FastMCP MCP client wrappers construct M2M OAuth providers without issuer binding — client secret redirectable to attacker-advertised authorization server

Maintainer assessment and remediation

This report is accepted as a FastMCP credential-binding vulnerability. Affected versions of the OAuth client credentials providers do not expose the authorization-server issuer that owns pre-registered credentials. An untrusted or compromised MCP server can therefore advertise a different authorization server and receive the client secret or a signed assertion. Signing keys themselves remain local; assertion replayability depends on the assertion source and audience policy.

Affected stable versions are 4.0.0–4.1.1 of fastmcp and fastmcp-slim. The wrappers were introduced in 4.0.0a1; 3.x does not include them.

The 4.1.2 fix adds issuer to ClientCredentialsOAuthProvider and PrivateKeyJWTOAuthProvider and requires MCP SDK 2.2.0. It also discards refresh credentials from reloaded tokens: without persisted authorization-server metadata, inherited refresh could otherwise transmit credentials before issuer discovery, including when a cached token was valid at startup and expired later. Cached tokens are isolated by configured issuer, so configuring or changing the issuer acquires a new token. Valid cached access tokens for the same issuer and their expiry remain intact.

Configuration required for protection: configure issuer with the issuer identifier of the authorization server that issued the credentials. Upgrading alone is insufficient: omission remains supported for compatibility and emits MCPDeprecationWarning. Rotate client secrets or other reusable credentials that may already have been sent to an untrusted endpoint. The fix is available in FastMCP 4.1.2.

from fastmcp.client.auth import ClientCredentialsOAuthProvider

auth = ClientCredentialsOAuthProvider(
    client_id="my-client-id",
    client_secret="my-client-secret",
    issuer="https://auth.example.com",
)