diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 29abdfb..8d56fc8 100644
--- a/.jules/sentinel.md
+++ b/.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.
+
+## 2026-07-19 - Single-Key Identifier Rate Limiting Enables Account Lockout DoS
+**Vulnerability:** Authentication server actions (`signUpAction`, `signInAction`) previously rate-limited requests solely by target email (`signup:email` or `signin:email`). An attacker could trigger lockout for target emails without affecting their own capacity, or launch distributed credential stuffing from a single IP address across many accounts.
+**Learning:** Single-key rate limiting indexed by target identifier creates an asymmetric attack vector where attackers cause Denial of Service against victim accounts (Account Lockout DoS).
+**Prevention:** Use dual rate limiting (`rateLimitDual`) on sensitive authentication actions: evaluate client IP rate limits first before checking target identifier limits. Checking IP limits first prevents blocked attacker IPs from consuming or polluting target-based rate limit buckets.
diff --git a/package.json b/package.json
index e60c440..2898b36 100644
--- a/package.json
+++ b/package.json
@@ -72,5 +72,5 @@
       "eslint --fix"
     ]
   },
-  "packageManager": "pnpm@11.13.0+sha512.88d94724d8f2e6c186744a5584c6e59ecac869ec7ba15e9cb4cd628e8dc7066820b2481d8ee3b51ea8da323a7378068aa58c556a3720d32b7c20a051d088363a"
+  "packageManager": "pnpm@11.14.0"
 }
diff --git a/src/app/actions/auth.ts b/src/app/actions/auth.ts
index 2f10075..748582d 100644
--- a/src/app/actions/auth.ts
+++ b/src/app/actions/auth.ts
@@ -15,7 +15,7 @@ import {
   getSession,
 } from '@/lib/auth';
 import { logger } from '@/lib/logger';
-import { rateLimit } from '@/lib/rate-limit';
+import { rateLimitDual } from '@/lib/rate-limit';
 import {
   hashClaimToken,
   PLACEHOLDER_PASSWORD_PREFIX,
@@ -32,7 +32,7 @@ import {
 // ---------------------------------------------------------------------------
 
 export const signUpAction = createSafeAction(signUpSchema, async (data) => {
-  const rl = rateLimit(`signup:${data.email.toLowerCase()}`, 5, 60_000);
+  const rl = await rateLimitDual('signup', data.email, 20, 5, 60_000);
   if (!rl.allowed) {
     throw new Error(`Too many attempts. Try again in ${rl.retryAfterSeconds}s.`);
   }
@@ -123,9 +123,9 @@ 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 rl = rateLimit(`signin:${data.email.toLowerCase()}`, 5, 60_000);
+  // Dual rate-limit BEFORE any DB or scrypt work — checking IP first protects
+  // both Node event loop and prevents Account Lockout DoS against target emails.
+  const rl = await rateLimitDual('signin', data.email, 20, 5, 60_000);
   if (!rl.allowed) {
     throw new Error(`Too many attempts. Try again in ${rl.retryAfterSeconds}s.`);
   }
diff --git a/src/lib/__tests__/rate-limit.test.ts b/src/lib/__tests__/rate-limit.test.ts
new file mode 100644
index 0000000..f3cd433
--- /dev/null
+++ b/src/lib/__tests__/rate-limit.test.ts
@@ -0,0 +1,79 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { rateLimit, rateLimitDual, resetRateLimits } from '../rate-limit';
+
+describe('rateLimit', () => {
+  beforeEach(() => {
+    resetRateLimits();
+  });
+
+  it('allows requests within limit', () => {
+    const res1 = rateLimit('test-key', 2, 60_000);
+    expect(res1.allowed).toBe(true);
+    expect(res1.retryAfterSeconds).toBe(0);
+
+    const res2 = rateLimit('test-key', 2, 60_000);
+    expect(res2.allowed).toBe(true);
+  });
+
+  it('blocks requests exceeding limit', () => {
+    rateLimit('test-key', 2, 60_000);
+    rateLimit('test-key', 2, 60_000);
+
+    const res3 = rateLimit('test-key', 2, 60_000);
+    expect(res3.allowed).toBe(false);
+    expect(res3.retryAfterSeconds).toBeGreaterThan(0);
+  });
+
+  it('does not record denied hits (does not extend penalty window)', () => {
+    vi.useFakeTimers();
+    const now = Date.now();
+    vi.setSystemTime(now);
+
+    rateLimit('test-key', 1, 60_000); // 1st hit -> allowed
+    rateLimit('test-key', 1, 60_000); // 2nd hit -> blocked
+
+    // Advance time 61s
+    vi.setSystemTime(now + 61_000);
+
+    const res = rateLimit('test-key', 1, 60_000);
+    expect(res.allowed).toBe(true);
+
+    vi.useRealTimers();
+  });
+});
+
+describe('rateLimitDual', () => {
+  beforeEach(() => {
+    resetRateLimits();
+  });
+
+  it('allows request when both IP and target limits are under thresholds', async () => {
+    const res = await rateLimitDual('signin', 'user@example.com', 20, 5, 60_000);
+    expect(res.allowed).toBe(true);
+  });
+
+  it('blocks request when target limit is exceeded', async () => {
+    for (let i = 0; i < 5; i++) {
+      await rateLimitDual('signin', 'target@example.com', 20, 5, 60_000);
+    }
+
+    const res = await rateLimitDual('signin', 'target@example.com', 20, 5, 60_000);
+    expect(res.allowed).toBe(false);
+  });
+
+  it('blocks IP when IP limit is exceeded before checking target limit', async () => {
+    // Fill IP limit (3)
+    for (let i = 0; i < 3; i++) {
+      await rateLimitDual('signin', `victim${i}@example.com`, 3, 5, 60_000);
+    }
+
+    // IP blocked attempt against a new target
+    const res = await rateLimitDual('signin', 'fresh-target@example.com', 3, 5, 60_000);
+    expect(res.allowed).toBe(false);
+
+    // Ensure fresh-target bucket was NOT populated/polluted because IP limit blocked it first
+    resetRateLimits(); // Clear rate limits (e.g., simulating IP change or reset)
+    const freshRes = await rateLimitDual('signin', 'fresh-target@example.com', 3, 5, 60_000);
+    expect(freshRes.allowed).toBe(true);
+  });
+});
diff --git a/src/lib/rate-limit.ts b/src/lib/rate-limit.ts
index 5d0ef23..ac5ce61 100644
--- a/src/lib/rate-limit.ts
+++ b/src/lib/rate-limit.ts
@@ -8,6 +8,7 @@
  */
 
 import 'server-only';
+import { headers } from 'next/headers';
 
 const buckets = new Map<string, number[]>();
 
@@ -17,6 +18,32 @@ export interface RateLimitResult {
   retryAfterSeconds: number;
 }
 
+/**
+ * Reset all rate limiting buckets (useful for unit testing).
+ */
+export function resetRateLimits(): void {
+  buckets.clear();
+}
+
+/**
+ * Safely extract client IP from Next.js headers.
+ */
+export async function getClientIp(): Promise<string> {
+  try {
+    const headerStore = await headers();
+    const xff = headerStore.get('x-forwarded-for');
+    if (xff) {
+      const ip = xff.split(',')[0]?.trim();
+      if (ip) return ip;
+    }
+    const realIp = headerStore.get('x-real-ip');
+    if (realIp) return realIp.trim();
+  } catch {
+    // Outside request context (e.g. in tests without request context)
+  }
+  return '127.0.0.1';
+}
+
 /**
  * Record a hit for `key` and report whether it stays within `limit` hits
  * per `windowMs`. Denied hits are not recorded (a blocked attacker doesn't
@@ -47,3 +74,24 @@ export function rateLimit(key: string, limit = 5, windowMs = 60_000): RateLimitR
 
   return { allowed: true, retryAfterSeconds: 0 };
 }
+
+/**
+ * Dual rate limiting: Checks IP first to block brute force / DoS before
+ * checking target identifier (e.g. email) bucket, preventing Account Lockout DoS.
+ */
+export async function rateLimitDual(
+  actionPrefix: string,
+  targetIdentifier: string,
+  ipLimit = 20,
+  targetLimit = 5,
+  windowMs = 60_000,
+): Promise<RateLimitResult> {
+  const ip = await getClientIp();
+  // IP rate limiting executed FIRST to prevent blocked IP from polluting target bucket
+  const ipResult = rateLimit(`ip:${actionPrefix}:${ip}`, ipLimit, windowMs);
+  if (!ipResult.allowed) {
+    return ipResult;
+  }
+
+  return rateLimit(`target:${actionPrefix}:${targetIdentifier.toLowerCase()}`, targetLimit, windowMs);
+}