MCP TypeScript SDK is the official TypeScript SDK for Model Context Protocol servers and clients. Starting in version 1.12.0 and prior to versions 1.31.0 and 2.2.0, the SDK's OAuth client support let the MCP server a client connected to decide which authorization server received the client's OAuth credentials. Stored and pre-provisioned credentials were not bound to the authorization server they belong to. A malicious or compromised MCP server could name its own authorization server in its protected resource metadata. Without any user interaction, the client would send that server the "refresh_token" and "client_secret" stored from an earlier sign-in (1.x), or the configured "client_secret" or signed assertion of a bundled non-interactive provider (1.x and 2.x). Only those applications that use the SDK as an MCP client over HTTP with an "authProvider": your own "OAuthClientProvider", or the bundled "ClientCredentialsProvider", "PrivateKeyJwtProvider", "StaticPrivateKeyJwtProvider" or (2.x) "CrossAppAccessProvider" and that may connect to an MCP server the owners does not fully trust while holding credentials for a legitimate authorization server are affected. "@modelcontextprotocol/sdk" 1.31.0 (1.x) and "@modelcontextprotocol/client" 2.2.0 (2.x) patch the issue. A workaround for those who cannot upgrade is available. 2.0.0 and 2.1.0 already accept "expectedIssuer". On 1.x, the only workaround is to connect OAuth-enabled clients only to MCP servers you trust.