fix(register): allowlist headers persisted at client registration

[sebastiondev]

Vulnerability summary

The POST /api/register endpoint in landing/app/api/register/route.ts iterates over all incoming request headers and persists them into the KV store via model.saveClientRegisterHeaders(). This endpoint is unauthenticated — it implements the OAuth 2.0 Dynamic Client Registration flow — and is reachable on the public internet at mcp.neon.tech.

Because every header is stored, sensitive values end up persisted:

• Authorization — bearer tokens from upstream proxies or mistaken client configurations
• Cookie — session cookies attached by browsers or intermediaries
• X-Forwarded-For / X-Forwarded-Proto / X-Real-IP — internal infrastructure topology
• Any other header injected by load balancers, CDNs, or clients

These stored headers are later read back in the /authorize flow, meaning the leaked data could also be reflected to other parties.

CWE-200: Exposure of Sensitive Information to an Unauthorized Actor

Affected file: landing/app/api/register/route.ts, lines 14–17 (the request.headers.forEach loop)

Data flow: request.headers → requestHeaders object (all keys) → model.saveClientRegisterHeaders(clientId, requestHeaders) → Postgres-backed KV store → read back in /authorize

Proof of Concept

curl -X POST https://mcp.neon.tech/api/register \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer leaked-secret-token" \
  -H "Cookie: session=sensitive-value" \
  -H "X-Forwarded-For: 10.0.0.42" \
  -H "X-Internal-Debug: should-not-persist" \
  -H "X-Read-Only: true" \
  -d '{
    "client_name": "test-client",
    "redirect_uris": ["https://example.com/callback"],
    "grant_types": ["authorization_code"],
    "response_types": ["code"],
    "token_endpoint_auth_method": "none"
  }'

All five custom headers above would be persisted into the KV store. Only x-read-only is actually consumed by the authorize handler — the rest are sensitive data leaking into persistent storage.

Fix description

The fix introduces a strict allowlist of headers that may be persisted:

const ALLOWED_REGISTER_HEADERS = [
  'x-read-only',
  'x-neon-read-only',
  'x-neon-project-id',
  'x-neon-scopes',
];

The forEach loop now checks ALLOWED_REGISTER_HEADERS.includes(lower) before adding a header to the requestHeaders object. This ensures only the four headers that the authorize flow actually consumes are stored — everything else is silently dropped.

This is a minimal, behavior-preserving change: the authorize handler only reads x-read-only (and potentially the other x-neon-* headers), so no legitimate functionality is affected.

Test results

A new integration test (does not persist sensitive or unknown headers) was added to landing/mcp-src/__tests__/oauth-register-route.integration.test.ts. The test sends a registration request with both allowed (x-read-only: true) and sensitive (authorization, cookie, x-forwarded-for, x-internal-trace) headers, then asserts:

• Only { 'x-read-only': 'true' } is passed to saveClientRegisterHeaders
• None of the sensitive headers appear in the persisted object
• The registration still succeeds (200 response with a valid client_id)

All existing tests continue to pass — the fix does not change behavior for legitimate registrations.

Security analysis

This is exploitable by any unauthenticated network client. The /api/register endpoint requires no authentication (by design — it implements RFC 7591 Dynamic Client Registration). An attacker can craft requests with arbitrary headers, and all of them are written to persistent storage. The risk compounds because:

1. Infrastructure headers (added by Vercel/CDN) reveal internal topology
2. Auth headers from legitimate users hitting the same endpoint could be captured if a proxy or browser attaches them
3. The stored headers are read back in the authorize flow, creating a secondary exposure surface

Before submitting, we verified that no existing middleware or framework protection filters headers before they reach the route handler — Next.js App Router passes the full NextRequest headers object through unchanged, and there is no header-stripping middleware in the Vercel deployment configuration.

---

_{Submitted by Sebastion — autonomous open-source security research from Foundation Machines. Free for public repos via the Sebastion AI GitHub App.}

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