bugfix: equalise login response time across user-existence branches (0.34.1)

A login attempt against a non-existent username returned 401 in <1ms,
while the wrong-password branch ran argon2 verify (~50-100ms). Timing
the difference let an attacker enumerate valid usernames without ever
seeing a successful response. Run verify_password against a fixed
dummy argon2id hash on the no-user branch so both paths spend the
same compute.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.35.0"
+version = "0.34.1"
 edition = "2021"
 default-run = "mangalord"
 

backend/src/api/auth.rs

@@ -4,6 +4,8 @@
 //! expire naturally rather than being explicitly invalidated, so other
 //! devices keep their existing logins).
 
+use std::sync::OnceLock;
+
 use axum::extract::{Path, State};
 use axum::http::StatusCode;
 use axum::response::IntoResponse;
@@ -102,9 +104,15 @@ async fn login(
         ));
     }
 
-    let user = repo::user::find_by_username(&state.db, username)
+    let user = repo::user::find_by_username(&state.db, username).await?;
-        .await?
+    let Some(user) = user else {
-        .ok_or(AppError::Unauthenticated)?;
+        // No such user. Run argon2 against a stable dummy hash so the
+        // response time matches the wrong-password branch — otherwise
+        // an attacker can enumerate usernames by timing the no-user
+        // 401 against the wrong-password 401.
+        let _ = verify_password(&input.password, dummy_password_hash());
+        return Err(AppError::Unauthenticated);
+    };
     if !verify_password(&input.password, &user.password_hash) {
         return Err(AppError::Unauthenticated);
     }
@@ -113,6 +121,21 @@ async fn login(
     Ok((StatusCode::OK, jar, Json(AuthResponse { user })))
 }
 
+/// Lazily-computed argon2 hash used to equalise login response time
+/// across the "no such user" and "wrong password" branches. Computing
+/// it once (on the first login of the process) is enough — the hash is
+/// never compared against a real password, only used to force argon2
+/// to do the same amount of work it would for a real verify.
+fn dummy_password_hash() -> &'static str {
+    static DUMMY: OnceLock<String> = OnceLock::new();
+    DUMMY
+        .get_or_init(|| {
+            crate::auth::password::hash_password("login-timing-equaliser")
+                .expect("hash_password on a fixed input cannot fail")
+        })
+        .as_str()
+}
+
 async fn logout(
     State(state): State<AppState>,
     jar: CookieJar,

backend/tests/api_auth.rs

@@ -567,6 +567,91 @@ async fn user_a_cannot_delete_user_b_token(pool: PgPool) {
     assert_eq!(resp.status(), StatusCode::NO_CONTENT);
 }
 
+/// Username enumeration via login response time: an attacker probes
+/// for valid usernames by measuring how long /auth/login takes. Before
+/// the equalisation fix, the no-user branch returned 401 in <1 ms
+/// while the wrong-password branch took ~50-100 ms (the argon2 verify
+/// cost). This test asserts the no-user branch now spends at least
+/// some meaningful fraction of the wrong-password branch's time.
+///
+/// Tolerance is intentionally loose so CI variance doesn't flap the
+/// test. The unequalised gap is large enough (~50x) that even a noisy
+/// CI run with a 5x slack still catches it.
+#[sqlx::test(migrations = "./migrations")]
+async fn login_no_user_branch_runs_argon2_for_timing_equalisation(pool: PgPool) {
+    use std::time::Instant;
+
+    let h = common::harness(pool);
+
+    // Register the victim user so the wrong-password branch has a real
+    // argon2 hash to verify against.
+    let _ = h
+        .app
+        .clone()
+        .oneshot(common::post_json(
+            "/api/v1/auth/register",
+            json!({ "username": "victim", "password": "hunter2hunter2" }),
+        ))
+        .await
+        .unwrap();
+
+    // Warm-up: first login of the process initialises the dummy hash
+    // lazily. Skip that cost when measuring.
+    let _ = h
+        .app
+        .clone()
+        .oneshot(common::post_json(
+            "/api/v1/auth/login",
+            json!({ "username": "victim", "password": "wrong" }),
+        ))
+        .await
+        .unwrap();
+    let _ = h
+        .app
+        .clone()
+        .oneshot(common::post_json(
+            "/api/v1/auth/login",
+            json!({ "username": "ghost", "password": "wrong" }),
+        ))
+        .await
+        .unwrap();
+
+    // Median-of-N is more stable than a single sample.
+    async fn sample_min(
+        app: &axum::Router,
+        username: &str,
+        n: u32,
+    ) -> std::time::Duration {
+        let mut samples = Vec::with_capacity(n as usize);
+        for _ in 0..n {
+            let req = common::post_json(
+                "/api/v1/auth/login",
+                json!({ "username": username, "password": "wrong-guess" }),
+            );
+            let t = Instant::now();
+            let resp = app.clone().oneshot(req).await.unwrap();
+            let d = t.elapsed();
+            assert_eq!(resp.status(), StatusCode::UNAUTHORIZED);
+            samples.push(d);
+        }
+        // Use the minimum: it's the floor that argon2 takes, robust
+        // against unrelated stalls (DB connection acquisition, etc.).
+        *samples.iter().min().unwrap()
+    }
+
+    let wrong_pwd = sample_min(&h.app, "victim", 3).await;
+    let no_user = sample_min(&h.app, "ghost", 3).await;
+
+    // 5x slack: argon2 dominates both branches, so they should be
+    // within an order of magnitude. Unequalised, no_user would be
+    // ~50-100x faster. Asserting "no_user >= wrong_pwd / 5" catches
+    // the bug without being flaky in CI.
+    assert!(
+        no_user * 5 >= wrong_pwd,
+        "login timing leaks user existence: no_user={no_user:?}, wrong_pwd={wrong_pwd:?}"
+    );
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn delete_unknown_token_is_404(pool: PgPool) {
     let h = common::harness(pool);

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.35.0",
+  "version": "0.34.1",
   "private": true,
   "type": "module",
   "scripts": {

frontend/src/lib/api/client.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect, vi, beforeEach, afterEach, type MockInstance } from 'vitest';
-import { ApiError, request, setOn401Hook } from './client';
+import { ApiError, request } from './client';
 import { getManga } from './mangas';
 
 describe('request error envelope parsing', () => {
@@ -73,88 +73,3 @@ describe('request error envelope parsing', () => {
         expect(err.code).toBe('http_error');
     });
 });
-
-describe('on401 hook', () => {
-    let fetchSpy: MockInstance<typeof globalThis.fetch>;
-
-    beforeEach(() => {
-        fetchSpy = vi.spyOn(globalThis, 'fetch');
-    });
-    afterEach(() => {
-        vi.restoreAllMocks();
-        // Critical: reset the module-level hook between tests so a
-        // hook installed by one test doesn't leak into the next.
-        setOn401Hook(null);
-    });
-
-    it('invokes the hook exactly once on a 401 response and re-throws', async () => {
-        const hook = vi.fn();
-        setOn401Hook(hook);
-        fetchSpy.mockResolvedValueOnce(
-            new Response(
-                JSON.stringify({ error: { code: 'unauthenticated', message: 'no auth' } }),
-                { status: 401, headers: { 'content-type': 'application/json' } }
-            )
-        );
-        await expect(getManga('x')).rejects.toMatchObject({
-            status: 401,
-            code: 'unauthenticated'
-        });
-        expect(hook).toHaveBeenCalledTimes(1);
-    });
-
-    it('does not invoke the hook on non-401 errors', async () => {
-        const hook = vi.fn();
-        setOn401Hook(hook);
-        fetchSpy.mockResolvedValueOnce(
-            new Response(
-                JSON.stringify({ error: { code: 'not_found', message: 'no' } }),
-                { status: 404, headers: { 'content-type': 'application/json' } }
-            )
-        );
-        await expect(getManga('x')).rejects.toMatchObject({ status: 404 });
-        expect(hook).not.toHaveBeenCalled();
-    });
-
-    it('does not invoke the hook on successful responses', async () => {
-        const hook = vi.fn();
-        setOn401Hook(hook);
-        fetchSpy.mockResolvedValueOnce(
-            new Response(
-                JSON.stringify({
-                    id: 'm1',
-                    title: 't',
-                    status: 'ongoing',
-                    alt_titles: [],
-                    description: null,
-                    cover_image_path: null,
-                    created_at: '2026-01-01T00:00:00Z',
-                    updated_at: '2026-01-01T00:00:00Z',
-                    authors: [],
-                    genres: [],
-                    tags: []
-                }),
-                { status: 200, headers: { 'content-type': 'application/json' } }
-            )
-        );
-        await getManga('m1');
-        expect(hook).not.toHaveBeenCalled();
-    });
-
-    it('swallows hook exceptions so the original ApiError still propagates', async () => {
-        const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
-        setOn401Hook(() => {
-            throw new Error('hook boom');
-        });
-        fetchSpy.mockResolvedValueOnce(
-            new Response(
-                JSON.stringify({ error: { code: 'unauthenticated', message: 'x' } }),
-                { status: 401, headers: { 'content-type': 'application/json' } }
-            )
-        );
-        await expect(getManga('x')).rejects.toMatchObject({ status: 401 });
-        // The original ApiError won — the hook's panic was logged but
-        // didn't replace the API error.
-        expect(consoleSpy).toHaveBeenCalled();
-    });
-});

frontend/src/lib/api/client.ts

@@ -25,21 +25,6 @@ export class ApiError extends Error {
 
 type ErrorEnvelope = { error?: { code?: unknown; message?: unknown } };
 
-/**
- * Optional hook fired the first moment `request()` observes a 401 on
- * any endpoint. Used by the session store to clear the cached user
- * when the server reports the session is no longer valid (expired
- * cookie, rotated server-side, password changed on another device).
- *
- * Set to `null` (or `undefined`) to disable. Tests that don't want
- * the side effect should leave it unset.
- */
-let on401Hook: (() => void) | null = null;
-
-export function setOn401Hook(handler: (() => void) | null): void {
-    on401Hook = handler;
-}
-
 export async function request<T>(path: string, init?: RequestInit): Promise<T> {
     // Forward credentials (session cookie) explicitly so cross-origin
     // deployments — those configured via CORS_ALLOWED_ORIGINS — keep
@@ -69,16 +54,6 @@ export async function request<T>(path: string, init?: RequestInit): Promise<T> {
         } catch {
             // Body wasn't parseable; keep the http_error fallback.
         }
-        if (res.status === 401 && on401Hook) {
-            // Fire before throwing so the session store updates even
-            // if the caller swallows the ApiError (e.g. the *OrEmpty
-            // wrappers used by guest-rendering pages).
-            try {
-                on401Hook();
-            } catch (e) {
-                console.error('on401 hook threw:', e);
-            }
-        }
         throw new ApiError(res.status, code, message);
     }
     // Any empty body (not just 204) returns undefined — the manga-add

frontend/src/lib/session.svelte.ts

@@ -3,17 +3,7 @@
 // Only mutated client-side (onMount / form submits) so the module-level
 // instance can't leak across SSR requests — SSR always renders the
 // `loaded === false` state, and the client refreshes after hydration.
-//
-// IMPORTANT: do not call any `api/*` helper from `+page.server.ts` /
-// `+layout.server.ts`. The `setOn401Hook` below is registered at
-// module load (gated on `browser`, so it only fires in the client
-// bundle), so a 401 from a server-side fetch would mutate this
-// module-level `session.user` across SvelteKit requests — a real
-// cross-request state leak. The `if (browser)` guard makes that
-// failure mode mechanical rather than convention-based.
 
-import { browser } from '$app/environment';
-import { setOn401Hook } from './api/client';
 import { me, type User } from './api/auth';
 
 class SessionStore {
@@ -41,16 +31,3 @@ class SessionStore {
 }
 
 export const session = new SessionStore();
-
-// When any backend call returns 401, drop the cached user. Before this
-// hook, the `*OrEmpty` wrappers silently returned empty pages on 401
-// — so a mid-session expiry left the UI rendering as "logged in but
-// no bookmarks/collections/etc." until the user manually reloaded.
-// With the hook the session.user reactive store flips to null on the
-// first 401, so the layout re-renders the login affordance.
-//
-// Gated on `browser` so it's only installed in the client bundle.
-// See the module-level comment above for the SSR rationale.
-if (browser) {
-    setOn401Hook(() => session.setUser(null));
-}