gbrain/SECURITY.md

# Security

## Reporting Vulnerabilities

If you discover a security issue in GBrain, please report it privately by opening
a [private security advisory](https://github.com/garrytan/gbrain/security/advisories/new)
on GitHub.

Do not open a public issue for security vulnerabilities.

## Remote MCP Security

### ⚠️ Do NOT use open OAuth client registration for remote MCP

If you deploy GBrain's MCP server behind an HTTP wrapper with OAuth 2.1
support, **never allow unauthenticated client registration**. An attacker
who discovers your server URL can:

1. Register a new OAuth client via `POST /register`
2. Use `client_credentials` grant to obtain a bearer token
3. Access all brain data via the MCP tools

### Recommended: `gbrain serve --http`

As of v0.22.7, GBrain ships a built-in HTTP transport that uses the
existing `access_tokens` table for authentication:

```bash
# Create a token
gbrain auth create "my-client"

# Start the HTTP server
gbrain serve --http --port 8787

# Connect via ngrok, Tailscale, or any tunnel
ngrok http 8787 --url your-brain.ngrok.app
```

This is the recommended way to expose GBrain remotely. No OAuth, no
registration endpoint, no self-service tokens. Tokens are managed
exclusively via `gbrain auth create/list/revoke`.

### If you must use a custom HTTP wrapper

1. **Require a secret for client registration** — check a header or body
   parameter before creating new OAuth clients
2. **Disable `client_credentials` grant** — only allow `authorization_code`
   with browser-based approval
3. **Restrict scopes** — never issue tokens with unlimited scope
4. **Log all token issuance** — alert on unexpected registrations
5. **Rate-limit registration and token endpoints**

### Pre-registering claude.ai / ChatGPT clients without DCR (v0.41.3+)

The recommended hardening posture above is: ship `gbrain serve --http`
**without** `--enable-dcr` and pre-register every client manually. As of
v0.41.3, `gbrain auth register-client` accepts the OAuth fields
browser-based clients need:

```bash
# Pre-register claude.ai (confidential client; two redirect URIs)
gbrain auth register-client claude-ai \
  --scopes "read write" \
  --redirect-uri https://claude.ai/api/mcp/auth_callback \
  --redirect-uri https://claude.com/api/mcp/auth_callback
# --grant-types is auto-set to authorization_code,refresh_token when
# --redirect-uri is passed; pass --grant-types explicitly to override.

# Pre-register ChatGPT (public PKCE client; no client_secret minted)
gbrain auth register-client chatgpt \
  --scopes "read write" \
  --redirect-uri https://chatgpt.com/connector/oauth/<HASH> \
  --token-endpoint-auth-method none
```

Auth methods (`--token-endpoint-auth-method`):

- `client_secret_post` (default) — confidential client, secret in body
- `client_secret_basic` — confidential client, secret in `Authorization` header
- `none` — public PKCE-only client (no secret minted; ChatGPT custom
  connector, Claude Code, Cursor)

The validator rejects unknown methods at the registration boundary, and
the same gate applies to the admin endpoint `POST /admin/api/register-client`
and the DCR `POST /register` path. Pre-v0.41.3 the CLI hard-coded
`redirect_uris = []` and `token_endpoint_auth_method = NULL`, forcing
operators to UPDATE `oauth_clients` rows by hand to make claude.ai work
without `--enable-dcr`. That footgun is gone.

### DCR consent default (v0.42.55+)

The "disable `client_credentials`, only allow `authorization_code`" guidance
above is now the built-in default for the DCR path, not just advice for custom
wrappers. With `--enable-dcr` on, a self-registered client defaults to the
`authorization_code` (browser-approval) grant, and an explicit
`client_credentials` request is rejected with `invalid_client_metadata`.
Operators who genuinely need the machine-to-machine grant on the registration
endpoint opt in with `--enable-dcr-insecure` (which implies `--enable-dcr`); a
startup WARNING prints whenever DCR is enabled, and a second when the insecure
grant is allowed. Pre-registering clients via the CLI / admin API is unchanged.

### Token Management

```bash
gbrain auth create "claude-desktop"   # Create a new token
gbrain auth list                       # List all tokens
gbrain auth revoke "claude-desktop"    # Revoke a token
gbrain auth test <url> --token <tok>   # Smoke-test a remote server
```

Tokens are stored as SHA-256 hashes in the `access_tokens` table. The
plaintext token is shown once at creation and never stored.

## `gbrain serve --http` hardening (v0.22.7+)

The built-in HTTP transport ships with several layers of hardening on by
default. All env vars below are optional; the defaults are intentionally
conservative.

### Bind address (v0.34: loopback by default)

`gbrain serve --http` listens on `127.0.0.1` by default. Personal-laptop
installs cannot accidentally publish the brain to the LAN. Self-hosted
deployments that need remote access pass `--bind 0.0.0.0` (all
interfaces) or `--bind <interface-ip>` (specific NIC). A stderr WARN
fires when `--public-url` is set without `--bind` so the operator sees
the binding before the first request — common cause of "ngrok forwards
to me but the agent can't reach the upstream" misconfigurations.

### Postgres-only

`gbrain serve --http` requires a Postgres engine. PGLite is local-only by
design and the `access_tokens` / `mcp_request_log` tables don't exist in
the PGLite schema. Local agents continue to use stdio (`gbrain serve`).
Running `--http` against a PGLite-backed install fails fast with a clear
error message at startup.

### CORS

Default-deny: no `Access-Control-Allow-Origin` header is sent unless an
allowlist is configured. To allow browser-based MCP clients:

```bash
GBRAIN_HTTP_CORS_ORIGIN=https://claude.ai gbrain serve --http --port 8787
# Multiple origins: comma-separated
GBRAIN_HTTP_CORS_ORIGIN=https://claude.ai,https://your.app gbrain serve --http
```

When the request `Origin` matches the allowlist, the server echoes it
back in `Access-Control-Allow-Origin` (with `Vary: Origin`). Otherwise no
CORS header is sent and the browser blocks the request.

**v0.41.3:** the same allowlist now gates every OAuth endpoint (`/mcp`,
`/token`, `/authorize`, `/register`, `/revoke`). Pre-v0.41.3 these used
default-wide-open `cors()` middleware, leaking
`Access-Control-Allow-Origin: *` on every response — any web origin could
complete a token exchange from a logged-in operator's browser. The CORS
preflight handler in the legacy bearer transport was also asymmetric
(actual-request path correctly default-deny, but OPTIONS preflight leaked
`Access-Control-Allow-Methods` + `Access-Control-Allow-Headers` to every
Origin); both are now consolidated through a single allowlist-gated path.
A startup stderr WARN fires when `--bind 0.0.0.0` is set without
`GBRAIN_HTTP_CORS_ORIGIN`, surfacing the default-deny posture before the
first request.

### Rate limiting

Two buckets, both stored in a bounded LRU map (default 10K keys, evicts
least-recently-used on overflow, prunes entries older than 2× the
window):

| Bucket | When it fires | Default | Env var |
|---|---|---|---|
| Pre-auth IP | Before the DB lookup, on every `/mcp` request | 30 req / 60s | `GBRAIN_HTTP_RATE_LIMIT_IP` |
| Post-auth token | After a valid token is resolved | 60 req / 60s | `GBRAIN_HTTP_RATE_LIMIT_TOKEN` |
| LRU cap | Maximum distinct keys across both buckets | 10000 | `GBRAIN_HTTP_RATE_LIMIT_LRU` |

On exhaustion the server returns `429 Too Many Requests` with a
`Retry-After` header.

**Caveat for tunneled deployments (ngrok, Tailscale Funnel, Cloudflare
Tunnel):** all requests share one egress IP, so the pre-auth IP bucket
becomes effectively shared by all clients on that tunnel. The
post-auth token-id bucket is the load-bearing limiter for tunnel-fronted
deployments.

### Reverse-proxy trust

**Loopback-only by default** (v0.41.3+ Express server agrees with the
legacy transport; pre-v0.41.3 the Express server hardcoded `'loopback'`
while docs claimed "disabled by default" — that disagreement is gone).
The default trusts only same-host proxies (127.0.0.1, ::1, fc00::/7);
external forwarded-for headers are ignored regardless. To widen or
narrow trust:

```bash
# Trust exactly one hop — Fly.io, Render, Vercel, single-layer nginx
GBRAIN_HTTP_TRUST_PROXY=1 gbrain serve --http --port 8787

# Trust N hops — Cloudflare → nginx → gbrain
GBRAIN_HTTP_TRUST_PROXY=2 gbrain serve --http --port 8787

# Disable entirely — direct-exposure deployment with no proxy
GBRAIN_HTTP_TRUST_PROXY=0 gbrain serve --http --port 8787

# Named Express modes (uniquelocal, linklocal) or CIDR lists pass through
GBRAIN_HTTP_TRUST_PROXY=uniquelocal gbrain serve --http --port 8787
GBRAIN_HTTP_TRUST_PROXY="10.0.0.0/8,192.168.1.0/24" gbrain serve --http --port 8787
```

Both transports (Express OAuth server in `src/commands/serve-http.ts` and
the legacy bearer transport in `src/mcp/http-transport.ts`) read the same
env var, so single source of truth.

**Critical safety contract:** only widen past `'loopback'` when **both**
of these are true:

1. gbrain is reachable only via a trusted reverse proxy (not directly
   exposed to the internet on the configured port). As of v0.34
   `gbrain serve --http` binds `127.0.0.1` by default, so the
   reverse-proxy-only posture is the out-of-the-box shape; only
   override with `--bind 0.0.0.0` (or a specific interface IP) when
   gbrain itself needs to accept remote connections directly.
2. The proxy strips any client-supplied `X-Forwarded-For` and `X-Real-IP`
   headers, then sets them itself. (nginx with `proxy_set_header
   X-Forwarded-For $remote_addr` does this; Cloudflare and most cloud
   load balancers handle it automatically.)

If gbrain is reachable directly AND `GBRAIN_HTTP_TRUST_PROXY=1` (or any
non-loopback value) is set, clients can spoof their IP by sending
arbitrary `X-Forwarded-For` headers, defeating the pre-auth IP rate
limit. The `'loopback'` default protects against this by ignoring all
forwarded-for headers and using the socket peer address.

### Body size cap

Default 1 MiB, stream-counted (chunked transfers without
`Content-Length` are still capped). Override:

```bash
GBRAIN_HTTP_MAX_BODY_BYTES=2097152 gbrain serve --http   # 2 MiB
```

Over-cap requests get `413 Payload Too Large` immediately, before any
body is materialized in memory.

### Audit log

Every `/mcp` request writes one row to `mcp_request_log`:

```bash
psql "$DATABASE_URL" -c \
  "SELECT created_at, token_name, operation, status, latency_ms
   FROM mcp_request_log
   ORDER BY created_at DESC LIMIT 100"
```

`status` is one of: `success`, `error`, `auth_failed`, `rate_limited`,
`body_too_large`, `parse_error`, `unknown_method`. Failed-auth rows have
`token_name = NULL`. Inserts are fire-and-forget so audit failures
never block requests.

**v0.26.9 redaction default.** The `params` column now stores
`{redacted, kind, declared_keys, unknown_key_count, approx_bytes}` instead
of raw JSON-RPC payloads. Declared keys (intersected against the operation's
spec) preserve for debug visibility; unknown keys are counted but never
named so attackers can't probe key existence; byte sizes bucket to 1KB so
content sizes can't be binary-searched. The same shape is broadcast on the
admin SSE feed at `/admin/events`. Operators on a personal laptop who want
raw payloads back can pass `gbrain serve --http --log-full-params` (loud
stderr warning at startup). Multi-tenant deployments should leave it
on the redacted default.