fix(transport-security): compare Host and Origin case-insensitively (#3437)

[amasen02]

Summary

Fixes #3437.

TransportSecurityMiddleware._validate_host() and _validate_origin() previously compared incoming Host and Origin headers against allowed_hosts and allowed_origins case-sensitively.

Per RFC 9110 §4.2.3 and RFC 6454 §4:

• "The scheme and host are case-insensitive and normally provided in lowercase; all other components are compared in a case-sensitive manner."

Every WHATWG URL-based HTTP client (browsers, Node fetch, undici, and mcp-remote) automatically lowercases hostnames before transmitting them. On Windows, where %COMPUTERNAME% is uppercase by default, configuring allowed_hosts=["MYHOST:*"] caused all incoming requests from standard fetch clients (e.g. Host: myhost:8000) to be rejected with 421 Misdirected Request.

Changes

• In src/mcp/server/transport_security.py:
  • _validate_host(): Lowercases both the incoming Host header and allowed_hosts items before exact and wildcard-port comparisons.
  • _validate_origin(): Lowercases both the incoming Origin header and allowed_origins items before exact and wildcard-port comparisons.
• In tests/server/test_transport_security.py:
  • Added test cases covering uppercase and mixed-case Host and Origin values against lowercase allowlists.
  • Added test_validate_request_case_insensitive_uppercase_settings asserting that uppercase allowlist configurations (such as Windows %COMPUTERNAME%) correctly permit lowercase client requests.

Verification

• Ran uv run pytest tests/server/test_transport_security.py: 31/31 passed in 0.11s.
• Checked with uv run ruff check and uv run ruff format --check: 100% clean.

[github-actions]

This PR has been closed automatically. This repo only keeps pull requests open when they come from a maintainer, or from a contributor a maintainer has assigned to the linked issue, and you aren't currently assigned to #3437.

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CONTRIBUTING.md has the full reasoning, but in short:

• We're a small team with very little capacity to review community PRs right now.
• Many recent PRs are AI-generated with little human review, and reviewing one carefully still costs a maintainer as much time as it ever did. A well-described issue is usually more useful to us than the code.

Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.