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

published 2026-03-31 7 mo ago modified 2026-09-10 1 mo ago also EUVD-2026-18758 , GHSA-rww4-4w9c-7733 , PYSEC-2026-2476

cvss: 6.1; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 0.2%; EPSS 0.2% chance of exploitation in 30 days, percentile 11%
public exploits: 0; none known

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.

Patch / fix

fixed in 2

pypi fastmcp 3.2.0
jlowin fastmcp 3.2.0

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-27124 updated 2026-10-27 cvss 6.5

FastMCP: FastMCP OAuthProxy: FastMCP OAuthProxy: Unauthorized actions due to improper consent validation in GitHub OAuth

Red Hat Developer Hub 1 rhdh/rhdh-hub-rhel9 Red Hat OpenShift AI (RHOAI) rhoai/odh-mlflow-rhel9/odh-mlflow-rhel9, rhoai/odh-pipeline-runtime-minimal-cpu-py312-rhel9/odh-pipeline-runtime-minimal-cpu-py312-rhel9, rhoai/odh-pipeline-runtime-pytorch-cuda-py312-rhel9/odh-pipeline-runtime-pytorch-cuda-py312-rhel9, rhoai/odh-pipeline-runtime-pytorch-rocm-py312-rhel9/odh-pipeline-runtime-pytorch-rocm-py312-rhel9, rhoai/odh-workbench-jupyter-minimal-cpu-py312-rhel9/odh-workbench-jupyter-minimal-cpu-py312-rhel9, rhoai/odh-workbench-jupyter-minimal-cuda-py312-rhel9/odh-workbench-jupyter-minimal-cuda-py312-rhel9, +13 more Red Hat Satellite 6 satellite/foreman-mcp-server-rhel9
affected: workaround, no fix available “Fix deferred”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation: Proof of concept is public
Automatable: yes
Technical impact: partial
Assessed: 7 mo ago (2026-04-03)

Exploit prediction

0.21% chance of exploitation in the next 30 days, higher than 11% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

github.com https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by	version	score	vector
CNA	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
CNA	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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
NVD	3.1	6.1	CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Affected packages

ecosystem	package	affected	fixed in
pypi	fastmcp	<3.2.0	3.2.0

Affected products

jlowin fastmcp

Classification

Weakness

CWE-441 Unintended Proxy or Intermediary

References

github.com /PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733
exploit, web, mitigation, vendor-advisory
github.com /jlowin/fastmcp/security/advisories/GHSA-rww4-4w9c-7733
web
nvd.nist.gov /vuln/detail/CVE-2026-27124
advisory
github.com /PrefectHQ/fastmcp
package
pypi.org /project/fastmcp
package
github.com /advisories/GHSA-rww4-4w9c-7733
advisory

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.

Summary