Advisories for Npm/@Modelcontextprotocol/Client package

In affected versions, the SDK's HTTP client transports and OAuth requests followed redirects to any origin. If an MCP server or authorization server redirected a request to another origin, that origin received custom request headers (such as an X-API-Key set in requestInit.headers) and the mcp-session-id header. On a 307 or 308 it also received the request body, including OAuth token requests with their refresh_token and client_secret. On current runtimes fetch …

In affected versions, the SDK's OAuth client let the MCP server decide which authorization server received the client's OAuth credentials. Credentials were not tied to the authorization server they belong to. A malicious or compromised MCP server could name its own authorization server. With no user interaction, the client would send it: the refresh_token and client_secret stored from an earlier sign-in the client_secret or signed assertion configured on a bundled …