FastMCP — Vulnerability (CVE-2026-27124)
Date: 2026-04
Affected: jlowin/fastmcp
Attack vector: other
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-27124
CWE: CWE-441
CVSS: 6.1 (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N)

FastMCP is the standard framework for building MCP applications. Prior to version 3.2.0, while testing the GitHubProvider OAuth integration, which allows authentication to a FastMCP MCP server via a FastMCP OAuthProxy using GitHub OAuth, it was discovered that the FastMCP OAuthProxy does not properly validate the user's consent upon receiving the authorization code from GitHub. In combination with GitHub’s behavior of skipping the consent page for previously authorized clients, this introduces a Confused Deputy vulnerability. This issue has been patched in version 3.2.0.


Mitigations:

Architectural layers (MAESTRO):