GitHub Security Advisory: GHSA-c2jp-c369-7pvx
Package: fastmcp (PIP)
Severity: HIGH (CVSS 0.0)
CWE: CWE-287: Improper Authentication
Vulnerable versions: < 2.13.0
Patched in: 2.13.0

Summary: FastMCP Auth Integration Allows for Confused Deputy Account Takeover

Details:
### Summary

FastMCP documentation [covers the scenario](https://gofastmcp.com/integrations/azure) where it is possible to use Entra ID or other providers for authentication. In this context, because Entra ID does not support Dynamic Client Registration (DCR), the FastMCP-hosted MCP server is acting as the authorization provider, as declared in the Protected Resource Metadata (PRM) document hosted on the server.

For example, on a local MCP server, it may be hosted here:

```http
http://localhost:8000/.well-known/oauth-protected-resource
```

And the JSON representation of the PRM document:

```json
{
  "resource": "http://localhost:8000/mcp",
  "authorization_servers": [
    "http://localhost:8000/"
  ],
  "scopes_supported": [
    "User.Read",
    "email",
    "openid",
    "profile"
  ],
  "bearer_methods_supported": [
    "header"
  ]
}
```

Notice that the `authorization_servers` field contains the MCP server itself - it acts as an **OAuth Client** to the downstream authorization server (e.g., Entra ID) and as a **Authorization Server** (AS) to the MCP client.

The FastMCP server also hosts the AS metadata:

```bash
http://localhost:8000/.well-known/oauth-authorization-server
```

With the following content:

```json
{
  "issuer": "http://localhost:8000/",
  "authorization_endpoint": "http://localhost:8000/authorize",
  "token_endpoint": "http://localhost:8000/token",
  "registration_endpoint": "http://localhost:8000/register",
  "scopes_supported": [
    "User.Read",
    "email",
    "openid",
    "profile"
  ],
  "response_types_supported": [
    "code"
  ],
  "grant_types_supported": [
    "authorization_code",
    "refresh_token"
  ],
  "token_endpoint_auth_methods_supported": [
    "client_secret_post"
  ],
  "code_challenge_methods_supported": [
    "S256"
  ]
}
```

All of this confirms that the FastMCP server is, in fact, handling the client-to-server authorization and then delegating the downstream effects (i.e., authorization with Entra ID) to its own redirect logic, with a call like this (as seen through MCP Inspector):

```http
http://localhost:8000/authorize?response_type=code&client_id=fdec0bb8-3423-40d0-aa2a-73de26bf6f93&code_challenge=2a9ZxAEr5NEsKPwFWuEFA1W-kFMXc-02u6qc8aLf_g4&code_challenge_method=S256&redirect_uri=http%3A%2F%2Flocalhost%3A6274%2Foauth%2Fcallback%2Fdebug&state=9f23fd47e2b8786b502f116bdbfd6ae3d7d2801167e24fea82f608bb52312bbd&scope=User.Read+email+openid+profile&resource=http%3A%2F%2Flocalhost%3A8000%2Fmcp
```

When using the built-in FastMCP `/authorize` endpoint, and in the example above, FastMCP server configured with Entra ID, it will then redirect the user here:

```http
https://login.microsoftonline.com/412e93fe-74e5-4ee6-9b67-1eeb1c79550e/oauth2/v2.0/authorize?response_type=code&client_id=7bac43f2-ca62-4148-93a5-fd5686cb16c0&redirect_uri=http%3A%2F%2Flocalhost%3A8000%2Fauth%2Fcallback&state=Tcv7bbg_v0Qi69RHbCzqR4tQHSHKPQuDDxjuo0wu5qU&scope=User.Read+email+openid+profile&code_challenge=bxICFAJDViuTTHIPU