Jesse Vincent e3fe480b29 feat(brainstorm-server): gate every endpoint behind a per-session key ...

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

2026-06-09 18:29:49 -07:00

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