fix(auth): validate redirect_uri schemes at DCR registration (RFC 9700 §4.1.1, RFC 7591 §2)

[CrypticCortex]

Closes #2629.

Summary

Adds validate_registered_redirect_uri(url: AnyUrl) and wires it into RegistrationHandler.handle so each submitted redirect_uri is validated immediately after model_validate_json succeeds. On failure, returns 400 invalid_redirect_uri per RFC 7591 §3.2.2.

The helper mirrors the existing validate_issuer_url policy:

• https is allowed.
• http is allowed only for loopback hosts (localhost, 127.0.0.1, [::1]) per RFC 8252 §7.3.
• Fragments are rejected per RFC 7591 §2 — including empty trailing-# fragments.
• Query strings are permitted (explicitly allowed by RFC 7591 §2).

The helper lives in a new mcp.server.auth._redirect_uri module to avoid a circular import with handlers.register (the existing mcp.server.auth.routes re-exports it so the public import path stays stable).

Why now

OAuthClientMetadata.redirect_uris is typed list[AnyUrl], which Pydantic accepts for any well-formed URL — including javascript:, data:, file:, vbscript:, ftp:, cleartext http:// to non-loopback hosts, and https://...#. The handler stores those values verbatim and authorize-time validation accepts them by exact match. Issue #<ISSUE_NUM> has the full reproduction.

Tests

tests/server/auth/test_routes.py: 13 unit tests covering each accept/reject path of the new helper, mirroring the existing test_validate_issuer_url_* cases (includes an explicit empty-fragment test).

tests/server/auth/test_error_handling.py: 5 end-to-end tests posting against /register — three rejection cases (javascript:, cleartext-http://, fragment) and two acceptance cases (HTTPS, loopback HTTP).

Local: ./scripts/test green; 1190 passed, 100.00% coverage, strict-no-cover clean.

Compatibility

No public-API removal. The change tightens what /register accepts; clients submitting non-spec-compliant URIs will now get 400 invalid_redirect_uri where they previously got 201. Existing stored client records are not re-validated.

Related

• Adjacent to Fix RFC 8252 Section 7.3 compliance for loopback redirect URIs #1934 (RFC 8252 loopback port relaxation at authorize-time) — no conflict; touches different code paths.
• Question raised earlier in Question: MCP Python SDK Redirect URI Validation - Implementation Granularity? #1446 (closed as duplicate).

Checks

• uv run --frozen ruff format .
• uv run --frozen ruff check .
• uv run --frozen pyright
• ./scripts/test — green, 100% coverage, strict-no-cover clean