OAuth HTTP routes set `CORSMiddleware(allow_origins="*")` on token/register/metadata endpoints |

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#3,563 opened on Sep 22, 2026 0 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty: 3/5
Estimated time: 1-2 days
Newbie friendliness: 55/100

Issue type: Bug
Clarity: Clearly specified
Activity status: Active
Tech stack: python
Domain: api, backend, security

Research direction

The issue is in src/mcp/server/auth/routes.py, focusing on the _cors function and its usage. First, understand how OAuth endpoints are configured and where TransportSecuritySettings.allowed_origins is defined. The task is to modify the CORS configuration for specific endpoints, ensuring the metadata handlers keep the wildcard while sensitive endpoints use an explicit allowlist. Run existing tests related to authentication and CORS to verify changes.

Written by the indexing model from the issue text.

Description

v1 v2

Static review of public source at commit 6affe5c0d358. No traffic was sent to any MCP environment.

OAuth route helpers wrap several endpoints with Starlette CORSMiddleware and a wildcard origin:

src/mcp/server/auth/routes.py:

def _cors(app: ASGIApp, allow_methods: list[str]) -> ASGIApp:
    return CORSMiddleware(
        app=app,
        allow_origins="*",
        allow_methods=allow_methods,
        allow_headers=[MCP_PROTOCOL_VERSION_HEADER],
    )

That wrapper is used for /.well-known/oauth-authorization-server, /token, and (when enabled) /register and /revoke. Wildcard ACAO is intentional for browser-based inspectors, and allow_credentials is not set here (browsers will not attach cookies under the * + credentials rule). Still, token and dynamic-registration endpoints are sensitive: a wildcard makes any future “reflect Origin + credentials” change, or a non-browser client that ignores CORS, easier to misuse.

Suggested change:

Keep * only on public metadata handlers.
For /token, /register, and /revoke, take an explicit allowed_origins list (or reuse TransportSecuritySettings.allowed_origins) and document the Inspector origin as an example allowlist entry.
Add a one-line comment that credentials must stay off while * remains.

Severity: low / configuration clarity and defense-in-depth. Not reported as an exploitable CORS bypass with the current header pair. No proof-of-concept.

Happy to send a focused PR if useful.

Dominant language: Python
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Forks: 4k
Avg merge: 1d 11h
Merged PRs (30d): 30

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