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CVE-2026-27124

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

Published: Apr 3, 2026Updated: Apr 22, 2026 Sources: CVE List NVDCWE-441
NVD MITRE
6.1CVSSMEDIUM
EPSS Score
0.2%
Top 89.3%
CISA KEV
Not in KEV
Exploits
No Known Exploits
Remediation
Workaround Only

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.

CVSS Vector Breakdown

AV:NAC:LPR:NUI:RS:CC:LI:LA:N
Exploitability
AV:NAttack Vector
Network
AC:LAttack Complexity
Low
PR:NPrivileges Required
None
UI:RUser Interaction
Required
Scope
S:CScope
Changed
Impact
C:LConfidentiality
Low
I:LIntegrity
Low
A:NAvailability
None
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Weaknesses

CWE-441

Affected Products

fastmcpjlowin
Library
AI / ML / ai-agent-tooling
jlowinindividual-devAI / MLaka fastmcp

Exploitability

Workaround Available

References

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

Timeline

Published
Apr 3, 2026
Last Updated
Apr 22, 2026

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