app_proxy/src/routes/assets_proxy.rs
@@ -1,7 +1,6 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
-use crate::csp::{RuntimeCspSources, build_asset_csp};
-use crate::state::AppState;
+use crate::state::{AppProxyBudgets, AppState};
 use axum::{
     body::Body,
     extract::{Path, State},
@@ -10,6 +9,9 @@ use axum::{
 };
 use std::path::{Path as FsPath, PathBuf};
 use std::time::Duration;
+use tokio::io::{AsyncWriteExt, DuplexStream};
+use tokio::sync::{OwnedSemaphorePermit, TryAcquireError};
+use tokio_util::io::ReaderStream;
 
 use super::file_stream::stream_file;
 use super::spa_static::{CORS_ALLOW_ANY_VALUE, asset_cache_control, guess_mime, is_font_mime};
@@ -17,6 +19,7 @@ use super::spa_static::{CORS_ALLOW_ANY_VALUE, asset_cache_control, guess_mime, i
 const ASSET_REQUEST_TIMEOUT: Duration = Duration::from_secs(15);
 const PRECOMPRESSED_VARIANTS: &[(&str, &str)] = &[("br", "br"), ("gzip", "gz")];
 const MAX_ASSET_SIZE_BYTES: u64 = 100 * 1024 * 1024;
+const UPSTREAM_ASSET_PUMP_BUFFER_BYTES: usize = 64 * 1024;
 const UPSTREAM_FAILURE_CACHE_CONTROL: &str = "no-store";
 const UPSTREAM_FAILURE_STRIPPED_HEADERS: &[&str] = &[
     "cdn-cache-control",
@@ -60,21 +63,38 @@ pub async fn proxy_assets(
 ) -> Response {
     let Some(cdn_endpoint) = &state.config.static_cdn_endpoint else {
         return serve_local_asset(
+            &state.budgets,
             &state.config.static_dir,
             &format!("assets/{path}"),
             request.headers(),
+            state.csp.asset_header(),
         )
         .await;
     };
 
-    let target_url = format!("{cdn_endpoint}/assets/{path}");
+    let target_url = format!("{}/assets/{path}", cdn_endpoint.as_str());
 
     let upstream_host = cdn_endpoint
-        .trim_start_matches("https://")
-        .trim_start_matches("http://")
-        .split('/')
-        .next()
-        .unwrap_or("localhost");
+        .as_url()
+        .host_str()
+        .map(|host| match cdn_endpoint.as_url().port() {
+            Some(port) => format!("{host}:{port}"),
+            None => host.to_owned(),
+        })
+        .unwrap_or_else(|| "localhost".to_owned());
+
+    let upstream_slot = match state
+        .budgets
+        .upstream_asset_slots
+        .clone()
+        .try_acquire_owned()
+    {
+        Ok(permit) => permit,
+        Err(TryAcquireError::NoPermits) => return super::capacity_refused_response(),
+        Err(TryAcquireError::Closed) => {
+            panic!("upstream asset slot semaphore closed unexpectedly")
+        }
+    };
 
     let mut request_builder = state
         .http_client
@@ -88,7 +108,7 @@ pub async fn proxy_assets(
         }
         request_builder = request_builder.header(name.clone(), value.clone());
     }
-    request_builder = request_builder.header("host", upstream_host);
+    request_builder = request_builder.header("host", upstream_host.as_str());
 
     let upstream_response = match request_builder.send().await {
         Ok(resp) => resp,
@@ -125,33 +145,54 @@ pub async fn proxy_assets(
     set_proxied_cache_control(&mut response_headers, &path, status);
     set_vary_on_accept_encoding(&mut response_headers);
 
-    let asset_csp = build_asset_csp(
-        &state.config.csp,
-        &RuntimeCspSources {
-            static_cdn_endpoint: state.config.static_cdn_endpoint.clone(),
-            media_endpoint: None,
-            s3_public_endpoint: None,
-            s3_uploads_bucket: None,
-            branding_image_origins: Vec::new(),
-        },
-    );
-    if let Ok(value) = HeaderValue::from_str(&asset_csp) {
-        response_headers.insert(header::CONTENT_SECURITY_POLICY, value);
-    }
+    response_headers.insert(header::CONTENT_SECURITY_POLICY, state.csp.asset_header());
     response_headers.remove("content-security-policy-report-only");
 
-    let body = Body::from_stream(upstream_response.bytes_stream());
+    let body = Body::from_stream(upstream_asset_body(upstream_response, upstream_slot));
     let mut response = Response::new(body);
     *response.status_mut() = status;
     *response.headers_mut() = response_headers;
     response
 }
 
+fn upstream_asset_body(
+    mut upstream_response: reqwest::Response,
+    upstream_slot: OwnedSemaphorePermit,
+) -> ReaderStream<DuplexStream> {
+    let (writer, reader) = tokio::io::duplex(UPSTREAM_ASSET_PUMP_BUFFER_BYTES);
+    tokio::spawn(async move {
+        let _upstream_slot = upstream_slot;
+        let mut writer = writer;
+        loop {
+            match upstream_response.chunk().await {
+                Ok(Some(chunk)) => {
+                    if writer.write_all(&chunk).await.is_err() {
+                        return;
+                    }
+                }
+                Ok(None) => break,
+                Err(err) => {
+                    tracing::warn!(%err, "upstream asset body ended early");
+                    return;
+                }
+            }
+        }
+        let _ = writer.shutdown().await;
+    });
+    ReaderStream::new(reader)
+}
+
 pub(super) async fn serve_local_asset(
+    budgets: &AppProxyBudgets,
     static_dir: &str,
     relative_path: &str,
     request_headers: &HeaderMap,
+    csp_asset_header: HeaderValue,
 ) -> Response {
+    let Ok(_read_slot) = budgets.local_read_slots.try_acquire() else {
+        return super::capacity_refused_response();
+    };
+
     let file_path = FsPath::new(static_dir).join(relative_path);
 
     let resolved = match tokio::fs::canonicalize(&file_path).await {
@@ -179,7 +220,12 @@ pub(super) async fn serve_local_asset(
         && if_none_match_matches(request_headers, entity_tag)
     {
         let mut response = StatusCode::NOT_MODIFIED.into_response();
-        set_local_asset_headers(response.headers_mut(), relative_path, Some(entity_tag));
+        set_local_asset_headers(
+            response.headers_mut(),
+            relative_path,
+            Some(entity_tag),
+            &csp_asset_header,
+        );
         return response;
     }
 
@@ -205,7 +251,12 @@ pub(super) async fn serve_local_asset(
             HeaderValue::from_static(content_encoding),
         );
     }
-    set_local_asset_headers(response.headers_mut(), relative_path, entity_tag.as_deref());
+    set_local_asset_headers(
+        response.headers_mut(),
+        relative_path,
+        entity_tag.as_deref(),
+        &csp_asset_header,
+    );
     response
 }
 
@@ -269,11 +320,17 @@ fn is_zero_quality(parameter: &str) -> bool {
             .is_ok_and(|quality| quality <= 0.0)
 }
 
-fn set_local_asset_headers(headers: &mut HeaderMap, relative_path: &str, entity_tag: Option<&str>) {
+fn set_local_asset_headers(
+    headers: &mut HeaderMap,
+    relative_path: &str,
+    entity_tag: Option<&str>,
+    csp_asset_header: &HeaderValue,
+) {
     headers.insert(
         header::CACHE_CONTROL,
         HeaderValue::from_static(asset_cache_control(relative_path)),
     );
+    headers.insert(header::CONTENT_SECURITY_POLICY, csp_asset_header.clone());
     set_vary_on_accept_encoding(headers);
     if is_font_mime(guess_mime(relative_path)) {
         headers.insert(
@@ -403,7 +460,9 @@ mod tests {
 
     fn upstream_backed_state(cdn_endpoint: &str) -> AppState {
         let mut config = AppProxyConfig::from_env();
-        config.static_cdn_endpoint = Some(cdn_endpoint.to_owned());
+        config.static_cdn_endpoint = Some(
+            crate::config::HttpEndpoint::parse("TEST_STATIC_CDN_ENDPOINT", cdn_endpoint).unwrap(),
+        );
         state_from_config(config)
     }
 
@@ -415,8 +474,13 @@ mod tests {
     }
 
     fn state_from_config(config: AppProxyConfig) -> AppState {
+        let csp = Arc::new(
+            crate::csp::CompiledCspPolicy::from_config(&config)
+                .expect("the test configuration must compile to a valid CSP"),
+        );
         AppState {
             config: Arc::new(config),
+            csp,
             http_client: build_http_client().unwrap(),
             discovery_cache: Arc::new(DiscoveryCache::new()),
             geoip: Arc::new(GeoipResolver::from_config(&GeoipConfig {
@@ -429,6 +493,7 @@ mod tests {
             })),
             invite_meta: Arc::new(OnceLock::new()),
             index_html: None,
+            budgets: crate::state::AppProxyBudgets::default(),
         }
     }
 
@@ -656,6 +721,18 @@ mod tests {
         }
     }
 
+    fn budgets() -> AppProxyBudgets {
+        AppProxyBudgets::default()
+    }
+
+    fn test_asset_csp() -> HeaderValue {
+        let mut config = AppProxyConfig::from_env();
+        config.static_cdn_endpoint = None;
+        crate::csp::CompiledCspPolicy::from_config(&config)
+            .expect("the test configuration must compile to a valid CSP")
+            .asset_header()
+    }
+
     fn entity_tag_of(response: &Response) -> Option<String> {
         response
             .headers()
@@ -676,9 +753,11 @@ mod tests {
         let fixture = LocalAssetDir::with_asset("0018072843a46dc4.woff2", b"wOF2stub");
 
         let first = serve_local_asset(
+            &budgets(),
             fixture.dir(),
             "assets/0018072843a46dc4.woff2",
             &HeaderMap::new(),
+            test_asset_csp(),
         )
         .await;
         assert_eq!(cors_origin_of(&first), Some(CORS_ALLOW_ANY_VALUE));
@@ -689,8 +768,14 @@ mod tests {
             header::IF_NONE_MATCH,
             HeaderValue::from_str(&entity_tag).unwrap(),
         );
-        let second =
-            serve_local_asset(fixture.dir(), "assets/0018072843a46dc4.woff2", &conditional).await;
+        let second = serve_local_asset(
+            &budgets(),
+            fixture.dir(),
+            "assets/0018072843a46dc4.woff2",
+            &conditional,
+            test_asset_csp(),
+        )
+        .await;
 
         assert_eq!(second.status(), StatusCode::NOT_MODIFIED);
         assert_eq!(
@@ -705,9 +790,11 @@ mod tests {
         let fixture = LocalAssetDir::with_asset("356aaade04a117b1.js", b"console.log(1)");
 
         let response = serve_local_asset(
+            &budgets(),
             fixture.dir(),
             "assets/356aaade04a117b1.js",
             &HeaderMap::new(),
+            test_asset_csp(),
         )
         .await;
 
@@ -728,7 +815,14 @@ mod tests {
 
         let mut resumed = HeaderMap::new();
         resumed.insert(header::RANGE, HeaderValue::from_static("bytes=10-"));
-        let response = serve_local_asset(fixture.dir(), "assets/fluxer-setup.exe", &resumed).await;
+        let response = serve_local_asset(
+            &budgets(),
+            fixture.dir(),
+            "assets/fluxer-setup.exe",
+            &resumed,
+            test_asset_csp(),
+        )
+        .await;
 
         assert_eq!(
             response.status(),
@@ -756,9 +850,11 @@ mod tests {
         let fixture = LocalAssetDir::with_asset("f00dcafe12345678.css", b"body{}");
 
         let first = serve_local_asset(
+            &budgets(),
             fixture.dir(),
             "assets/f00dcafe12345678.css",
             &HeaderMap::new(),
+            test_asset_csp(),
         )
         .await;
         let entity_tag = entity_tag_of(&first).expect("first response carries a validator");
@@ -768,8 +864,14 @@ mod tests {
             header::IF_NONE_MATCH,
             HeaderValue::from_str(&entity_tag).unwrap(),
         );
-        let second =
-            serve_local_asset(fixture.dir(), "assets/f00dcafe12345678.css", &conditional).await;
+        let second = serve_local_asset(
+            &budgets(),
+            fixture.dir(),
+            "assets/f00dcafe12345678.css",
+            &conditional,
+            test_asset_csp(),
+        )
+        .await;
 
         assert_eq!(second.status(), StatusCode::NOT_MODIFIED);
         assert_eq!(entity_tag_of(&second).as_deref(), Some(entity_tag.as_str()));
@@ -788,8 +890,14 @@ mod tests {
             header::IF_NONE_MATCH,
             HeaderValue::from_static("\"stale-from-a-previous-build\""),
         );
-        let response =
-            serve_local_asset(fixture.dir(), "assets/voice_engine_bg.wasm", &conditional).await;
+        let response = serve_local_asset(
+            &budgets(),
+            fixture.dir(),
+            "assets/voice_engine_bg.wasm",
+            &conditional,
+            test_asset_csp(),
+        )
+        .await;
 
         assert_eq!(response.status(), StatusCode::OK);
         assert_eq!(
@@ -807,9 +915,11 @@ mod tests {
         let fixture = LocalAssetDir::with_asset("2d715e4730758083.worker.js", b"self.onmessage=0");
 
         let response = serve_local_asset(
+            &budgets(),
             fixture.dir(),
             "assets/2d715e4730758083.worker.js",
             &HeaderMap::new(),
+            test_asset_csp(),
         )
         .await;
 
@@ -859,9 +969,11 @@ mod tests {
             .and_sibling("356aaade04a117b1.js.br", b"brotli-bytes");
 
         let response = serve_local_asset(
+            &budgets(),
             fixture.dir(),
             "assets/356aaade04a117b1.js",
             &accept_encoding("gzip, deflate, br, zstd"),
+            test_asset_csp(),
         )
         .await;
 
@@ -887,9 +999,11 @@ mod tests {
         let fixture = LocalAssetDir::with_asset("469e0b8f10c496a1.css", b"body{color:red}");
 
         let response = serve_local_asset(
+            &budgets(),
             fixture.dir(),
             "assets/469e0b8f10c496a1.css",
             &accept_encoding("gzip, deflate, br"),
+            test_asset_csp(),
         )
         .await;
 
@@ -906,18 +1020,22 @@ mod tests {
             .and_sibling("488b87159423ca35.js.gz", b"gzip-bytes");
 
         let gzip_only = serve_local_asset(
+            &budgets(),
             fixture.dir(),
             "assets/488b87159423ca35.js",
             &accept_encoding("gzip, deflate"),
+            test_asset_csp(),
         )
         .await;
         assert_eq!(content_encoding_of(&gzip_only), Some("gzip"));
         assert_eq!(body_bytes(gzip_only).await, b"gzip-bytes");
 
         let identity = serve_local_asset(
+            &budgets(),
             fixture.dir(),
             "assets/488b87159423ca35.js",
             &HeaderMap::new(),
+            test_asset_csp(),
         )
         .await;
         assert_eq!(
@@ -934,9 +1052,11 @@ mod tests {
             .and_sibling("2d715e4730758083.worker.js.br", b"brotli-bytes");
 
         let response = serve_local_asset(
+            &budgets(),
             fixture.dir(),
             "assets/2d715e4730758083.worker.js",
             &accept_encoding("br;q=0, gzip"),
+            test_asset_csp(),
         )
         .await;
 
@@ -950,17 +1070,21 @@ mod tests {
             .and_sibling("f00dcafe12345678.css.br", b"brotli-bytes-are-longer");
 
         let brotli = serve_local_asset(
+            &budgets(),
             fixture.dir(),
             "assets/f00dcafe12345678.css",
             &accept_encoding("br"),
+            test_asset_csp(),
         )
         .await;
         let brotli_tag = entity_tag_of(&brotli).expect("the brotli variant carries a validator");
 
         let identity = serve_local_asset(
+            &budgets(),
             fixture.dir(),
             "assets/f00dcafe12345678.css",
             &HeaderMap::new(),
+            test_asset_csp(),
         )
         .await;
         let identity_tag = entity_tag_of(&identity).expect("the raw file carries a validator");
@@ -975,8 +1099,14 @@ mod tests {
             header::IF_NONE_MATCH,
             HeaderValue::from_str(&brotli_tag).unwrap(),
         );
-        let revalidated =
-            serve_local_asset(fixture.dir(), "assets/f00dcafe12345678.css", &conditional).await;
+        let revalidated = serve_local_asset(
+            &budgets(),
+            fixture.dir(),
+            "assets/f00dcafe12345678.css",
+            &conditional,
+            test_asset_csp(),
+        )
+        .await;
         assert_eq!(revalidated.status(), StatusCode::NOT_MODIFIED);
         assert!(varies_on_accept_encoding(&revalidated));
     }
@@ -988,8 +1118,14 @@ mod tests {
 
         let mut ranged = accept_encoding("br");
         ranged.insert(header::RANGE, HeaderValue::from_static("bytes=4-6"));
-        let response =
-            serve_local_asset(fixture.dir(), "assets/356aaade04a117b1.js", &ranged).await;
+        let response = serve_local_asset(
+            &budgets(),
+            fixture.dir(),
+            "assets/356aaade04a117b1.js",
+            &ranged,
+            test_asset_csp(),
+        )
+        .await;
 
         assert_eq!(response.status(), StatusCode::PARTIAL_CONTENT);
         assert_eq!(content_encoding_of(&response), Some("br"));
@@ -1305,4 +1441,84 @@ mod tests {
             Some("application/octet-stream")
         );
     }
+
+    #[tokio::test]
+    async fn a_local_asset_is_refused_once_the_read_slots_are_gone() {
+        let fixture = LocalAssetDir::with_asset("356aaade04a117b1.js", b"console.log(1)");
+        let budgets = AppProxyBudgets::default();
+        let held = budgets
+            .local_read_slots
+            .clone()
+            .try_acquire_many_owned(
+                u32::try_from(crate::state::LOCAL_FILE_READS_IN_FLIGHT_MAX).unwrap(),
+            )
+            .expect("a fresh budget holds every local read slot");
+
+        let refused = serve_local_asset(
+            &budgets,
+            fixture.dir(),
+            "assets/356aaade04a117b1.js",
+            &HeaderMap::new(),
+            test_asset_csp(),
+        )
+        .await;
+        assert_eq!(refused.status(), StatusCode::SERVICE_UNAVAILABLE);
+        assert_eq!(
+            refused
+                .headers()
+                .get(header::CACHE_CONTROL)
+                .and_then(|value| value.to_str().ok()),
+            Some("no-store"),
+            "a cached refusal would pin the outage for every later reader"
+        );
+
+        drop(held);
+        let served = serve_local_asset(
+            &budgets,
+            fixture.dir(),
+            "assets/356aaade04a117b1.js",
+            &HeaderMap::new(),
+            test_asset_csp(),
+        )
+        .await;
+        assert_eq!(served.status(), StatusCode::OK);
+    }
+
+    #[tokio::test]
+    async fn an_open_local_asset_response_never_holds_a_read_slot() {
+        let fixture = LocalAssetDir::with_asset("356aaade04a117b1.js", b"console.log(1)");
+        let budgets = AppProxyBudgets::default();
+
+        let mut open = Vec::with_capacity(crate::state::LOCAL_FILE_READS_IN_FLIGHT_MAX + 1);
+        for _ in 0..=crate::state::LOCAL_FILE_READS_IN_FLIGHT_MAX {
+            open.push(
+                serve_local_asset(
+                    &budgets,
+                    fixture.dir(),
+                    "assets/356aaade04a117b1.js",
+                    &HeaderMap::new(),
+                    test_asset_csp(),
+                )
+                .await,
+            );
+        }
+
+        let refused = open
+            .iter()
+            .filter(|response| response.status() != StatusCode::OK)
+            .count();
+        assert_eq!(
+            refused, 0,
+            "{refused} readers were turned away while earlier responses were still open"
+        );
+
+        let body = axum::body::to_bytes(open.pop().unwrap().into_body(), usize::MAX)
+            .await
+            .unwrap();
+        assert_eq!(
+            body.as_ref(),
+            b"console.log(1)",
+            "a response served past the read slot count carried the wrong bytes"
+        );
+    }
 }
diff --git a/fluxer_app_proxy/src/routes/health.rs b/fluxer_app_proxy/src/routes/health.rs
index 19a2f108c..12667509a 100644
--- a/fluxer_app_proxy/src/routes/health.rs
+++ b/fluxer_app_proxy/src/routes/health.rs
@@ -81,8 +81,13 @@ mod tests {
         let mut config = AppProxyConfig::from_env();
         config.invite_meta_enabled = invite_meta_enabled;
         config.discovery_upstream_url = spawn_discovery_origin().await;
+        let csp = Arc::new(
+            crate::csp::CompiledCspPolicy::from_config(&config)
+                .expect("the test configuration must compile to a valid CSP"),
+        );
         AppState {
             config: Arc::new(config),
+            csp,
             http_client: build_http_client().unwrap(),
             discovery_cache: Arc::new(DiscoveryCache::new()),
             geoip: Arc::new(GeoipResolver::from_config(&GeoipConfig {
@@ -95,6 +100,7 @@ mod tests {
             })),
             invite_meta: Arc::new(OnceLock::new()),
             index_html: None,
+            budgets: crate::state::AppProxyBudgets::default(),
         }
     }
 
diff --git a/fluxer_app_proxy/src/routes/mod.rs b/fluxer_app_proxy/src/routes/mod.rs
index af11ea4a8..29e971eed 100644
--- a/fluxer_app_proxy/src/routes/mod.rs
+++ b/fluxer_app_proxy/src/routes/mod.rs
@@ -14,7 +14,7 @@ use axum::{
     extract::Request,
     http::{HeaderName, HeaderValue, header},
     middleware::{Next, from_fn, from_fn_with_state},
-    response::Response,
+    response::{IntoResponse, Response},
     routing::get,
 };
 use rand::RngExt;
@@ -127,6 +127,14 @@ fn generate_request_id() -> String {
     hex::encode(bytes)
 }
 
+pub(super) fn capacity_refused_response() -> Response {
+    let mut response = axum::http::StatusCode::SERVICE_UNAVAILABLE.into_response();
+    let headers = response.headers_mut();
+    headers.insert(header::RETRY_AFTER, HeaderValue::from_static("1"));
+    headers.insert(header::CACHE_CONTROL, HeaderValue::from_static("no-store"));
+    response
+}
+
 fn set_static_header(headers: &mut axum::http::HeaderMap, name: HeaderName, value: &'static str) {
     headers
         .entry(name)
diff --git a/fluxer_app_proxy/src/routes/spa_index.rs b/fluxer_app_proxy/src/routes/spa_index.rs
index d10419707..c1fc9a046 100644
--- a/fluxer_app_proxy/src/routes/spa_index.rs
+++ b/fluxer_app_proxy/src/routes/spa_index.rs
@@ -1,23 +1,29 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
 use crate::bootstrap::{build_bootstrap_script, inject_bootstrap};
-use crate::csp::{RuntimeCspSources, build_csp, generate_nonce, http_origin};
-use crate::discovery_cache::DiscoveryResponse;
+use crate::config::HttpEndpoint;
+use crate::csp::{RuntimeCspSources, generate_nonce};
+use crate::discovery_cache::{DiscoveryResponse, discovery_endpoint};
 use crate::geoip::build_geoip_response;
 use crate::invite_meta::{
     InviteMetaEndpoints, InvitePageMeta, inject_invite_meta, invite_code_from_path,
 };
-use crate::state::AppState;
+use crate::state::{
+    AppProxyBudgets, AppState, MAX_RENDERED_SPA_INDEX_BYTES, MAX_SPA_INDEX_BYTES,
+    SPA_DOCUMENT_RENDER_RESERVATION_BYTES, read_bounded_text_file,
+};
 use crate::time_freeze::{
     load_time_freeze_config_for_request, should_serve_frozen, time_freeze_debug_header,
 };
 use axum::{
+    body::{Body, Bytes},
     extract::{Request, State},
     http::{HeaderMap, HeaderName, HeaderValue, StatusCode, header},
     response::{IntoResponse, Response},
 };
 use std::path::Path;
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
+use tokio::sync::{OwnedSemaphorePermit, TryAcquireError};
 
 use super::assets_proxy::serve_local_asset;
 use super::file_stream::stream_file;
@@ -36,6 +42,7 @@ pub async fn spa_catch_all(
 
     if let Some(cache_control) = static_root_file_cache_control(request_path) {
         return serve_static_file(
+            &state.budgets,
             &state.config.static_dir,
             request_path,
             cache_control,
@@ -45,9 +52,11 @@ pub async fn spa_catch_all(
     }
     if is_static_asset_path(request_path) {
         return serve_local_asset(
+            &state.budgets,
             &state.config.static_dir,
             request_path.trim_start_matches('/'),
             &headers,
+            state.csp.asset_header(),
         )
         .await;
     }
@@ -83,11 +92,16 @@ fn is_static_asset_path(request_path: &str) -> bool {
 }
 
 async fn serve_static_file(
+    budgets: &AppProxyBudgets,
     static_dir: &str,
     request_path: &str,
     cache_control: &'static str,
     request_headers: &HeaderMap,
 ) -> Response {
+    let Ok(_read_slot) = budgets.local_read_slots.try_acquire() else {
+        return super::capacity_refused_response();
+    };
+
     let file_path = Path::new(static_dir).join(request_path.trim_start_matches('/'));
 
     let resolved = match tokio::fs::canonicalize(&file_path).await {
@@ -149,35 +163,40 @@ async fn serve_spa_index(state: &AppState, headers: &HeaderMap, request_path: &s
     let invite_meta = resolve_invite_meta(state, request_path, &runtime_csp_sources).await;
     let static_cdn_endpoint = runtime_csp_sources
         .static_cdn_endpoint
-        .as_deref()
-        .unwrap_or("");
-    let media_endpoint = runtime_csp_sources.media_endpoint.as_deref().unwrap_or("");
-    let csp = build_csp(&state.config.csp, &nonce, &runtime_csp_sources);
+        .as_ref()
+        .map_or("", HttpEndpoint::as_str);
+    let media_endpoint = runtime_csp_sources
+        .media_endpoint
+        .as_ref()
+        .map_or("", HttpEndpoint::as_str);
+    let csp = state.csp.spa_header(&nonce, &runtime_csp_sources);
     let geoip = build_geoip_response(state.geoip.lookup(headers));
     let script_tag = build_bootstrap_script(&state.config, &discovery, &geoip, &nonce);
 
-    if let Some(snapshot) = should_serve_frozen(&time_freeze) {
-        let frozen_html = String::from_utf8_lossy(&snapshot.index_html);
-        let dev_buster = should_bust_dev_assets.then(current_dev_asset_cache_buster);
-        let html = render_spa_document(
-            &frozen_html,
-            &nonce,
-            &script_tag,
-            static_cdn_endpoint,
-            media_endpoint,
-            invite_meta.as_ref(),
-            dev_buster.as_deref(),
-        );
-        return build_spa_response(html, &csp, debug_header.as_deref(), should_bust_dev_assets);
-    }
+    let raw_html = if let Some(snapshot) = should_serve_frozen(&time_freeze) {
+        String::from_utf8_lossy(&snapshot.index_html).into_owned()
+    } else {
+        match load_spa_index_html(state).await {
+            Ok(content) => content,
+            Err(response) => return response,
+        }
+    };
 
-    let raw_html = match load_spa_index_html(state).await {
-        Ok(content) => content,
-        Err(response) => return response,
+    let mut document_budget = match state
+        .budgets
+        .spa_document_memory
+        .clone()
+        .try_acquire_many_owned(SPA_DOCUMENT_RENDER_RESERVATION_BYTES)
+    {
+        Ok(permit) => permit,
+        Err(TryAcquireError::NoPermits) => return super::capacity_refused_response(),
+        Err(TryAcquireError::Closed) => {
+            panic!("SPA document memory budget semaphore closed unexpectedly")
+        }
     };
 
     let dev_buster = should_bust_dev_assets.then(current_dev_asset_cache_buster);
-    let html = render_spa_document(
+    let html = match render_spa_document(
         &raw_html,
         &nonce,
         &script_tag,
@@ -185,8 +204,60 @@ async fn serve_spa_index(state: &AppState, headers: &HeaderMap, request_path: &s
         media_endpoint,
         invite_meta.as_ref(),
         dev_buster.as_deref(),
-    );
-    build_spa_response(html, &csp, debug_header.as_deref(), should_bust_dev_assets)
+    ) {
+        Ok(html) => html,
+        Err(error) => {
+            tracing::error!(%error, "failed to render SPA document within its size limit");
+            return StatusCode::INTERNAL_SERVER_ERROR.into_response();
+        }
+    };
+    let html = html.into_boxed_str();
+    let retained_bytes = u32::try_from(html.len()).expect("bounded SPA document size must fit u32");
+    let released_bytes = SPA_DOCUMENT_RENDER_RESERVATION_BYTES
+        .checked_sub(retained_bytes)
+        .expect("rendered SPA document must fit its memory reservation");
+    if released_bytes > 0 {
+        let released_permits =
+            usize::try_from(released_bytes).expect("SPA document permit count must fit usize");
+        drop(
+            document_budget
+                .split(released_permits)
+                .expect("SPA document memory reservation must contain its unused permits"),
+        );
+    }
+    build_spa_response(
+        html,
+        csp,
+        debug_header.as_deref(),
+        should_bust_dev_assets,
+        document_budget,
+    )
+}
+
+#[derive(Debug)]
+struct SpaDocumentSizeLimitError {
+    attempted_bytes: usize,
+}
+
+impl std::fmt::Display for SpaDocumentSizeLimitError {
+    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(
+            formatter,
+            "rendered SPA document would be {} bytes, exceeding the {MAX_RENDERED_SPA_INDEX_BYTES} byte limit",
+            self.attempted_bytes
+        )
+    }
+}
+
+impl std::error::Error for SpaDocumentSizeLimitError {}
+
+fn bounded_document(document: String) -> Result<String, SpaDocumentSizeLimitError> {
+    if document.len() > MAX_RENDERED_SPA_INDEX_BYTES {
+        return Err(SpaDocumentSizeLimitError {
+            attempted_bytes: document.len(),
+        });
+    }
+    Ok(document)
 }
 
 fn render_spa_document(
@@ -197,16 +268,21 @@ fn render_spa_document(
     media_endpoint: &str,
     invite_meta: Option<&InvitePageMeta>,
     dev_asset_cache_buster: Option<&str>,
-) -> String {
-    let mut document =
-        inject_bootstrap(html, nonce, script_tag, static_cdn_endpoint, media_endpoint);
+) -> Result<String, SpaDocumentSizeLimitError> {
+    let mut document = bounded_document(inject_bootstrap(
+        html,
+        nonce,
+        script_tag,
+        static_cdn_endpoint,
+        media_endpoint,
+    ))?;
     if let Some(meta) = invite_meta {
-        document = inject_invite_meta(&document, meta);
+        document = bounded_document(inject_invite_meta(&document, meta))?;
     }
     if let Some(buster) = dev_asset_cache_buster {
-        document = append_dev_asset_cache_buster(&document, buster);
+        document = bounded_document(append_dev_asset_cache_buster(&document, buster))?;
     }
-    document
+    Ok(document)
 }
 
 async fn refresh_discovery_for_spa(state: &AppState) -> Option<DiscoveryResponse> {
@@ -224,8 +300,14 @@ async fn resolve_invite_meta(
     let code = invite_code_from_path(request_path)?;
     let resolver = state.invite_meta.get()?;
     let endpoints = InviteMetaEndpoints {
-        media_endpoint: runtime_csp_sources.media_endpoint.clone(),
-        static_cdn_endpoint: runtime_csp_sources.static_cdn_endpoint.clone(),
+        media_endpoint: runtime_csp_sources
+            .media_endpoint
+            .as_ref()
+            .map(|endpoint| endpoint.as_str().to_owned()),
+        static_cdn_endpoint: runtime_csp_sources
+            .static_cdn_endpoint
+            .as_ref()
+            .map(|endpoint| endpoint.as_str().to_owned()),
     };
 
     match resolver.resolve(code, &endpoints).await {
@@ -243,7 +325,7 @@ fn build_runtime_csp_sources(state: &AppState, discovery: &DiscoveryResponse) ->
             .or_else(|| state.config.static_cdn_endpoint.clone()),
         media_endpoint: discovery_endpoint(discovery, "media"),
         s3_public_endpoint: state.config.s3_public_endpoint.clone(),
-        s3_uploads_bucket: Some(state.config.s3_uploads_bucket.clone()),
+        s3_uploads_endpoint: state.config.s3_uploads_endpoint.clone(),
         branding_image_origins: branding_image_origins(discovery),
     }
 }
@@ -256,7 +338,7 @@ const BRANDING_IMAGE_KEYS: &[&str] = &[
     "favicon_url",
 ];
 
-fn branding_image_origins(discovery: &DiscoveryResponse) -> Vec<String> {
+fn branding_image_origins(discovery: &DiscoveryResponse) -> Vec<HttpEndpoint> {
     let Some(branding) = discovery
         .data
         .get("app_public")
@@ -264,39 +346,39 @@ fn branding_image_origins(discovery: &DiscoveryResponse) -> Vec<String> {
     else {
         return Vec::new();
     };
-    let mut origins: Vec<String> = Vec::new();
+    let mut origins: Vec<HttpEndpoint> = Vec::new();
     for key in BRANDING_IMAGE_KEYS {
-        let Some(origin) = branding
+        let Some(raw) = branding
             .get(*key)
             .and_then(|value| value.as_str())
-            .and_then(http_origin)
+            .map(str::trim)
+            .filter(|value| !value.is_empty())
         else {
             continue;
         };
-        if !origins.contains(&origin) {
+        let origin = match HttpEndpoint::parse(key, raw) {
+            Ok(origin) => origin,
+            Err(error) => {
+                tracing::warn!(%error, "ignoring invalid branding image origin");
+                continue;
+            }
+        };
+        if !origins
+            .iter()
+            .any(|existing| existing.csp_origin() == origin.csp_origin())
+        {
             origins.push(origin);
         }
     }
     origins
 }
 
-fn discovery_endpoint(discovery: &DiscoveryResponse, key: &str) -> Option<String> {
-    discovery
-        .data
-        .get("endpoints")
-        .and_then(|endpoints| endpoints.get(key))
-        .and_then(|value| value.as_str())
-        .map(str::trim)
-        .filter(|value| !value.is_empty())
-        .map(ToOwned::to_owned)
-}
-
 #[allow(clippy::result_large_err)]
 async fn load_spa_index_html(state: &AppState) -> Result<String, Response> {
     if let Some(index_upstream_url) = &state.config.index_upstream_url {
         let response = state
             .http_client
-            .get(index_upstream_url)
+            .get(index_upstream_url.as_url().clone())
             .timeout(Duration::from_secs(10))
             .send()
             .await
@@ -309,8 +391,31 @@ async fn load_spa_index_html(state: &AppState) -> Result<String, Response> {
             tracing::error!(url = %index_upstream_url, %status, "upstream index.html returned non-success status");
             return Err(StatusCode::BAD_GATEWAY.into_response());
         }
-        return response.text().await.map_err(|err| {
-            tracing::error!(url = %index_upstream_url, %err, "failed to read upstream index.html body");
+        if response
+            .content_length()
+            .is_some_and(|length| length > MAX_SPA_INDEX_BYTES as u64)
+        {
+            tracing::error!(url = %index_upstream_url, "upstream index.html exceeds the size limit");
+            return Err(StatusCode::BAD_GATEWAY.into_response());
+        }
+        let mut response = response;
+        let mut bytes: Vec<u8> = Vec::new();
+        loop {
+            let chunk = response.chunk().await.map_err(|err| {
+                tracing::error!(url = %index_upstream_url, %err, "failed to read upstream index.html body");
+                StatusCode::BAD_GATEWAY.into_response()
+            })?;
+            let Some(chunk) = chunk else {
+                break;
+            };
+            if chunk.len() > MAX_SPA_INDEX_BYTES - bytes.len() {
+                tracing::error!(url = %index_upstream_url, "upstream index.html exceeds the size limit");
+                return Err(StatusCode::BAD_GATEWAY.into_response());
+            }
+            bytes.extend_from_slice(&chunk);
+        }
+        return String::from_utf8(bytes).map_err(|err| {
+            tracing::error!(url = %index_upstream_url, %err, "upstream index.html is not valid UTF-8");
             StatusCode::BAD_GATEWAY.into_response()
         });
     }
@@ -319,25 +424,44 @@ async fn load_spa_index_html(state: &AppState) -> Result<String, Response> {
         return Ok(cached.to_string());
     }
 
+    let Ok(_read_slot) = state.budgets.local_read_slots.try_acquire() else {
+        return Err(super::capacity_refused_response());
+    };
     let index_path = Path::new(&state.config.static_dir).join("index.html");
-    tokio::fs::read_to_string(&index_path).await.map_err(|err| {
-        tracing::error!(path = ?index_path, %err, "failed to read index.html");
-        StatusCode::INTERNAL_SERVER_ERROR.into_response()
-    })
+    read_bounded_text_file(&index_path, MAX_SPA_INDEX_BYTES)
+        .await
+        .map_err(|err| {
+            tracing::error!(path = ?index_path, %err, "failed to read index.html");
+            StatusCode::INTERNAL_SERVER_ERROR.into_response()
+        })
+}
+
+struct SpaDocumentBody {
+    html: Box<str>,
+    _budget: OwnedSemaphorePermit,
+}
+
+impl AsRef<[u8]> for SpaDocumentBody {
+    fn as_ref(&self) -> &[u8] {
+        self.html.as_bytes()
+    }
 }
 
 fn build_spa_response(
-    html: String,
-    csp: &str,
+    html: Box<str>,
+    csp: HeaderValue,
     time_freeze_header: Option<&str>,
     dev_no_store: bool,
+    document_budget: OwnedSemaphorePermit,
 ) -> Response {
-    let mut response = html.into_response();
+    let body = Bytes::from_owner(SpaDocumentBody {
+        html,
+        _budget: document_budget,
+    });
+    let mut response = Response::new(Body::from(body));
     let headers = response.headers_mut();
 
-    if let Ok(v) = HeaderValue::from_str(csp) {
-        headers.insert(header::CONTENT_SECURITY_POLICY, v);
-    }
+    headers.insert(header::CONTENT_SECURITY_POLICY, csp);
     headers.insert(
         header::CONTENT_TYPE,
         HeaderValue::from_static("text/html; charset=utf-8"),
@@ -609,7 +733,8 @@ mod tests {
             "",
             None,
             None,
-        );
+        )
+        .expect("test SPA document must render within its size limit");
 
         assert!(!rendered.contains("{{CSP_NONCE_PLACEHOLDER}}"));
         assert!(rendered.contains(r#"nonce="reqnonce""#));
@@ -627,7 +752,8 @@ mod tests {
             "",
             Some(&meta),
             None,
-        );
+        )
+        .expect("test SPA document must render within its size limit");
         let without_meta = render_spa_document(
             SHELL_WITH_A_NONCE_HOLE,
             "reqnonce",
@@ -636,7 +762,8 @@ mod tests {
             "",
             None,
             None,
-        );
+        )
+        .expect("test SPA document must render within its size limit");
 
         assert!(with_meta.contains("Join Sample Space"));
         assert!(with_meta.contains("og:title"));
@@ -654,7 +781,8 @@ mod tests {
             "",
             None,
             Some("9911"),
-        );
+        )
+        .expect("test SPA document must render within its size limit");
         let untouched = render_spa_document(
             SHELL_WITH_A_NONCE_HOLE,
             "reqnonce",
@@ -663,7 +791,8 @@ mod tests {
             "",
             None,
             None,
-        );
+        )
+        .expect("test SPA document must render within its size limit");
 
         assert!(busted.contains(r#"src="/assets/app.js?_=9911""#));
         assert!(untouched.contains(r#"src="/assets/app.js""#));
@@ -688,7 +817,8 @@ mod tests {
             "",
             None,
             None,
-        );
+        )
+        .expect("test SPA document must render within its size limit");
 
         assert!(
             !served.contains("{{CSP_NONCE_PLACEHOLDER}}"),
@@ -719,7 +849,8 @@ mod tests {
             "https://media.example.test",
             None,
             None,
-        );
+        )
+        .expect("test SPA document must render within its size limit");
 
         assert!(
             rendered
@@ -752,7 +883,8 @@ mod tests {
             "https://media.example.test/",
             None,
             None,
-        );
+        )
+        .expect("test SPA document must render within its size limit");
         assert!(
             distinct.contains(r#"<link rel="preconnect" href="https://media.example.test">"#),
             "the media argument never reached the media preconnect"
@@ -768,7 +900,8 @@ mod tests {
             "https://cdn.example.test",
             None,
             None,
-        );
+        )
+        .expect("test SPA document must render within its size limit");
         assert!(
             shared.contains(r#"<link rel="preconnect" href="https://cdn.example.test">"#),
             "the static preconnects must survive a media endpoint that collapses onto them"
@@ -844,13 +977,24 @@ mod tests {
         let mut config = AppProxyConfig::from_env();
         config.release_channel = channel;
         config.time_freeze_enabled = true;
-        config.index_upstream_url = index_upstream_url;
-        config.static_cdn_endpoint = static_cdn_fallback.map(ToOwned::to_owned);
+        config.index_upstream_url = index_upstream_url.map(|url| {
+            crate::config::HttpUrl::parse("TEST_INDEX_UPSTREAM_URL", &url)
+                .expect("test index upstream URL must be a valid HTTP URL")
+        });
+        config.static_cdn_endpoint = static_cdn_fallback.map(|endpoint| {
+            HttpEndpoint::parse("TEST_STATIC_CDN_ENDPOINT", endpoint)
+                .expect("test static CDN endpoint must be a valid HTTP endpoint")
+        });
         config.trust_client_ip_header = false;
         config.discovery_upstream_url = discovery_upstream_url;
 
+        let csp = Arc::new(
+            crate::csp::CompiledCspPolicy::from_config(&config)
+                .expect("the test configuration must compile to a valid CSP"),
+        );
         AppState {
             config: Arc::new(config),
+            csp,
             http_client: reqwest::Client::new(),
             discovery_cache: Arc::new(DiscoveryCache::new()),
             geoip: Arc::new(GeoipResolver::from_config(&GeoipConfig {
@@ -863,6 +1007,7 @@ mod tests {
             })),
             invite_meta: Arc::new(OnceLock::new()),
             index_html: cached_shell.map(Arc::from),
+            budgets: crate::state::AppProxyBudgets::default(),
         }
     }
 
@@ -1111,8 +1256,14 @@ mod tests {
             "an application route was mistaken for a static root file"
         );
 
-        let response =
-            serve_static_file(static_dir, "/robots.txt", policy, &HeaderMap::new()).await;
+        let response = serve_static_file(
+            &AppProxyBudgets::default(),
+            static_dir,
+            "/robots.txt",
+            policy,
+            &HeaderMap::new(),
+        )
+        .await;
         assert_eq!(response.status(), StatusCode::OK);
         let cache_control = response
             .headers()
diff --git a/fluxer_app_proxy/src/routes/spa_static.rs b/fluxer_app_proxy/src/routes/spa_static.rs
index 01a7ba870..22f611ece 100644
--- a/fluxer_app_proxy/src/routes/spa_static.rs
+++ b/fluxer_app_proxy/src/routes/spa_static.rs
@@ -1,7 +1,8 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
-use crate::discovery_cache::DiscoveryResponse;
-use crate::state::AppState;
+use crate::config::HttpEndpoint;
+use crate::discovery_cache::discovery_endpoint;
+use crate::state::{AppState, MAX_STATIC_TEXT_FILE_BYTES, read_bounded_file};
 use crate::time_freeze::{
     TimeFreezeConfig, describe_decision, load_time_freeze_config_for_request,
     time_freeze_debug_header,
@@ -50,8 +51,7 @@ pub async fn version_json(State(state): State<AppState>, headers: HeaderMap) ->
         return resp;
     }
 
-    let mut result =
-        serve_static_text_file(&state.config.static_dir, "version.json", "application/json");
+    let mut result = serve_static_text_file(&state, "version.json", "application/json").await;
 
     if result.status() == StatusCode::NOT_FOUND && !state.config.build_version.is_empty() {
         let body = serde_json::json!({ "version": state.config.build_version });
@@ -68,21 +68,23 @@ pub async fn version_json(State(state): State<AppState>, headers: HeaderMap) ->
 pub async fn manifest_json(State(state): State<AppState>) -> Response {
     let static_cdn_endpoint = runtime_static_cdn_endpoint(&state).await;
     serve_static_text_file_with_cdn(
-        &state.config.static_dir,
+        &state,
         "manifest.json",
         "application/manifest+json",
-        static_cdn_endpoint.as_deref(),
+        static_cdn_endpoint.as_ref(),
     )
+    .await
 }
 
 pub async fn browserconfig_xml(State(state): State<AppState>) -> Response {
     let static_cdn_endpoint = runtime_static_cdn_endpoint(&state).await;
     serve_static_text_file_with_cdn(
-        &state.config.static_dir,
+        &state,
         "browserconfig.xml",
         "application/xml; charset=utf-8",
-        static_cdn_endpoint.as_deref(),
+        static_cdn_endpoint.as_ref(),
     )
+    .await
 }
 
 pub async fn service_worker(State(state): State<AppState>, headers: HeaderMap) -> Response {
@@ -96,17 +98,14 @@ pub async fn service_worker(State(state): State<AppState>, headers: HeaderMap) -
     if let Some(resp) = frozen {
         return resp;
     }
-    let mut result = serve_static_text_file(
-        &state.config.static_dir,
-        "sw.js",
-        "application/javascript; charset=utf-8",
-    );
+    let mut result =
+        serve_static_text_file(&state, "sw.js", "application/javascript; charset=utf-8").await;
     set_time_freeze_header(&mut result, debug_header.as_deref());
     result
 }
 
 pub async fn service_worker_map(State(state): State<AppState>) -> Response {
-    serve_static_text_file(&state.config.static_dir, "sw.js.map", "application/json")
+    serve_static_text_file(&state, "sw.js.map", "application/json").await
 }
 
 fn set_time_freeze_header(response: &mut Response, value: Option<&str>) {
@@ -125,7 +124,7 @@ fn set_time_freeze_header(response: &mut Response, value: Option<&str>) {
     let _ = (response, value);
 }
 
-async fn runtime_static_cdn_endpoint(state: &AppState) -> Option<String> {
+async fn runtime_static_cdn_endpoint(state: &AppState) -> Option<HttpEndpoint> {
     if let Some(discovery) = state.discovery_cache.get().await
         && let Some(endpoint) = discovery_endpoint(&discovery, "static_cdn")
     {
@@ -135,34 +134,28 @@ async fn runtime_static_cdn_endpoint(state: &AppState) -> Option<String> {
     state.config.static_cdn_endpoint.clone()
 }
 
-fn discovery_endpoint(discovery: &DiscoveryResponse, key: &str) -> Option<String> {
-    discovery
-        .data
-        .get("endpoints")
-        .and_then(|endpoints| endpoints.get(key))
-        .and_then(|value| value.as_str())
-        .map(str::trim)
-        .filter(|value| !value.is_empty())
-        .map(ToOwned::to_owned)
+async fn serve_static_text_file(state: &AppState, filename: &str, content_type: &str) -> Response {
+    serve_static_text_file_with_cdn(state, filename, content_type, None).await
 }
 
-fn serve_static_text_file(static_dir: &str, filename: &str, content_type: &str) -> Response {
-    serve_static_text_file_with_cdn(static_dir, filename, content_type, None)
-}
-
-fn serve_static_text_file_with_cdn(
-    static_dir: &str,
+async fn serve_static_text_file_with_cdn(
+    state: &AppState,
     filename: &str,
     content_type: &str,
-    static_cdn_endpoint: Option<&str>,
+    static_cdn_endpoint: Option<&HttpEndpoint>,
 ) -> Response {
+    let static_dir = state.config.static_dir.as_str();
     let file_path = Path::new(static_dir).join(filename);
 
-    let resolved = match file_path.canonicalize() {
+    let Ok(_read_slot) = state.budgets.local_read_slots.try_acquire() else {
+        return super::capacity_refused_response();
+    };
+
+    let resolved = match tokio::fs::canonicalize(&file_path).await {
         Ok(p) => p,
         Err(_) => return StatusCode::NOT_FOUND.into_response(),
     };
-    let base = match Path::new(static_dir).canonicalize() {
+    let base = match tokio::fs::canonicalize(static_dir).await {
         Ok(p) => p,
         Err(_) => return StatusCode::NOT_FOUND.into_response(),
     };
@@ -170,12 +163,16 @@ fn serve_static_text_file_with_cdn(
         return StatusCode::NOT_FOUND.into_response();
     }
 
-    let content = match std::fs::read(&resolved) {
+    let content = match read_bounded_file(&resolved, MAX_STATIC_TEXT_FILE_BYTES).await {
         Ok(bytes) => bytes,
-        Err(_) => return StatusCode::NOT_FOUND.into_response(),
+        Err(error) if error.is_not_found() => return StatusCode::NOT_FOUND.into_response(),
+        Err(error) => {
+            tracing::error!(file = filename, %error, "refusing to serve static text file");
+            return StatusCode::NOT_FOUND.into_response();
+        }
     };
 
-    let replacement = static_cdn_endpoint.unwrap_or("").trim_end_matches('/');
+    let replacement = static_cdn_endpoint.map_or("", HttpEndpoint::as_str);
     let body: axum::body::Body = match std::str::from_utf8(&content) {
         Ok(text) => text
             .replace("{{STATIC_CDN_ENDPOINT}}", replacement)
diff --git a/fluxer_app_proxy/src/state.rs b/fluxer_app_proxy/src/state.rs
index aa0e29caf..1e94f0d13 100644
--- a/fluxer_app_proxy/src/state.rs
+++ b/fluxer_app_proxy/src/state.rs
@@ -1,20 +1,136 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
 use crate::config::AppProxyConfig;
+use crate::csp::CompiledCspPolicy;
 use crate::discovery_cache::DiscoveryCache;
 use crate::invite_meta::InviteMetaResolver;
 use fluxer_common::geoip::GeoipResolver;
 use std::sync::{Arc, OnceLock};
 use std::time::Duration;
+use tokio::io::AsyncReadExt;
+use tokio::sync::Semaphore;
+
+pub const MAX_SPA_INDEX_BYTES: usize = 4 * 1024 * 1024;
+pub const MAX_RENDERED_SPA_INDEX_BYTES: usize = 8 * 1024 * 1024;
+pub const MAX_STATIC_TEXT_FILE_BYTES: usize = 4 * 1024 * 1024;
+pub const SPA_DOCUMENT_MEMORY_BUDGET_BYTES: usize = 160 * 1024 * 1024;
+pub const SPA_DOCUMENT_RENDER_RESERVATION_BYTES: u32 = 40 * 1024 * 1024;
+pub const UPSTREAM_ASSET_RESPONSES_IN_FLIGHT_MAX: usize = 32;
+pub const LOCAL_FILE_READS_IN_FLIGHT_MAX: usize = 256;
+
+const _: () = assert!(
+    MAX_RENDERED_SPA_INDEX_BYTES <= SPA_DOCUMENT_RENDER_RESERVATION_BYTES as usize,
+    "a rendered SPA document must fit inside the memory reserved to render it"
+);
+
+#[derive(Clone)]
+pub struct AppProxyBudgets {
+    pub spa_document_memory: Arc<Semaphore>,
+    pub upstream_asset_slots: Arc<Semaphore>,
+    pub local_read_slots: Arc<Semaphore>,
+}
+
+impl AppProxyBudgets {
+    pub fn new() -> Self {
+        Self {
+            spa_document_memory: Arc::new(Semaphore::new(SPA_DOCUMENT_MEMORY_BUDGET_BYTES)),
+            upstream_asset_slots: Arc::new(Semaphore::new(UPSTREAM_ASSET_RESPONSES_IN_FLIGHT_MAX)),
+            local_read_slots: Arc::new(Semaphore::new(LOCAL_FILE_READS_IN_FLIGHT_MAX)),
+        }
+    }
+}
+
+impl Default for AppProxyBudgets {
+    fn default() -> Self {
+        Self::new()
+    }
+}
 
 #[derive(Clone)]
 pub struct AppState {
     pub config: Arc<AppProxyConfig>,
+    pub csp: Arc<CompiledCspPolicy>,
     pub http_client: reqwest::Client,
     pub discovery_cache: Arc<DiscoveryCache>,
     pub geoip: Arc<GeoipResolver>,
     pub invite_meta: Arc<OnceLock<InviteMetaResolver>>,
     pub index_html: Option<Arc<str>>,
+    pub budgets: AppProxyBudgets,
+}
+
+#[derive(Debug)]
+pub enum BoundedFileReadError {
+    TooLarge { actual: u64, maximum: usize },
+    Io(std::io::Error),
+}
+
+impl std::fmt::Display for BoundedFileReadError {
+    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::TooLarge { actual, maximum } => write!(
+                formatter,
+                "file is {actual} bytes, exceeding the {maximum} byte limit"
+            ),
+            Self::Io(source) => source.fmt(formatter),
+        }
+    }
+}
+
+impl std::error::Error for BoundedFileReadError {}
+
+impl BoundedFileReadError {
+    pub fn is_not_found(&self) -> bool {
+        matches!(self, Self::Io(source) if source.kind() == std::io::ErrorKind::NotFound)
+    }
+}
+
+pub async fn read_bounded_file(
+    path: &std::path::Path,
+    max_bytes: usize,
+) -> Result<Vec<u8>, BoundedFileReadError> {
+    let file = tokio::fs::File::open(path)
+        .await
+        .map_err(BoundedFileReadError::Io)?;
+    let metadata = file.metadata().await.map_err(BoundedFileReadError::Io)?;
+    if !metadata.is_file() {
+        return Err(BoundedFileReadError::Io(std::io::Error::new(
+            std::io::ErrorKind::NotFound,
+            "not a regular file",
+        )));
+    }
+    let declared_length = metadata.len();
+    if declared_length > max_bytes as u64 {
+        return Err(BoundedFileReadError::TooLarge {
+            actual: declared_length,
+            maximum: max_bytes,
+        });
+    }
+    let expected_bytes =
+        usize::try_from(declared_length).expect("a length within a usize limit must fit usize");
+    let mut bytes = Vec::with_capacity(expected_bytes);
+    tokio::io::AsyncReadExt::read_to_end(&mut file.take(declared_length + 1), &mut bytes)
+        .await
+        .map_err(BoundedFileReadError::Io)?;
+    if bytes.len() > expected_bytes {
+        return Err(BoundedFileReadError::TooLarge {
+            actual: bytes.len() as u64,
+            maximum: max_bytes,
+        });
+    }
+    Ok(bytes)
+}
+
+pub async fn read_bounded_text_file(
+    path: &std::path::Path,
+    max_bytes: usize,
+) -> Result<String, BoundedFileReadError> {
+    let bytes = read_bounded_file(path, max_bytes).await?;
+    String::from_utf8(bytes).map_err(|error| {
+        BoundedFileReadError::Io(std::io::Error::new(
+            std::io::ErrorKind::InvalidData,
+            error.utf8_error(),
+        ))
+    })
 }
 
 pub fn build_http_client() -> reqwest::Result<reqwest::Client> {