🛡️ Sentinel: [HIGH] Fix authentication credential stuffing via dual rate-limiting

.jules/sentinel.md

@@ -4,3 +4,8 @@
 **Vulnerability:** The application used `scryptSync` (synchronous CPU-intensive password hashing) inside Next.js server action handlers for registration and login. Because Node.js runs on a single main event loop, a small number of concurrent authentication requests (or a distributed credential stuffing attack) completely blocks the event loop, starving all other concurrent requests and causing a full Denial of Service (DoS).
 **Learning:** Next.js Server Actions and Route Handlers run on Node's main thread by default. Using synchronous cryptography operations (such as `scryptSync` or `pbkdf2Sync`) prevents the server from processing other concurrent connections.
 **Prevention:** Always use asynchronous password-hashing implementations (such as async `scrypt` wrapped in a Promise or bcrypt/argon2 async variants) inside Next.js/Node.js web entry points to delegate heavy hashing computations to the Node.js libuv thread pool, keeping the main event loop responsive.
+
+## 2026-07-16 - Credential Stuffing and Distributed Brute-Force Risks in Single-Key Rate Limiting
+**Vulnerability:** The application previously relied on a single-dimension rate limiter keyed solely on the user's lowercase email address for authentication endpoints (`signUpAction` and `signInAction`). While this protects any individual email account from being single-target brute-forced, it leaves the platform entirely vulnerable to distributed credential stuffing attacks (where an attacker tests thousands of unique email/password pairs, making only a single attempt per email) or distributed IP-based DoS attacks, both of which can exhaust database/CPU resources without triggering the email-keyed rate limiter.
+**Learning:** Single-key rate limiting based only on username/email does not prevent horizontal brute-force (credential stuffing) or service exhaustion. Defensive authentication security requires dual layers of control to restrict abuse from individual clients while still allowing legitimate access patterns.
+**Prevention:** Always implement dual rate-limiting in critical user authentication and mutation entry points by combining IP-based tracking (extracted safely from headers like `x-forwarded-for` and `x-real-ip`) and target-based tracking (like lowercase emails). This prevents single clients from sweeping across multiple user accounts.

src/__tests__/setup.ts

@@ -8,6 +8,7 @@ vi.mock('next/headers', () => ({
     set: vi.fn(),
     delete: vi.fn(),
   }),
+  headers: () => Promise.resolve({ get: () => null }),
 }));
 
 vi.mock('next/navigation', () => ({

src/app/actions/__tests__/auth-actions.test.ts

@@ -29,6 +29,7 @@ vi.mock('next/headers', () => ({
     set: vi.fn(),
     delete: vi.fn(),
   }),
+  headers: () => Promise.resolve({ get: () => null }),
 }));
 
 vi.mock('next/navigation', () => ({

src/app/actions/auth.ts

@@ -16,6 +16,7 @@ import {
 } from '@/lib/auth';
 import { logger } from '@/lib/logger';
 import { rateLimit } from '@/lib/rate-limit';
+import { headers } from 'next/headers';
 import {
   hashClaimToken,
   PLACEHOLDER_PASSWORD_PREFIX,
@@ -32,6 +33,15 @@ import {
 // ---------------------------------------------------------------------------
 
 export const signUpAction = createSafeAction(signUpSchema, async (data) => {
+  const heads = await headers();
+  const xff = heads.get('x-forwarded-for');
+  const ip = (xff ? xff.split(',')[0]?.trim() : null) || heads.get('x-real-ip') || 'unknown';
+
+  const ipRl = rateLimit(`signup:ip:${ip}`, 10, 60_000);
+  if (!ipRl.allowed) {
+    throw new Error(`Too many attempts from your IP. Try again in ${ipRl.retryAfterSeconds}s.`);
+  }
+
   const rl = rateLimit(`signup:${data.email.toLowerCase()}`, 5, 60_000);
   if (!rl.allowed) {
     throw new Error(`Too many attempts. Try again in ${rl.retryAfterSeconds}s.`);
@@ -125,6 +135,15 @@ export const signUpAction = createSafeAction(signUpSchema, async (data) => {
 export const signInAction = createSafeAction(signInSchema, async (data) => {
   // Rate-limit BEFORE any DB or scrypt work — the sync scrypt verify is
   // exactly what an attacker would use to burn the event loop.
+  const heads = await headers();
+  const xff = heads.get('x-forwarded-for');
+  const ip = (xff ? xff.split(',')[0]?.trim() : null) || heads.get('x-real-ip') || 'unknown';
+
+  const ipRl = rateLimit(`signin:ip:${ip}`, 10, 60_000);
+  if (!ipRl.allowed) {
+    throw new Error(`Too many attempts from your IP. Try again in ${ipRl.retryAfterSeconds}s.`);
+  }
+
   const rl = rateLimit(`signin:${data.email.toLowerCase()}`, 5, 60_000);
   if (!rl.allowed) {
     throw new Error(`Too many attempts. Try again in ${rl.retryAfterSeconds}s.`);

src/lib/__tests__/rate-limit.test.ts

@@ -0,0 +1,112 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { rateLimit } from '../rate-limit';
+
+describe('rate-limit.ts', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date('2026-07-16T12:00:00Z'));
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it('allows hits within the limit', () => {
+    const key = 'user1';
+    const limit = 3;
+    const windowMs = 60_000;
+
+    // First 3 hits are allowed
+    for (let i = 0; i < limit; i++) {
+      const res = rateLimit(key, limit, windowMs);
+      expect(res.allowed).toBe(true);
+      expect(res.retryAfterSeconds).toBe(0);
+      vi.advanceTimersByTime(1000); // 1s apart
+    }
+  });
+
+  it('blocks hits exceeding the limit and returns correct retryAfterSeconds', () => {
+    const key = 'user2';
+    const limit = 3;
+    const windowMs = 60_000;
+
+    // Hit 1: t=0
+    expect(rateLimit(key, limit, windowMs).allowed).toBe(true);
+    vi.advanceTimersByTime(10_000); // Now t=10s
+
+    // Hit 2: t=10s
+    expect(rateLimit(key, limit, windowMs).allowed).toBe(true);
+    vi.advanceTimersByTime(10_000); // Now t=20s
+
+    // Hit 3: t=20s
+    expect(rateLimit(key, limit, windowMs).allowed).toBe(true);
+
+    // Hit 4: t=20s (Exceeds limit!)
+    const blockedRes = rateLimit(key, limit, windowMs);
+    expect(blockedRes.allowed).toBe(false);
+    // Oldest hit was at t=0. Window is 60s.
+    // So the oldest hit will fall out at t=60s.
+    // Current time is t=20s.
+    // Remaining time: 60 - 20 = 40 seconds.
+    expect(blockedRes.retryAfterSeconds).toBe(40);
+  });
+
+  it('denied hits are not recorded and do not extend lockout window', () => {
+    const key = 'user3';
+    const limit = 2;
+    const windowMs = 60_000;
+
+    // Hit 1: t=0
+    expect(rateLimit(key, limit, windowMs).allowed).toBe(true);
+    vi.advanceTimersByTime(10_000); // t=10s
+
+    // Hit 2: t=10s
+    expect(rateLimit(key, limit, windowMs).allowed).toBe(true);
+
+    // Hit 3: t=10s (Blocked)
+    expect(rateLimit(key, limit, windowMs).allowed).toBe(false);
+
+    // If denied hits were recorded, the sliding window would have hits at t=0, t=10s, t=10s.
+    // Since it's not recorded, advancing by 51s (t=61s) means hit 1 (t=0) fell out.
+    // Now only 1 hit remains (t=10s). Hit 4 should be allowed.
+    vi.advanceTimersByTime(51_000); // t=61s
+    expect(rateLimit(key, limit, windowMs).allowed).toBe(true);
+  });
+
+  it('sliding window lets hits fall out and allows new requests', () => {
+    const key = 'user4';
+    const limit = 2;
+    const windowMs = 60_000;
+
+    // Hit 1: t=0
+    expect(rateLimit(key, limit, windowMs).allowed).toBe(true);
+    vi.advanceTimersByTime(40_000); // t=40s
+
+    // Hit 2: t=40s
+    expect(rateLimit(key, limit, windowMs).allowed).toBe(true);
+
+    // Hit 3: t=40s (Blocked)
+    expect(rateLimit(key, limit, windowMs).allowed).toBe(false);
+
+    // Advance 21 seconds to t=61s (first hit at t=0 has expired)
+    vi.advanceTimersByTime(21_000); // t=61s
+    const res = rateLimit(key, limit, windowMs);
+    expect(res.allowed).toBe(true);
+  });
+
+  it('performs opportunistic cleanup of the internal map when size exceeds threshold', () => {
+    const windowMs = 60_000;
+
+    // Fill the limiter with 10,001 entries to trigger size cleanup
+    for (let i = 0; i < 10_005; i++) {
+      rateLimit(`key-${i}`, 5, windowMs);
+    }
+
+    // Now advance time so all those keys are expired
+    vi.advanceTimersByTime(windowMs + 1000);
+
+    // Trigger another rateLimit call to trigger cleanup
+    const res = rateLimit('new-key', 5, windowMs);
+    expect(res.allowed).toBe(true);
+  });
+});