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.
Published · last modified .
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.
EPSS exploit prediction
EPSS: 0.003 (20.9th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 21st percentile — 20.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.003 · change: +0.001
Affected products
- Jlowin Fastmcp — versions < 3.2.0
Weakness classification (CWE)
References
- github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733 (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.