diff --git a/.jules/sentinel.md b/.jules/sentinel.md
new file mode 100644
index 0000000..48d42f2
--- /dev/null
+++ b/.jules/sentinel.md
@@ -0,0 +1,4 @@
+## 2025-12-23 - Missing Security Headers
+**Vulnerability:** The Express application was missing standard security headers (HSTS, CSP, X-Frame-Options, etc.), increasing risk of XSS, clickjacking, and MIME-sniffing.
+**Learning:** Even with manual CORS handling, standard security headers are often overlooked in custom server implementations.
+**Prevention:** Implemented a reusable `securityHeaders` middleware in `src/security-headers.ts` that enforces these headers on all responses.
diff --git a/src/http-transport.test.ts b/src/http-transport.test.ts
index 9f4371a..cbfd778 100644
--- a/src/http-transport.test.ts
+++ b/src/http-transport.test.ts
@@ -1876,4 +1876,30 @@ describeIfListen('HttpTransport', () => {
       expect(isAllowed).toBe(true); // localhost always allowed
     });
   });
+
+  describe('Security Headers', () => {
+    it('should include standard security headers in responses', async () => {
+      const response = await request(app).get('/health');
+
+      expect(response.headers['x-content-type-options']).toBe('nosniff');
+      expect(response.headers['x-frame-options']).toBe('DENY');
+      expect(response.headers['content-security-policy']).toContain("default-src 'self'");
+      expect(response.headers['referrer-policy']).toBe('no-referrer');
+      expect(response.headers['permissions-policy']).toBe('interest-cohort=()');
+    });
+
+    it('should include HSTS header when X-Forwarded-Proto is https', async () => {
+      const response = await request(app)
+        .get('/health')
+        .set('X-Forwarded-Proto', 'https');
+
+      expect(response.headers['strict-transport-security']).toContain('max-age=31536000');
+    });
+
+    it('should not include HSTS header on plain HTTP', async () => {
+      const response = await request(app).get('/health');
+
+      expect(response.headers['strict-transport-security']).toBeUndefined();
+    });
+  });
 });
diff --git a/src/http-transport.ts b/src/http-transport.ts
index f3b67f9..b8119cf 100644
--- a/src/http-transport.ts
+++ b/src/http-transport.ts
@@ -27,6 +27,7 @@ import type {
 import { isInitializeRequest } from './jsonrpc-validator.js';
 import { MetricsCollector } from './metrics.js';
 import { ExternalOAuthProvider } from './oauth-provider.js';
+import { securityHeaders } from './security-headers.js';
 import type { AuthInterceptor } from './types/profile.js';
 import { HTTP_STATUS, MIME_TYPES, OAUTH_PATHS, TIMEOUTS, OAUTH_RATE_LIMIT } from './constants.js';
 import { escapeHtmlSafe } from './validation-utils.js';
@@ -106,6 +107,9 @@ export class HttpTransport {
       next();
     });
 
+    // Security headers (standard hardening)
+    this.app.use(securityHeaders);
+
     // DNS rebinding protection when binding to localhost
     // Deny requests with mismatched Host headers to prevent DNS rebinding attacks
     // Applies when server host is localhost/127.0.0.1, regardless of auth configuration
diff --git a/src/security-headers.ts b/src/security-headers.ts
new file mode 100644
index 0000000..9cb81d9
--- /dev/null
+++ b/src/security-headers.ts
@@ -0,0 +1,37 @@
+import { Request, Response, NextFunction } from 'express';
+
+/**
+ * Middleware to add security headers to all responses
+ *
+ * Why:
+ * - X-Content-Type-Options: nosniff - Prevents MIME-sniffing
+ * - X-Frame-Options: DENY - Prevents clickjacking
+ * - Content-Security-Policy: default-src 'self' - Reduces XSS risk
+ * - Referrer-Policy: no-referrer - Protects user privacy
+ * - Permissions-Policy: interest-cohort=() - Disables FLoC
+ */
+export function securityHeaders(req: Request, res: Response, next: NextFunction): void {
+  // Prevent browser from guessing the MIME type
+  res.setHeader('X-Content-Type-Options', 'nosniff');
+
+  // Prevent the site from being embedded in a frame (clickjacking protection)
+  res.setHeader('X-Frame-Options', 'DENY');
+
+  // Restrict resources to the same origin (XSS mitigation)
+  // Note: This is a strict policy; might need adjustment if external resources are needed later
+  res.setHeader('Content-Security-Policy', "default-src 'self'; frame-ancestors 'none'; object-src 'none'; base-uri 'self'");
+
+  // Do not send referrer header
+  res.setHeader('Referrer-Policy', 'no-referrer');
+
+  // Disable FLoC (Federated Learning of Cohorts)
+  res.setHeader('Permissions-Policy', 'interest-cohort=()');
+
+  // HSTS (Strict-Transport-Security)
+  // Only set if the request is secure (HTTPS) or signaled as such by a proxy
+  if (req.secure || req.headers['x-forwarded-proto'] === 'https') {
+    res.setHeader('Strict-Transport-Security', 'max-age=31536000; includeSubDomains');
+  }
+
+  next();
+}