CVE-2025-69196: FastMCP OAuth Proxy token reuse across MCP servers

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:

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