GHSA-vqqp-x5qm-5qp4: FastMCP MCP client wrappers construct M2M OAuth providers without issuer binding — client secret redirectable to attacker-advertised authorization server

fastmcp’s ClientCredentialsOAuthProvider and PrivateKeyJWTOAuthProvider do not expose or validate OAuth issuer binding when connecting to MCP servers. A client secret configured for a legitimate authorization server can be transmitted to an attacker-controlled authorization server if the MCP server URL is attacker-supplied or compromised. This is the same vulnerability class as GHSA-qx49-fqc8-xw99 (Python MCP SDK) and GHSA-6qxp-vccf-f47h (TypeScript MCP SDK), applied to fastmcp’s wrapper layer.