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feat(brainstorm-server): gate every endpoint behind a per-session key

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The companion server is reachable by any local browser tab (default loopback
bind) and by any host that can route to it (remote --host bind). It served
screens, files, and accepted event-injecting WebSocket connections with no
authentication, so a malicious browser tab or a direct remote client could read
brainstorm content or inject events that the agent reads as the user's input
(prompt injection into a live session).

Generate a per-session secret token, carry it in the served URL as ?key=, and
mirror it into an HttpOnly SameSite=Strict per-port cookie on first load so
same-origin subresources and the WebSocket handshake authenticate automatically.
Every HTTP request and WebSocket upgrade now requires a valid key (query or
cookie, constant-time compared); unauthenticated requests get a friendly 403
explaining they need the full URL. A secret authenticates the client uniformly
across loopback, tunnel, and remote binds and defeats DNS rebinding, which a
Host/Origin allowlist cannot.

Also guard handleMessage against a null JSON payload that crashed the process.

Tests: new auth.test.js (13 cases) covering the key on /, /files/*, and WS plus
cookie bootstrap and the null-payload guard; server.test.js threads the key;
ws-protocol.test.js + auth.test.js wired into npm test.

Closes #1014
Refs #1110, #1553, #1504
This commit is contained in:
Jesse Vincent
2026-06-09 12:22:53 -07:00
parent 3e3c10e671
commit e3fe480b29
5 changed files with 308 additions and 20 deletions
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84
skills/brainstorming/scripts/server.cjs
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@@ -104,6 +104,15 @@ const CONTENT_DIR = path.join(SESSION_DIR, 'content');
const STATE_DIR = path.join(SESSION_DIR, 'state');
let ownerPid = process.env.BRAINSTORM_OWNER_PID ? Number(process.env.BRAINSTORM_OWNER_PID) : null;
// Per-session secret key. The companion is reachable by any local browser tab
// and, when bound to a non-loopback host, by any host that can route to it.
// The key authenticates the real client uniformly across loopback, tunnel, and
// remote binds — and defeats DNS rebinding — where a Host/Origin allowlist
// cannot. It rides the served URL as ?key= and is mirrored into a cookie on
// first load so same-origin subresources and the WebSocket carry it for free.
const TOKEN = process.env.BRAINSTORM_TOKEN || crypto.randomBytes(32).toString('hex');
const COOKIE_NAME = 'brainstorm-key-' + PORT;
const MIME_TYPES = {
'.html': 'text/html', '.css': 'text/css', '.js': 'application/javascript',
'.json': 'application/json', '.png': 'image/png', '.jpg': 'image/jpeg',
@@ -121,6 +130,16 @@ h1 { color: #333; } p { color: #666; }</style>
<body><h1>Brainstorm Companion</h1>
<p>Waiting for the agent to push a screen...</p></body></html>`;
const FORBIDDEN_PAGE = `<!DOCTYPE html>
<html>
<head><meta charset="utf-8"><title>Session key required</title>
<style>body { font-family: system-ui, sans-serif; padding: 2rem; max-width: 800px; margin: 0 auto; }
h1 { color: #333; } p { color: #666; } code { background: #f0f0f0; padding: 0.1em 0.3em; border-radius: 4px; }</style>
</head>
<body><h1>Session key required</h1>
<p>This page needs the full URL your coding agent gave you, including the
<code>?key=&hellip;</code> part. Copy the complete URL and open it again.</p></body></html>`;
const frameTemplate = fs.readFileSync(path.join(__dirname, 'frame-template.html'), 'utf-8');
const helperScript = fs.readFileSync(path.join(__dirname, 'helper.js'), 'utf-8');
const helperInjection = '<script>\n' + helperScript + '\n</script>';
@@ -147,11 +166,64 @@ function getNewestScreen() {
return files.length > 0 ? files[0].path : null;
}
// ========== Authentication ==========
function timingSafeEqualStr(a, b) {
const ab = Buffer.from(String(a));
const bb = Buffer.from(String(b));
if (ab.length !== bb.length) return false;
return crypto.timingSafeEqual(ab, bb);
}
function parseCookies(header) {
const out = {};
if (!header) return out;
for (const part of header.split(';')) {
const eq = part.indexOf('=');
if (eq < 0) continue;
out[part.slice(0, eq).trim()] = part.slice(eq + 1).trim();
}
return out;
}
// A request is authorized if it carries the session key as ?key= or as the
// session cookie. Both are compared in constant time.
function isAuthorized(req) {
const q = req.url.indexOf('?');
if (q >= 0) {
const key = new URLSearchParams(req.url.slice(q + 1)).get('key');
if (key && timingSafeEqualStr(key, TOKEN)) return true;
}
const cookie = parseCookies(req.headers['cookie'])[COOKIE_NAME];
if (cookie && timingSafeEqualStr(cookie, TOKEN)) return true;
return false;
}
function pathnameOf(url) {
const q = url.indexOf('?');
return q >= 0 ? url.slice(0, q) : url;
}
// ========== HTTP Request Handler ==========
function handleRequest(req, res) {
touchActivity();
if (req.method === 'GET' && req.url === '/') {
if (!isAuthorized(req)) {
res.writeHead(403, { 'Content-Type': 'text/html; charset=utf-8' });
res.end(FORBIDDEN_PAGE);
return;
}
// Mirror the key into a cookie so same-origin subresources (/files/*) and the
// WebSocket handshake carry it automatically, whatever URL style the agent
// writes. SameSite=Strict: a cross-site page can neither read the key nor ride
// the cookie; HttpOnly: page scripts can't exfiltrate it.
res.setHeader('Set-Cookie',
COOKIE_NAME + '=' + TOKEN + '; HttpOnly; SameSite=Strict; Path=/');
const pathname = pathnameOf(req.url);
if (req.method === 'GET' && pathname === '/') {
const screenFile = getNewestScreen();
let html = screenFile
? (raw => isFullDocument(raw) ? raw : wrapInFrame(raw))(fs.readFileSync(screenFile, 'utf-8'))
@@ -165,8 +237,8 @@ function handleRequest(req, res) {
res.writeHead(200, { 'Content-Type': 'text/html; charset=utf-8' });
res.end(html);
} else if (req.method === 'GET' && req.url.startsWith('/files/')) {
const fileName = path.basename(req.url.slice(7));
} else if (req.method === 'GET' && pathname.startsWith('/files/')) {
const fileName = path.basename(pathname.slice(7));
const filePath = path.join(CONTENT_DIR, fileName);
if (fileName.startsWith('.') || !fs.existsSync(filePath)) {
res.writeHead(404);
@@ -188,6 +260,8 @@ function handleRequest(req, res) {
const clients = new Set();
function handleUpgrade(req, socket) {
if (!isAuthorized(req)) { socket.destroy(); return; }
const key = req.headers['sec-websocket-key'];
if (!key) { socket.destroy(); return; }
@@ -254,7 +328,7 @@ function handleMessage(text) {
}
touchActivity();
console.log(JSON.stringify({ source: 'user-event', ...event }));
if (event.choice) {
if (event && event.choice) {
const eventsFile = path.join(STATE_DIR, 'events');
fs.appendFileSync(eventsFile, JSON.stringify(event) + '\n');
}
@@ -418,7 +492,7 @@ function startServer() {
}
const info = JSON.stringify({
type: 'server-started', port: Number(PORT), host: HOST,
url_host: URL_HOST, url: 'http://' + URL_HOST + ':' + PORT,
url_host: URL_HOST, url: 'http://' + URL_HOST + ':' + PORT + '/?key=' + TOKEN,
screen_dir: CONTENT_DIR, state_dir: STATE_DIR, idle_timeout_ms: IDLE_TIMEOUT_MS
});
console.log(info);