CVE-2026-27124CWE-441

FastMCP: Missing Consent Verification in OAuth Proxy Callback Facilitates Confused Deputy Vulnerabilities

High · published April 3, 2026

CVSS v4.0
8.2
EPSS
0%
Percentile
22.1
In the wild
Unconfirmed
What it is

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.

The record
Technical detail
CVSS v4.0
8.2 · HIGH
Vector
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
EPSS
0.00305 · 22.1th percentile
Weakness
CWE-441 · Unintended Proxy or Intermediary ('Confused Deputy')
Published
2026-04-03T15:22Z
EPSS history
Timeline
  • 03 APR 15:22Z
    FastMCP: Missing Consent Verification in OAuth Proxy Callback Facilitates Confused Deputy Vulnerabilities
    cvelistv5