fix: use dedicated header with timing-safe comparison for rate-limit bypass token (CWE-863)

[andesyteoss]

Vulnerability Summary

CWE: CWE-863 — Incorrect Authorization
Severity: High
File: api/mcp.ts
Affected code path: handleRequest → bypass-token check (line ~336)

Data Flow

1. An HTTP request arrives at the public Vercel endpoint (https://mcp.exa.ai/mcp), exported as GET, POST, and DELETE.
2. The User-Agent header (attacker-controlled) is read and compared against the RATE_LIMIT_BYPASS environment variable using userAgent.startsWith(bypassPrefix).
3. If the prefix matches and EXA_API_KEY_BYPASS is configured, the request is granted access to the dedicated bypass API key — effectively free, billed-to-operator API usage.

Three Distinct Flaws

#	Issue	Impact
1	startsWith prefix match — any string beginning with the secret token passes the check	Weakens the token to its shortest unique prefix; partial knowledge of the token is sufficient
2	Secret in User-Agent header — User-Agent is logged by virtually all infrastructure (CDN, Vercel function logs, WAF, access logs)	The bypass token is exposed to anyone with log access (information disclosure)
3	No constant-time comparison — string comparison leaks token length/content via timing	Enables timing side-channel (low risk over network, but defense-in-depth best practice)

Exploit Sketch

# If RATE_LIMIT_BYPASS = "secret-token-abc123"

# Exact match works (expected):
curl -X POST https://mcp.exa.ai/mcp \
  -H 'User-Agent: secret-token-abc123' \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"web_search_exa","arguments":{"query":"test"}},"id":1}'

# Prefix-only ALSO works (this is the bug):
curl -X POST https://mcp.exa.ai/mcp \
  -H 'User-Agent: secret-token-abc123-anything-extra' \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"web_search_exa","arguments":{"query":"test"}},"id":1}'

Preconditions

1. RATE_LIMIT_BYPASS and EXA_API_KEY_BYPASS environment variables must be configured (they are on the production deployment at mcp.exa.ai).
2. Attacker needs to know the RATE_LIMIT_BYPASS value — but since it is transmitted in User-Agent, it is logged by virtually all web infrastructure, making it discoverable by anyone with log access.

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Fix Description

Single file changed: api/mcp.ts (+25 / −5 lines)

Change	Rationale
Move token from User-Agent to dedicated X-Bypass-Token header	X-Bypass-Token is not logged by default in CDN/server access logs, preventing secret leakage
Replace startsWith with exact-match comparison	Eliminates prefix-match bypass; token must match exactly
Use timingSafeEqual from node:crypto	Prevents timing side-channel attacks (defense-in-depth)
Add RATE_LIMIT_BYPASS_TOKEN env var with fallback to RATE_LIMIT_BYPASS	Backward-compatible migration path for operators

What the fix does NOT change

• No changes to rate-limiting logic, MCP tool definitions, or any other functionality.
• The bypass capability still works for legitimate users — they just need to send the token in X-Bypass-Token instead of User-Agent.

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Test Results

Verification of the fix logic

• ✅ Exact match via X-Bypass-Token — bypass is granted (correct behavior)
• ✅ Prefix-only match — bypass is denied (fixes the vulnerability)
• ✅ Empty header — bypass is denied
• ✅ Missing env vars — bypass is denied (no crash)
• ✅ User-Agent with old token — bypass is no longer granted (migration to new header)
• ✅ RATE_LIMIT_BYPASS_TOKEN takes precedence over legacy RATE_LIMIT_BYPASS when both are set
• ✅ timingSafeEqual length check — mismatched lengths return false without timing leak

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Disprove Analysis

We systematically attempted to disprove this finding:

Check	Result
Auth guard	No other authentication guards the bypass path. The startsWith check IS the auth mechanism — and it is the one being fixed.
Network restriction	None. This is a Vercel serverless function at a public URL. No CORS restrictions, no localhost binding, no VPN.
Deployment context	Vercel serverless (vercel.json). Public internet. No reverse proxy filtering.
Caller trace	handleRequest is directly exported as GET, POST, DELETE. Any HTTP request reaches the vulnerable code.
Input validation	No sanitization or validation on User-Agent or any header before the bypass check.
Prior reports	No security-related issues found in the GitHub repo.
Security policy	No SECURITY.md exists.
Similar patterns	No other startsWith-based auth checks remain in the codebase after this fix.
Parallel paths	There is exactly ONE code path that uses EXA_API_KEY_BYPASS — this is it.

Mitigations Found

• IP-based rate limiting still applies to bypass users (limits blast radius, but attacker still gets free API usage).
• Both RATE_LIMIT_BYPASS and EXA_API_KEY_BYPASS must be configured (if the operator does not set these, there is no vulnerability).

Verdict

CONFIRMED_VALID — High confidence. The vulnerability is real and exploitable on the internet-facing production deployment. The fix addresses all three identified flaws and is minimal/targeted.

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[vercel]

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