CVE-2026-27124 FastMCP: Missing Consent Verification in OAuth Proxy Callback Facilitates Confused Deputy Vulnerabilities
Attackers can bypass OAuth consent checks in FastMCP, leading to confused deputy attacks and unauthorized account access.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-27124?
CVE-2026-27124 is a high-severity authentication bypass vulnerability in jlowin fastmcp, published on 3 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-441
Which versions are affected by CVE-2026-27124?
3 open source packages ship an affected version, including fastmcp. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
fastmcp PyPI | All versions | 3.2.0 |
fastmcp PyPI | All versions | 3.2.0 |
fastmcp PyPI | All versions | 3.2.0 |
Products include Jlowin Fastmcp.
How do I fix CVE-2026-27124?
Upgrade fastmcp to 3.2.0 or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-27124 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-27124 exists.
Other vulnerabilities in jlowin fastmcp
- CVE-2026-32871 FastMCP OpenAPI Provider has an SSRF & Path Traversal Vulnerability
- CVE-2025-69196 FastMCP OAuth Proxy token reuse across MCP servers
- CVE-2025-64340 FastMCP has a Command Injection vulnerability - Gemini CLI
- CVE-2025-62801 FastMCP vulnerable to windows command injection in FastMCP Cursor installer via server_name
- CVE-2025-62800 FastMCP vulnerable to reflected XSS in client's callback page
Sources
Also known as ECHO-86a3-9d3c-b622, GHSA-rww4-4w9c-7733, PYSEC-2026-2476, ROOT-APP-PYPI-CVE-2026-27124.