What is CVE-2026-27124?

CVE-2026-27124 is a medium-severity vulnerability in Jlowin Fastmcp, classified under Unintended Proxy or Intermediary (Confused Deputy). CVSS score: 6.1/10. Published 2026-04-03.

How severe is CVE-2026-27124?

Medium severity. CVSS v3 base score is 6.1 out of 10.

Vulnerability in Jlowin Fastmcp

CVE-2026-27124

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.

EPSS: 0.002 (11.0th percentile) — read the EPSS interpretation.

CVSS v3 metric

CVSS v3 base score 6.1 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.

Affected products

Jlowin Fastmcp — versions < 3.2.0

Weakness classification (CWE)

CWE-441 — Unintended Proxy or Intermediary (Confused Deputy)

References

security-advisories@github.com (x_refsource_CONFIRM, Exploit, Mitigation, Vendor Advisory)

Frequently asked questions

What is CVE-2026-27124?: CVE-2026-27124 is a medium-severity vulnerability in Jlowin Fastmcp, classified under Unintended Proxy or Intermediary (Confused Deputy). CVSS score: 6.1/10. Published 2026-04-03.
How severe is CVE-2026-27124?: Medium severity. CVSS v3 base score is 6.1 out of 10.