diff --git a/.jules/sentinel.md b/.jules/sentinel.md
new file mode 100644
index 0000000..a75b462
--- /dev/null
+++ b/.jules/sentinel.md
@@ -0,0 +1,4 @@
+## 2024-05-23 - [IDOR in Newsletter Unsubscribe]
+**Vulnerability:** The unsubscribe endpoint `/api/newsletter/unsubscribe` accepted a JSON payload with `userId` and directly unsubscribed that user without any authentication or signature verification.
+**Learning:** Manual scripts that generate public links (like email newsletters) often skip standard security practices (like authentication) because they are "internal tools", but the resulting links expose public endpoints.
+**Prevention:** Always require a cryptographic signature (HMAC) for any action performed via a public link that acts on a specific user's data without a session.
diff --git a/app/api/newsletter/unsubscribe/route.ts b/app/api/newsletter/unsubscribe/route.ts
index 2280c61..c3d83b6 100644
--- a/app/api/newsletter/unsubscribe/route.ts
+++ b/app/api/newsletter/unsubscribe/route.ts
@@ -1,9 +1,10 @@
 import { createServiceRoleClient } from '../../../../lib/supabase/admin';
 import { NextResponse } from 'next/server';
+import { verifyUnsubscribeToken } from '@/lib/newsletter-security';
 
 export async function POST(request: Request) {
   try {
-    const { userId } = await request.json();
+    const { userId, token } = await request.json();
 
     if (!userId) {
       return NextResponse.json(
@@ -12,6 +13,14 @@ export async function POST(request: Request) {
       );
     }
 
+    // Validate token to prevent unauthorized unsubscriptions
+    if (!token || !verifyUnsubscribeToken(userId, token)) {
+      return NextResponse.json(
+        { error: 'Invalid or missing unsubscribe token' },
+        { status: 403 }
+      );
+    }
+
     // Force casting to any to bypass the restrictive "never" inference on the Supabase client
     // which likely stems from a mismatch in generated types vs actual usage in this context.
     const supabase = createServiceRoleClient() as any;
diff --git a/app/unsubscribe/page.tsx b/app/unsubscribe/page.tsx
index 033fe85..506de89 100644
--- a/app/unsubscribe/page.tsx
+++ b/app/unsubscribe/page.tsx
@@ -7,6 +7,7 @@ import Link from 'next/link';
 function UnsubscribeContent() {
   const searchParams = useSearchParams();
   const userId = searchParams.get('uid');
+  const token = searchParams.get('token');
   const [status, setStatus] = useState<'loading' | 'success' | 'error'>('loading');
 
   useEffect(() => {
@@ -22,7 +23,7 @@ function UnsubscribeContent() {
           headers: {
             'Content-Type': 'application/json',
           },
-          body: JSON.stringify({ userId }),
+          body: JSON.stringify({ userId, token }),
         });
 
         if (response.ok) {
diff --git a/lib/newsletter-security.ts b/lib/newsletter-security.ts
new file mode 100644
index 0000000..c956382
--- /dev/null
+++ b/lib/newsletter-security.ts
@@ -0,0 +1,35 @@
+import crypto from 'crypto';
+
+const SECRET_KEY = process.env.CSRF_SALT || process.env.SUPABASE_SERVICE_ROLE_KEY || 'default-secret-key-change-me';
+
+/**
+ * Generates a signed token for unsubscribing a user.
+ * The token is an HMAC-SHA256 of the userId.
+ */
+export function generateUnsubscribeToken(userId: string): string {
+  if (!userId) throw new Error('userId is required');
+
+  const hmac = crypto.createHmac('sha256', SECRET_KEY);
+  hmac.update(userId);
+  return hmac.digest('hex');
+}
+
+/**
+ * Verifies if the token is valid for the given userId.
+ */
+export function verifyUnsubscribeToken(userId: string, token: string): boolean {
+  if (!userId || !token) return false;
+
+  const expectedToken = generateUnsubscribeToken(userId);
+
+  // Use constant-time comparison to prevent timing attacks
+  try {
+    return crypto.timingSafeEqual(
+      Buffer.from(token),
+      Buffer.from(expectedToken)
+    );
+  } catch (e) {
+    // Length mismatch or other error
+    return false;
+  }
+}
diff --git a/scripts/send-newsletter.ts b/scripts/send-newsletter.ts
index bba75a7..8c3fd6f 100644
--- a/scripts/send-newsletter.ts
+++ b/scripts/send-newsletter.ts
@@ -2,6 +2,7 @@ import * as dotenv from 'dotenv';
 import * as postmark from 'postmark';
 import { createServiceRoleClient } from '../lib/supabase/admin';
 import { getHtmlBody, getSubject } from '../lib/email/templates/monthly-update';
+import { generateUnsubscribeToken } from '../lib/newsletter-security';
 
 // Load environment variables from .env.local
 dotenv.config({ path: '.env.local' });
@@ -79,7 +80,8 @@ async function sendNewsletter() {
   for (const profile of profiles) {
     if (!profile.email) continue;
 
-    const unsubscribeUrl = `${NEXT_PUBLIC_APP_URL}/unsubscribe?uid=${profile.id}`;
+    const token = generateUnsubscribeToken(profile.id);
+    const unsubscribeUrl = `${NEXT_PUBLIC_APP_URL}/unsubscribe?uid=${profile.id}&token=${token}`;
     const htmlBody = getHtmlBody(unsubscribeUrl);
     const subject = getSubject();