diff --git a/lib/csrf.js b/lib/csrf.js
new file mode 100644
index 0000000..7d02db2
--- /dev/null
+++ b/lib/csrf.js
@@ -0,0 +1,49 @@
+// Cross-site request forgery guard for the state-changing API.
+//
+// AgentXRay has no cookies and no auth, so a CSRF *token* would prove nothing —
+// the actual attack is a page on another origin driving the visitor's browser
+// to POST at http://127.0.0.1:3800 (rewrite prompts, install library items,
+// trigger a backup). Browsers always attach `Origin` (and `Sec-Fetch-Site`) to
+// such requests, so the boundary is: unsafe methods are accepted only when the
+// request is same-origin or comes from a non-browser client (no Origin header).
+//
+// Same-origin is decided against the Host the request arrived on, not a fixed
+// allow-list, so `HOST=0.0.0.0` LAN deployments keep working from any hostname
+// the operator actually serves.
+
+const SAFE = new Set(['GET', 'HEAD', 'OPTIONS']);
+
+function originHost(value) {
+  try {
+    return new URL(value).host.toLowerCase();
+  } catch {
+    return null;
+  }
+}
+
+// Exported for unit tests: returns null when allowed, else a short reason.
+function crossSiteReason(req) {
+  if (SAFE.has(req.method)) return null;
+  const fetchSite = req.get('sec-fetch-site');
+  if (fetchSite && fetchSite !== 'same-origin' && fetchSite !== 'none') return `sec-fetch-site=${fetchSite}`;
+  const origin = req.get('origin');
+  if (origin) {
+    const host = originHost(origin);
+    if (host !== (req.get('host') || '').toLowerCase()) return `origin ${origin} != host ${req.get('host')}`;
+    return null;
+  }
+  const referer = req.get('referer');
+  if (referer) {
+    const host = originHost(referer);
+    if (host && host !== (req.get('host') || '').toLowerCase()) return `referer ${referer} != host ${req.get('host')}`;
+  }
+  return null;
+}
+
+function csrfGuard(req, res, next) {
+  const reason = crossSiteReason(req);
+  if (reason) return res.status(403).json({ error: 'cross-site request rejected', reason });
+  next();
+}
+
+module.exports = { csrfGuard, crossSiteReason };
diff --git a/package.json b/package.json
index 3e8af6d..57a1d0c 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@alloevil/agent-xray",
-  "version": "1.17.0",
+  "version": "1.17.1",
   "description": "Web dashboard for viewing AI agent session logs — supports OpenClaw, Codex, Claude Code, Hermes, OMP, DeepSeek Harness, and Gemini CLI",
   "main": "server.js",
   "bin": {
diff --git a/server.js b/server.js
index 0e46ec7..a5a3d0f 100644
--- a/server.js
+++ b/server.js
@@ -11,6 +11,7 @@ const mountLibraryRoutes = require('./lib/routes/library');
 const mountBackupRoutes = require('./lib/routes/backup');
 const mountWatchRoutes = require('./lib/routes/watch');
 const mountLlmRoutes = require('./lib/routes/llm');
+const { csrfGuard } = require('./lib/csrf');
 
 const app = express();
 const PORT = process.env.PORT || 3800;
@@ -35,6 +36,8 @@ if (HAS_DIST) {
 } else {
   app.use(express.static(PUBLIC_DIR, { maxAge: 0, etag: false, lastModified: false }));
 }
+// Reject cross-site POST/PUT/DELETE before the body is parsed (lib/csrf.js).
+app.use('/api', csrfGuard);
 app.use(express.json({ limit: '256kb' }));
 
 // Disable all caching
diff --git a/test/api.test.js b/test/api.test.js
index e7374f1..31f456d 100644
--- a/test/api.test.js
+++ b/test/api.test.js
@@ -449,3 +449,45 @@ describe('backup', () => {
     assert.ok(typeof status.lastBackup === 'string');
   });
 });
+
+describe('csrf guard', () => {
+  let srv;
+  before(async () => {
+    srv = await startServer();
+  });
+  after(async () => {
+    await srv.stop();
+  });
+
+  const send = (method, headers, pathname = '/api/prompts/hidden') =>
+    fetch(srv.base + pathname, {
+      method,
+      headers: { 'Content-Type': 'application/json', ...headers },
+      body: method === 'GET' ? undefined : JSON.stringify({ hash: 'deadbeef' }),
+    });
+
+  it('rejects a state-changing request whose Origin is another site', async () => {
+    const res = await send('POST', { Origin: 'https://evil.example' });
+    assert.equal(res.status, 403);
+    const body = await res.json();
+    assert.match(body.reason, /origin https:\/\/evil\.example/);
+  });
+
+  it('rejects when Sec-Fetch-Site says cross-site even without Origin', async () => {
+    const res = await send('DELETE', { 'Sec-Fetch-Site': 'cross-site' }, '/api/prompts/hidden/deadbeef');
+    assert.equal(res.status, 403);
+  });
+
+  it('accepts same-origin browser requests and origin-less clients', async () => {
+    const host = new URL(srv.base).host;
+    const same = await send('POST', { Origin: `http://${host}`, 'Sec-Fetch-Site': 'same-origin' });
+    assert.notEqual(same.status, 403);
+    const curl = await send('POST', {});
+    assert.notEqual(curl.status, 403);
+  });
+
+  it('never blocks reads, whatever the origin', async () => {
+    const res = await send('GET', { Origin: 'https://evil.example', 'Sec-Fetch-Site': 'cross-site' }, '/api/version');
+    assert.equal(res.status, 200);
+  });
+});