CVE-2026-27124

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

State: Published
Severity: 8.2 High
Published: Apr 3, 2026
Updated: Apr 3, 2026
Reserved: Feb 17, 2026

Description

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.

Scores

CVSS	Score	Vector	Source
4.0	8.2 High	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	GitHub_M

Weaknesses

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

Affected Products

Product	Versions
fastmcpjlowin	< 3.2.0Affected

References

https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733

Every record here comes from the CVE Program’s CVE List. © 1999–2026 The MITRE Corporation. CVE is a registered trademark of The MITRE Corporation. The records are reproduced under the CVE Program terms of use.