diff --git a/CLAUDE.md b/CLAUDE.md
index ea9f48e1..3097db75 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -304,6 +304,15 @@ REDIS_URL=redis://localhost:6379
 
 CODEFRAME_API_KEY_SECRET=<secret>     # API key hashing
 
+# Session token lifetime (#657 — defense-in-depth)
+JWT_LIFETIME_SECONDS=86400            # JWT validity window; default 24h (was 7d).
+                                      # Shorter window limits exposure of the
+                                      # localStorage-stored web-UI token. No
+                                      # refresh token, so users re-login after
+                                      # expiry. The web UI also ships a CSP
+                                      # (web-ui/security-headers.js) to contain
+                                      # XSS-based token exfiltration.
+
 # Outbound webhook SSRF guard (#656) — default OFF (block)
 CODEFRAME_ALLOW_PRIVATE_WEBHOOKS=1    # Allow webhook URLs whose host resolves to
                                       # private/loopback/link-local/metadata IPs.
diff --git a/codeframe/auth/manager.py b/codeframe/auth/manager.py
index 42bfc6f3..d4324e68 100644
--- a/codeframe/auth/manager.py
+++ b/codeframe/auth/manager.py
@@ -68,7 +68,7 @@ def refresh_secret() -> str:
 # These must match the JWTStrategy defaults from FastAPI Users
 JWT_ALGORITHM = "HS256"
 JWT_AUDIENCE = ["fastapi-users:auth"]
-JWT_LIFETIME_SECONDS = int(os.getenv("JWT_LIFETIME_SECONDS", "604800"))  # 7 days
+JWT_LIFETIME_SECONDS = int(os.getenv("JWT_LIFETIME_SECONDS", "86400"))  # 24h (#657)
 
 # NOTE: The default-secret warning is intentionally NOT emitted at import time.
 # Importing this module must stay silent so it never leaks onto the CLI (the
diff --git a/web-ui/__tests__/security-headers.test.js b/web-ui/__tests__/security-headers.test.js
new file mode 100644
index 00000000..5328b19f
--- /dev/null
+++ b/web-ui/__tests__/security-headers.test.js
@@ -0,0 +1,52 @@
+/**
+ * Security headers / CSP builder tests (#657).
+ *
+ * The CSP's exfil-containment value lives in connect-src / img-src /
+ * object-src / base-uri / frame-ancestors — these tests pin that lockdown so a
+ * future edit can't silently widen it (e.g. to `connect-src *`).
+ */
+import { buildCsp, buildConnectSrc, securityHeaders } from '../security-headers';
+
+describe('security headers (#657)', () => {
+  test('CSP locks down the exfil-relevant directives', () => {
+    const csp = buildCsp({});
+    expect(csp).toContain("default-src 'self'");
+    expect(csp).toContain("object-src 'none'");
+    expect(csp).toContain("base-uri 'self'");
+    expect(csp).toContain("frame-ancestors 'none'");
+    // The GitHub owner avatar is the only allowed external image host;
+    // anything else would re-open a GET-based exfil channel.
+    expect(csp).toContain('https://avatars.githubusercontent.com');
+    expect(csp).not.toContain('img-src *');
+  });
+
+  test('connect-src includes self and the configured backend + ws origins', () => {
+    const cs = buildConnectSrc({
+      apiUrl: 'https://api.example.com',
+      wsUrl: 'wss://api.example.com',
+    });
+    expect(cs).toContain("'self'");
+    expect(cs).toContain('https://api.example.com');
+    expect(cs).toContain('wss://api.example.com');
+  });
+
+  test('connect-src never falls back to a wildcard', () => {
+    // Empty env (same-origin API, default ws) must still be a closed list.
+    const cs = buildConnectSrc({ apiUrl: '', wsUrl: '' });
+    expect(cs).not.toContain('*');
+    expect(cs).toContain("'self'");
+    expect(cs).toContain('ws://localhost:8000');
+  });
+
+  test('securityHeaders ships the CSP plus the hardening header set', () => {
+    const keys = securityHeaders({}).map((h) => h.key);
+    expect(keys).toEqual(
+      expect.arrayContaining([
+        'Content-Security-Policy',
+        'X-Content-Type-Options',
+        'X-Frame-Options',
+        'Referrer-Policy',
+      ])
+    );
+  });
+});
diff --git a/web-ui/next.config.js b/web-ui/next.config.js
index cd430eae..02dd4351 100644
--- a/web-ui/next.config.js
+++ b/web-ui/next.config.js
@@ -1,5 +1,12 @@
+const { securityHeaders } = require('./security-headers');
+
 /** @type {import('next').NextConfig} */
 const nextConfig = {
+  // Defense-in-depth CSP + hardening headers (#657): contains any future XSS
+  // so an injected script can't exfiltrate the localStorage JWT.
+  async headers() {
+    return [{ source: '/:path*', headers: securityHeaders() }];
+  },
   async rewrites() {
     return {
       beforeFiles: [
diff --git a/web-ui/security-headers.js b/web-ui/security-headers.js
new file mode 100644
index 00000000..752721f2
--- /dev/null
+++ b/web-ui/security-headers.js
@@ -0,0 +1,60 @@
+/**
+ * Content-Security-Policy + hardening headers for the web UI (#657).
+ *
+ * Defense-in-depth: the JWT lives in localStorage (the EventSource header
+ * limitation drove the `?token=` design), so a CSP contains any future XSS by
+ * locking down where injected JS can send data. connect-src is built from the
+ * SAME build-time env the app uses for its API/WS calls, so it matches the
+ * real backend without hardcoding a deploy URL.
+ *
+ * Required by next.config.js (CommonJS) — keep this file dependency-free.
+ */
+
+const DEFAULT_WS_URL = 'ws://localhost:8000';
+const AVATAR_HOST = 'https://avatars.githubusercontent.com';
+
+/**
+ * Closed allow-list of origins the browser may talk to. 'self' covers the
+ * same-origin REST/SSE traffic (NEXT_PUBLIC_API_URL defaults to '' = proxied);
+ * the WebSocket hooks dial NEXT_PUBLIC_WS_URL (or the localhost default).
+ */
+function buildConnectSrc({ apiUrl, wsUrl } = {}) {
+  const sources = new Set(["'self'"]);
+  if (apiUrl) sources.add(apiUrl);
+  sources.add(wsUrl || DEFAULT_WS_URL);
+  return Array.from(sources).join(' ');
+}
+
+function buildCsp(env = process.env) {
+  const connectSrc = buildConnectSrc({
+    apiUrl: env.NEXT_PUBLIC_API_URL,
+    wsUrl: env.NEXT_PUBLIC_WS_URL,
+  });
+  return [
+    "default-src 'self'",
+    // ponytail: 'unsafe-inline'/'unsafe-eval' are required by the Next.js App
+    // Router without a per-request nonce middleware (a much larger change).
+    // Exfil containment comes from connect-src/img-src/object-src below — not
+    // script-src — so the token can't be POSTed/GET'd to an attacker origin.
+    "script-src 'self' 'unsafe-inline' 'unsafe-eval'",
+    "style-src 'self' 'unsafe-inline'",
+    `img-src 'self' data: blob: ${AVATAR_HOST}`,
+    "font-src 'self' data:",
+    `connect-src ${connectSrc}`,
+    "object-src 'none'",
+    "base-uri 'self'",
+    "frame-ancestors 'none'",
+    "form-action 'self'",
+  ].join('; ');
+}
+
+function securityHeaders(env = process.env) {
+  return [
+    { key: 'Content-Security-Policy', value: buildCsp(env) },
+    { key: 'X-Content-Type-Options', value: 'nosniff' },
+    { key: 'X-Frame-Options', value: 'DENY' },
+    { key: 'Referrer-Policy', value: 'strict-origin-when-cross-origin' },
+  ];
+}
+
+module.exports = { buildCsp, buildConnectSrc, securityHeaders };