feat: add DNS rebinding protection conformance tests#115
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Add a new server conformance scenario to verify that localhost MCP servers properly validate Host headers to prevent DNS rebinding attacks. Checks: - localhost-host-rebinding-rejected: Verifies server returns 403 for non-localhost Host headers (e.g., evil.example.com) - localhost-host-valid-accepted: Verifies server accepts requests with valid localhost Host headers Also updates the everything-server example to use createMcpExpressApp() which includes DNS rebinding protection by default. Closes #103 Co-Authored-By: Claude <noreply@anthropic.com>
Add a minimal MCP server that intentionally omits DNS rebinding protection to serve as a negative test case. This server is expected to FAIL the dns-rebinding-protection scenario. Also update README to document the negative test case. Co-Authored-By: Claude <noreply@anthropic.com>
Update the DNS rebinding protection test to accept any 4xx status code as a valid rejection response, not just 403. This accommodates different HTTP semantics: - 403 Forbidden: Per MCP spec - 421 Misdirected Request: RFC 7540 (semantically correct for DNS rebinding) - Other 4xx: Implementation-specific error codes
commit: |
pcarleton
commented
Jan 22, 2026
| EventId, | ||
| StreamId | ||
| } from '@modelcontextprotocol/sdk/server/streamableHttp.js'; | ||
| import { createMcpExpressApp } from '@modelcontextprotocol/sdk/server/express.js'; |
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note: this example is going to get deleted in a follow-up PR to make the typescript-sdk example the source of truth. updating it in place for now.
- Simplify negative test server (remove event store, sessions) - Update spec reference URL to security_best_practices - Clarify scope: localhost servers without HTTPS/auth - DRY up checks.push using spread operator - Fix valid host check to require 2xx response
maxisbey
requested changes
Jan 22, 2026
…tion - Send both Host and Origin headers in test requests so servers checking either header will pass the conformance test - Add second spec reference for Origin check (transports#security-warning) - Update descriptions to mention both Host and Origin headers - Fix protocol version to 2025-11-25 - Remove unnecessary 'as const' type assertions
maxisbey
reviewed
Jan 22, 2026
Comment on lines
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| hostHeader: validHost, | ||
| originHeader: `http://${validHost}`, |
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Maybe we should actually be testing these separately? Otherwise we don't know whether the server will reject both a bad host and a bad origin, we only know that it will reject either:
- both
- only header
- only origin
but not which specifically
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that's intentional, we want either protection to be valid. Checking host or origin will prevent the attack.
maxisbey
approved these changes
Jan 22, 2026
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Summary
Add conformance tests to verify localhost MCP servers properly validate Host headers to prevent DNS rebinding attacks.
Closes #103
Changes
New scenario:
dns-rebinding-protectionTwo checks:
localhost-host-rebinding-rejected: Server returns 4xx for non-localhost Host headers (e.g.,Host: evil.example.com)localhost-host-valid-accepted: Server accepts valid localhost Host headersImplementation details
undicifor making HTTP requests with custom Host headers (native fetch doesn't allow this)Additional changes
everything-serverexample to usecreateMcpExpressApp()for DNS rebinding protectionno-dns-rebinding-protection.tsthat intentionally omits protectionTesting
Verified against:
createMcpExpressApp())Related