While testing the OAuth Proxy implementation, it was noticed that the server does not properly respect the `resource` parameter submitted by the client in the authorization and token request. Instead of issuing the token explicitly for this MCP server, the token is issued for the `base_url` passed to the `OAuthProxy` during initialization.

**Affected File:**
*https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L828*

**Affected Code:**
```python
self._jwt_issuer: JWTIssuer = JWTIssuer(
issuer=str(self.base_url),
audience=f"{str(self.base_url).rstrip('/')}/mcp",
signing_key=jwt_signing_key,
)
```

Since the issued access and refresh tokens do not include information about the resource the token was issued for, it is impossible for the MCP server to properly verify whether the token was issued for it, hence violating the requirement of doing so demanded by the [specification](https://mcp.mintlify.app/specification/2025-11-25/basic/authorization#token-audience-binding-and-validation). Being able to verify whether the token was issued for the target MCP server enforces the protection offered by the, per MCP specification mandatory, Protected Resource Metadata OAuth extension.

Therefore, this misconfiguration exposes all MCP server setups using the FastMCP OAuth Proxy to an attack where an adversary creates a malicious MCP server that advertises the benign OAuth Proxy authorization server as its own authorization server. Once a victim completes an OAuth flow with this malicious MCP server, authenticating against the AS, the adversary can extract the token received at the malicious MCP server and use it to access other MCP servers (the benign ones) that also use the same AS, including the tools and resources they expose.

**Steps to reproduce:**
1. Extract the provided [PoC environment](https://github.com/user-attachments/files/23839983/improper_resource_validation_fastmcp.tgz).
2. Enter the *client_id* and *client_secret* of a GitHub App you control into the `mcp-server-proxy.py` script.
3. Start the benign MCP server using an OAuth Proxy (in this case the *GitHubProvider*): `python3 mcp-server-proxy.py`.
4. Start the malicious AS: `python3 mal_auth_server.py`.
5. Start the malicious MCP server: `python3 attacker_server.py`.
6. Connect the client to the malicious MCP server: `python3 client.py`.
7. Complete the OAuth flow.
8. Observe in the logs of the malicious MCP server that the request to the benign MCP server with the stolen token returned a 200 status code.

## Impact

This vulnerability allows an adversary to steal a victim’s authentication material for a benign MCP server using the FastMCP OAuth Proxy. The severity of this issue was decreased to _Medium_ due to the consent screen showing the name of the MCP server the OAuth Proxy was intended for. However, a victim might not see it or get otherwise convinced by the attacker to ignore it, and overall this does not act as a proper mitigation for this issue.

## Mitigation

To mitigate this vulnerability, it is recommended to issue tokens specifically for the MCP server submitted in the authorization URL’s `resource` GET parameter. In this way, the receiving MCP server will be able to properly verify that the token was indeed issued for it, allowing it to reject tokens stolen by an attack like the one demonstrated above.

CWE-863 Incorrect Authorization