MCP TypeScript SDK: OAuth client could send credentials to an authorization server chosen by the MCP server

Summary

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 provider

Am I affected?

Yes, if both of these hold:

• your application uses the SDK's OAuth client over HTTP, through any of:
  • an authProvider on a transport
  • the withOAuth() middleware
  • direct calls to auth() or fetchToken()
• it may connect to an MCP server you do not fully trust while holding credentials for a legitimate authorization server

Affected versions:

• @modelcontextprotocol/sdk 1.12.0 through 1.30.1
• @modelcontextprotocol/client 2.0.0 through 2.1.0, only for:
  • bundled providers without expectedIssuer
  • credentials stored or supplied without issuer
  • direct calls to fetchToken()
  • providers that read storage back through OAuthTokensSchema or OAuthClientInformationSchema

Not affected:

• MCP servers built with the SDK
• stdio clients

Fix

Upgrade to:

• 1.x: 1.31.0 or later