CVE-2026-104850

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: @modelcontextprotocol/sdk 1.31.0 or later
• 2.x: @modelcontextprotocol/client 2.2.0 or later, and @modelcontextprotocol/core 2.2.0 or later if you import it directly

The client now records the authorization server as issuer on saved credentials and does not send them to a different one. The user signs in again, or the call throws.

In the following cases, upgrading to the patched version is not enough. You also need to make a change:

• Bundled providers (ClientCredentialsProvider, PrivateKeyJwtProvider, StaticPrivateKeyJwtProvider, CrossAppAccessProvider): pass expectedIssuer, for example expectedIssuer: 'https://auth.example.com'. Without it they still use whichever authorization server the MCP server names.
• Credentials saved without issuer: tokens and client information your OAuthClientProvider persisted (file, keychain, database) before upgrading, including everything 1.x saved before 1.31.0. They still go to whichever authorization server is named at first use. Add issuer to them, or clear them so users sign in again.
• Your own OAuthClientProvider: save exactly what saveTokens() and saveClientInformation() are given, including issuer. For pre-registered credentials, include issuer in what clientInformation() returns.

Not covered by this fix:

• a new interactive sign-in, which still goes to the authorization server the MCP server names. Only complete sign-ins for servers you trust.
• refreshAuthorization() and exchangeAuthorization() called directly
• 2.x with skipIssuerMetadataValidation: true

If an affected client may have connected to an untrusted MCP server, rotate its client secret or signing key and revoke its tokens.

If you cannot upgrade yet, connect OAuth clients only to MCP servers you trust. 2.0.0 and 2.1.0 already accept expectedIssuer.