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.

Metrics

Attack Vector Network

Attack Complexity Low

Privileges Required None

Attack Requirements Present

User Interaction None

Vulnerable System Confidentiality Impact None

Vulnerable System Integrity Impact High

Vulnerable System Availability Impact None

Subsequent System Confidentiality Impact None

Subsequent System Integrity Impact None

Subsequent System Availability Impact None

No CVSS v3.1

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

Exploitation poc

Automatable yes

Technical Impact partial

Affected Vendors & Products

Vendors Products
Jlowin
  • Fastmcp

No data.

No data.

References
Link Providers
https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733 cve-icon cve-icon cve-icon cve-icon
https://nvd.nist.gov/vuln/detail/CVE-2026-27124 cve-icon
https://www.cve.org/CVERecord?id=CVE-2026-27124 cve-icon
History

Tue, 07 Apr 2026 00:00:00 +0000

Type Values Removed Values Added
Weaknesses CWE-303
References
Metrics threat_severity

None

 cvssV3_1

{'score': 6.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N'}

threat_severity

Moderate

Fri, 03 Apr 2026 21:30:00 +0000

Type Values Removed Values Added
First Time appeared Jlowin
Jlowin fastmcp
Vendors & Products Jlowin
Jlowin fastmcp

Fri, 03 Apr 2026 17:45:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'poc', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Fri, 03 Apr 2026 16:30:00 +0000

Type Values Removed Values Added
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.
Title FastMCP: Missing Consent Verification in OAuth Proxy Callback Facilitates Confused Deputy Vulnerabilities
Weaknesses CWE-441
References
Metrics cvssV4_0

{'score': 8.2, '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'}
cve-icon MITRE

Status: PUBLISHED

Assigner: GitHub_M

Published: 2026-04-03T15:22:17.028Z

Updated: 2026-04-03T16:44:20.749Z

Reserved: 2026-02-17T18:42:27.043Z

Link: CVE-2026-27124

cve-icon Vulnrichment

Updated: 2026-04-03T16:44:08.349Z

cve-icon NVD

Status : Received

Published: 2026-04-03T16:16:36.453

Modified: 2026-04-03T17:16:41.970

Link: CVE-2026-27124

cve-icon Redhat

Severity : Moderate

Publid Date: 2026-04-03T15:22:17Z

Links: CVE-2026-27124 - Bugzilla