Official MCP Python SDK Flaw Can Let Malicious Servers Steal OAuth Credentials

A security vulnerability in the official MCP Python SDK allows malicious MCP servers to steal OAuth credentials from client applications. The flaw causes affected versions of the SDK to send sensitive authentication data—including the client secret, authorization code, and PKCE proof key—to an attacker-controlled token endpoint. This means any application built on the vulnerable SDK could be tricked into surrendering its OAuth credentials to a malicious server. The issue was disclosed by the SDK's maintainers via a security advisory. The vulnerability has been patched in version 1.30.0 and later. Developers using the MCP Python SDK are urged to upgrade immediately to prevent credential theft.