fix(admin): sign CSRF tokens and escape them in scripts (#2071)

fluxer_admin/src/middleware/csrf.rs

@@ -2,24 +2,43 @@
 
 use axum::{
     body::{Body, to_bytes},
-    extract::Request,
+    extract::{Request, State},
     http::{HeaderValue, Method, StatusCode, header},
     middleware::Next,
     response::{IntoResponse, Response},
 };
-use rand::RngExt;
+
+use crate::middleware::auth::AuthContext;
+use crate::session::{create_csrf_token, verify_csrf_token};
+use crate::state::AppState;
 
 const CSRF_COOKIE_NAME: &str = "csrf_token";
 pub const CSRF_FORM_FIELD: &str = "_csrf";
 const CSRF_HEADER_NAME: &str = "x-csrf-token";
-const TOKEN_LENGTH: usize = 32;
+const HOST_CSRF_COOKIE_NAME: &str = "__Host-csrf_token";
 const MAX_CSRF_FORM_BYTES: usize = 8 * 1024 * 1024;
 
 const IGNORED_PATH_SUFFIXES: &[&str] = &["/oauth2_callback", "/auth/start"];
 
-pub async fn csrf_protection(mut request: Request, next: Next) -> Response {
-    let existing_token = extract_csrf_cookie(&request);
-    let token = existing_token.unwrap_or_else(generate_csrf_token);
+pub async fn csrf_protection(
+    State(state): State<AppState>,
+    mut request: Request,
+    next: Next,
+) -> Response {
+    let config = state.config();
+    let secret = config.secret_key_base.clone();
+    let admin_endpoint = config.admin_endpoint.clone();
+    let is_production = config.is_production();
+
+    let user_id = request
+        .extensions()
+        .get::<AuthContext>()
+        .map(|ctx| ctx.session.user_id.clone())
+        .unwrap_or_default();
+
+    let token = extract_csrf_cookie(&request)
+        .filter(|cookie| verify_csrf_token(cookie, &user_id, &secret))
+        .unwrap_or_else(|| create_csrf_token(&user_id, &secret));
     request.extensions_mut().insert(CsrfToken(token.clone()));
 
     if matches!(
@@ -31,6 +50,9 @@ pub async fn csrf_protection(mut request: Request, next: Next) -> Response {
             .iter()
             .any(|suffix| path.ends_with(suffix));
         if !is_ignored {
+            if !is_same_site_request(&request, &admin_endpoint) {
+                return StatusCode::FORBIDDEN.into_response();
+            }
             let header_token = extract_csrf_header(&request);
             let query_token = extract_csrf_from_query(&request);
             let mut submitted = query_token.or(header_token);
@@ -43,21 +65,24 @@ pub async fn csrf_protection(mut request: Request, next: Next) -> Response {
                 request = restored_request;
                 submitted = body_token;
             }
-            match submitted {
-                Some(ref submitted_token) if submitted_token == &token => {}
-                _ => {
-                    return StatusCode::FORBIDDEN.into_response();
-                }
+            let accepted = submitted.as_deref().is_some_and(|submitted_token| {
+                submitted_token == token && verify_csrf_token(submitted_token, &user_id, &secret)
+            });
+            if !accepted {
+                return StatusCode::FORBIDDEN.into_response();
             }
         }
     }
 
     let mut response = next.run(request).await;
 
-    let cookie_value = format!(
-        "{}={}; Path=/; SameSite=Lax; HttpOnly",
-        CSRF_COOKIE_NAME, token
-    );
+    let cookie_name = if is_production {
+        HOST_CSRF_COOKIE_NAME
+    } else {
+        CSRF_COOKIE_NAME
+    };
+    let secure = if is_production { "; Secure" } else { "" };
+    let cookie_value = format!("{cookie_name}={token}; Path=/; SameSite=Lax; HttpOnly{secure}");
     if let Ok(value) = HeaderValue::from_str(&cookie_value) {
         response.headers_mut().append(header::SET_COOKIE, value);
     }
@@ -69,7 +94,10 @@ fn extract_csrf_cookie(request: &Request) -> Option<String> {
     let cookie_header = request.headers().get(header::COOKIE)?.to_str().ok()?;
     for pair in cookie_header.split(';') {
         let pair = pair.trim();
-        if let Some(value) = pair.strip_prefix("csrf_token=") {
+        let value = pair
+            .strip_prefix("__Host-csrf_token=")
+            .or_else(|| pair.strip_prefix("csrf_token="));
+        if let Some(value) = value {
             let trimmed = value.trim();
             if !trimmed.is_empty() {
                 return Some(trimmed.to_owned());
@@ -127,18 +155,22 @@ async fn extract_csrf_from_form_body(
     Ok((request, token))
 }
 
-fn generate_csrf_token() -> String {
-    let mut rng = rand::rng();
-    let bytes: [u8; TOKEN_LENGTH] = rng.random();
-    hex_encode(&bytes)
-}
-
-fn hex_encode(bytes: &[u8]) -> String {
-    let mut s = String::with_capacity(bytes.len() * 2);
-    for byte in bytes {
-        s.push_str(&format!("{byte:02x}"));
+fn is_same_site_request(request: &Request, admin_endpoint: &str) -> bool {
+    if let Some(site) = request
+        .headers()
+        .get("sec-fetch-site")
+        .and_then(|value| value.to_str().ok())
+    {
+        return matches!(site, "same-origin" | "same-site" | "none");
+    }
+    match request
+        .headers()
+        .get(header::ORIGIN)
+        .and_then(|value| value.to_str().ok())
+    {
+        Some(origin) => origin == admin_endpoint,
+        None => true,
     }
-    s
 }
 
 #[derive(Clone, Debug)]
@@ -156,40 +188,6 @@ pub fn get_csrf_token(request: &Request) -> String {
 mod tests {
     use super::*;
 
-    #[test]
-    fn generate_csrf_token_correct_length() {
-        let token = generate_csrf_token();
-        assert_eq!(
-            token.len(),
-            TOKEN_LENGTH * 2,
-            "token must be {} hex chars",
-            TOKEN_LENGTH * 2
-        );
-    }
-
-    #[test]
-    fn generate_csrf_token_is_valid_hex() {
-        let token = generate_csrf_token();
-        assert!(
-            token.chars().all(|c| c.is_ascii_hexdigit()),
-            "token must contain only hex chars: {token}"
-        );
-    }
-
-    #[test]
-    fn generate_csrf_token_is_unique() {
-        let a = generate_csrf_token();
-        let b = generate_csrf_token();
-        assert_ne!(a, b, "consecutive tokens must differ");
-    }
-
-    #[test]
-    fn hex_encode_produces_correct_output() {
-        assert_eq!(hex_encode(&[0x00, 0xff, 0x0a]), "00ff0a");
-        assert_eq!(hex_encode(&[]), "");
-        assert_eq!(hex_encode(&[0xde, 0xad]), "dead");
-    }
-
     #[test]
     fn oauth2_callback_is_exempt() {
         let exempt = IGNORED_PATH_SUFFIXES

fluxer_admin/src/routes/auth.rs

@@ -348,7 +348,7 @@ fn oauth_callback_page(config: &AdminConfig, code: Option<&str>, state: Option<&
     )
 }
 
-fn json_string(value: &str) -> String {
+pub(crate) fn json_string(value: &str) -> String {
     serde_json::to_string(value)
         .expect("JSON string serialization cannot fail")
         .replace('<', "\\u003c")

fluxer_admin/src/routes/mod.rs

@@ -73,7 +73,10 @@ pub fn build_router(config: AdminConfig) -> Router {
         .route("/", get(dashboard))
         .route("/dashboard", get(dashboard))
         .layer(from_fn(middleware::htmx::flash_redirect_to_toast))
-        .layer(from_fn(middleware::csrf::csrf_protection))
+        .layer(from_fn_with_state(
+            state.clone(),
+            middleware::csrf::csrf_protection,
+        ))
         .layer(from_fn(middleware::self_hosted::self_hosted_override))
         .layer(from_fn_with_state(
             state.clone(),

fluxer_admin/src/session.rs

@@ -3,6 +3,7 @@
 use base64::{Engine as _, engine::general_purpose::URL_SAFE_NO_PAD};
 use hmac::{Hmac, KeyInit, Mac};
 use serde::{Deserialize, Serialize};
+use rand::RngExt;
 use sha2::Sha256;
 use std::time::{SystemTime, UNIX_EPOCH};
 
@@ -78,6 +79,36 @@ fn verify_signature<'a>(signed_data: &'a str, secret_key: &str) -> Option<&'a st
     if diff == 0 { Some(data) } else { None }
 }
 
+pub fn create_csrf_token(user_id: &str, secret_key: &str) -> String {
+    let mut rng = rand::rng();
+    let nonce: [u8; 16] = rng.random();
+    let payload = URL_SAFE_NO_PAD.encode(format!(
+        "{}:{}",
+        URL_SAFE_NO_PAD.encode(user_id.as_bytes()),
+        URL_SAFE_NO_PAD.encode(nonce)
+    ));
+    sign_data(&payload, secret_key)
+}
+
+pub fn verify_csrf_token(token: &str, user_id: &str, secret_key: &str) -> bool {
+    let Some(data) = verify_signature(token, secret_key) else {
+        return false;
+    };
+    let Ok(decoded) = URL_SAFE_NO_PAD.decode(data.as_bytes()) else {
+        return false;
+    };
+    let Ok(payload) = String::from_utf8(decoded) else {
+        return false;
+    };
+    let Some((encoded_uid, _nonce)) = payload.split_once(':') else {
+        return false;
+    };
+    match URL_SAFE_NO_PAD.decode(encoded_uid.as_bytes()) {
+        Ok(uid_bytes) => uid_bytes == user_id.as_bytes(),
+        Err(_) => false,
+    }
+}
+
 pub const LEGACY_SESSION_COOKIE_NAME: &str = "session";
 pub const SESSION_COOKIE_NAME: &str = "admin_session";
 pub const SESSION_MAX_AGE: i64 = MAX_AGE_SECONDS as i64;

fluxer_admin/src/templates/components/message_list.rs

@@ -7,6 +7,7 @@ use super::media::user_avatar_url;
 use super::nsfw_indicators::{attachment_nsfw_badge, channel_nsfw_state_badge};
 use super::user_display::format_user_display;
 use crate::config::AdminConfig;
+use crate::routes::auth::json_string;
 
 pub struct Attachment {
     pub id: String,
@@ -309,7 +310,7 @@ pub fn message_list(
 }
 
 pub fn message_deletion_script(csrf_token: &str) -> Markup {
-    let csrf = serde_json::to_string(csrf_token).unwrap_or_else(|_| "\"\"".into());
+    let csrf = json_string(csrf_token);
     let script = r#"(function() {
     var csrf = __CSRF__;
     function bp() {