FastMCP OAuth credential binding vulnerability in versions 4.0.0-4.1.1

FastMCP's OAuth client credential providers lacked issuer binding, allowing untrusted MCP servers to redirect client secrets to attacker-controlled authorization servers. The vulnerability affects fastmcp and fastmcp-slim versions 4.0.0 through 4.1.1 and is fixed in version 4.1.2.

Disclosed 23 October 2026 · Record updated 23 October 2026

Impact

Client credentials could be transmitted to attacker-controlled authorization servers due to missing issuer validation in OAuth provider configuration.

Our coverage

A vault door swings open as a rogue server's hand reaches through, siphoning glowing credential tokens into a shadowy funnel while legitimate machines stand helpless nearby.

Vulnerabilities

FastMCP flaw could leak OAuth secrets to rogue MCP servers

A credential-binding bug in FastMCP's machine-to-machine OAuth providers let untrusted servers redirect client secrets to an attacker's authorisation server.

23 Oct 2026

Sources

github.comhttps://github.com/advisories/GHSA-vqqp-x5qm-5qp4