diff --git a/apps/backend/src/routes/auth.ts b/apps/backend/src/routes/auth.ts
index febc41d..a6b2518 100644
--- a/apps/backend/src/routes/auth.ts
+++ b/apps/backend/src/routes/auth.ts
@@ -1,5 +1,6 @@
 import type { FastifyInstance, FastifyRequest, FastifyReply } from 'fastify';
 import { randomBytes } from 'crypto';
+
 const GITHUB_AUTH_URL = 'https://github.com/login/oauth/authorize';
 const GITHUB_TOKEN_URL = 'https://github.com/login/oauth/access_token';
 const GITHUB_USER_URL = 'https://api.github.com/user';
@@ -20,6 +21,15 @@ export async function authRoutes(app: FastifyInstance) {
     const clientState = (request.query as any).state || '';
     const state = clientState ? `${clientState}_${generateState()}` : generateState();
 
+    // Store state in a short-lived cookie so the callback can verify it
+    reply.setCookie('oauth_state', state, {
+      httpOnly: true,
+      secure: process.env.NODE_ENV === 'production',
+      sameSite: 'lax',
+      path: '/',
+      maxAge: 10 * 60,
+    });
+
     const params = new URLSearchParams({
       client_id: (process.env.GITHUB_CLIENT_ID || '').trim(),
       redirect_uri: redirectUri,
@@ -27,13 +37,20 @@ export async function authRoutes(app: FastifyInstance) {
       state,
     });
     const authUrl = `${GITHUB_AUTH_URL}?${params}`;
-    console.log('--- GITHUB OAUTH REDIRECT ---');
-    console.log('URL:', authUrl);
     return reply.redirect(authUrl);
   });
 
   app.get('/github/callback', async (request: FastifyRequest<{ Querystring: OAuthCallbackQuery }>, reply: FastifyReply) => {
-    const { code } = request.query;
+    const { code, state } = request.query;
+
+    // Validate state to prevent CSRF attacks
+    const storedState = (request.cookies as any).oauth_state;
+    reply.clearCookie('oauth_state', { path: '/' });
+
+    if (!storedState || !state || state !== storedState) {
+      return reply.status(400).send({ error: 'Invalid OAuth state parameter.' });
+    }
+
     if (!code) {
       return reply.status(400).send({ error: 'Missing authorization code' });
     }
@@ -146,6 +163,15 @@ export async function authRoutes(app: FastifyInstance) {
     const clientState = (request.query as any).state || '';
     const state = clientState ? `${clientState}_${generateState()}` : generateState();
 
+    // Store state in a short-lived cookie so the callback can verify it
+    reply.setCookie('oauth_state', state, {
+      httpOnly: true,
+      secure: process.env.NODE_ENV === 'production',
+      sameSite: 'lax',
+      path: '/',
+      maxAge: 10 * 60,
+    });
+
     const params = new URLSearchParams({
       client_id: (process.env.GOOGLE_CLIENT_ID || '').trim(),
       redirect_uri: redirectUri,
@@ -155,13 +181,20 @@ export async function authRoutes(app: FastifyInstance) {
       access_type: 'offline',
     });
     const authUrl = `${GOOGLE_AUTH_URL}?${params}`;
-    console.log('--- GOOGLE OAUTH REDIRECT ---');
-    console.log('URL:', authUrl);
     return reply.redirect(authUrl);
   });
 
   app.get('/google/callback', async (request: FastifyRequest<{ Querystring: OAuthCallbackQuery }>, reply: FastifyReply) => {
-    const { code } = request.query;
+    const { code, state } = request.query;
+
+    // Validate state to prevent CSRF attacks
+    const storedState = (request.cookies as any).oauth_state;
+    reply.clearCookie('oauth_state', { path: '/' });
+
+    if (!storedState || !state || state !== storedState) {
+      return reply.status(400).send({ error: 'Invalid OAuth state parameter.' });
+    }
+
     if (!code) {
       return reply.status(400).send({ error: 'Missing authorization code' });
     }
diff --git a/apps/backend/src/routes/connect.ts b/apps/backend/src/routes/connect.ts
index 68f8671..96ab5f6 100644
--- a/apps/backend/src/routes/connect.ts
+++ b/apps/backend/src/routes/connect.ts
@@ -1,4 +1,5 @@
 import type { FastifyInstance, FastifyRequest, FastifyReply } from 'fastify';
+import { randomBytes } from 'crypto';
 
 const GITHUB_AUTH_URL = 'https://github.com/login/oauth/authorize';
 const GITHUB_TOKEN_URL = 'https://github.com/login/oauth/access_token';
@@ -52,7 +53,9 @@ export async function connectRoutes(app: FastifyInstance) {
     return reply.redirect(`${GITHUB_AUTH_URL}?${params}`);
   });
 
-  app.get('/github/callback', async (request: FastifyRequest<{ Querystring: OAuthCallbackQuery }>, reply: FastifyReply) => {
+  app.get('/github/callback', {
+    preHandler: [app.authenticate],
+  }, async (request: FastifyRequest<{ Querystring: OAuthCallbackQuery }>, reply: FastifyReply) => {
     const { code, state } = request.query;
 
     if (!code || !state) {
@@ -66,12 +69,18 @@ export async function connectRoutes(app: FastifyInstance) {
       if (!decodedState) {
         return reply.redirect(`${process.env.PUBLIC_APP_URL}/settings?error=connect_failed`);
       }
-      const userId = decodedState.userId;
 
-      if (!userId) {
+      // Verify the userId embedded in the state matches the authenticated session.
+      // Without this check an attacker who obtains any valid GitHub OAuth code can
+      // craft an arbitrary state payload (base64-encoded JSON) pointing to a victim's
+      // userId and have the server store the attacker's token under the victim's account.
+      const sessionUserId = (request.user as any).id;
+      if (!decodedState.userId || decodedState.userId !== sessionUserId) {
         return reply.redirect(`${process.env.PUBLIC_APP_URL}/settings?error=invalid_state`);
       }
 
+      const userId = decodedState.userId;
+
       // Exchange code for token
       const tokenRes = await fetch(GITHUB_TOKEN_URL, {
         method: 'POST',