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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.2 · 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 · score source

EPSS

• CVE-2026-27124: 0.21% estimated exploitation activity in the next 30 days; percentile 11.38 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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. Technical Details An adversary can initiate an authentication flow by connecting their malicious MCP client to a benign MCP server using the GitHubProvider OAuth integration. During this flow, the attacker consents to connect their client to the MCP server and, at that point, can capture the GitHub authorization URL they are redirected to after granting consent. The attacker can then lure a victim, who is already logged into GitHub and has previously connected an MCP client to the benign MCP server, to open this captured URL. As a result,…

Published by the source

31 Mar 2026, 22:32 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:40 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-rww4-4w9c-7733

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-rww4-4w9c-7733

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# Patch Next advisory brief: CVE-2026-27124

Advisory: FastMCP: Missing Consent Verification in OAuth Proxy Callback Facilitates Confused Deputy Vulnerabilities
IDs: CVE-2026-27124, GHSA-rww4-4w9c-7733
Record: https://patchnext.com/advisories/cve-2026-27124 (revision 3, source checked 26 Oct 2026, 13:40 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fastmcp 3.2.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI fastmcp: affected < 3.2.0; fixed in 3.2.0

## Suggested inspection (read-only)
- `python -m pip show fastmcp`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on fastmcp.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-rww4-4w9c-7733 (OSV record GHSA-rww4-4w9c-7733 lists affected ranges: PyPI fastmcp >=0 <3.2.0 (fixed))
- Verified: https://github.com/advisories/GHSA-rww4-4w9c-7733 (GitHub-reviewed advisory GHSA-rww4-4w9c-7733 lists affected ranges: PyPI fastmcp >=0 <3.2.0)

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