feat: add DNS rebinding protection conformance tests (#115)

* feat: add DNS rebinding protection conformance tests

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>

* feat: add negative test case for DNS rebinding protection

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>

* fix: accept any 4xx status code for DNS rebinding rejection

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

* refactor: address PR review comments

- 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

* fix: include error details in catch block

* refactor: simplify negative test server to stateless

* refactor: check both Host and Origin headers for DNS rebinding protection

- 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

---------

Co-authored-by: Claude <noreply@anthropic.com>

package-lock.json

@@ -14,6 +14,7 @@
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         "jose": "^6.1.2",
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         "yaml": "^2.8.2",
         "zod": "^3.25.76"
       },
@@ -4926,6 +4927,15 @@
         "url": "https://github.com/sponsors/antfu"
       }
     },
+    "node_modules/undici": {
+      "version": "7.19.0",
+      "resolved": "https://registry.npmjs.org/undici/-/undici-7.19.0.tgz",
+      "integrity": "sha512-Heho1hJD81YChi+uS2RkSjcVO+EQLmLSyUlHyp7Y/wFbxQaGb4WXVKD073JytrjXJVkSZVzoE2MCSOKugFGtOQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=20.18.1"
+      }
+    },
     "node_modules/undici-types": {
       "version": "6.21.0",
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