Is CVE-2026-27124 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-27124 being exploited in the wild.

How severe is CVE-2026-27124?

Medium severity, with a CVSS score of 6.1, published 3 April 2026.

Which products and versions are affected?

fastmcp (PyPI) ECOSYSTEM:introduced=0,fixed=3.2.0.

Is there a fix for CVE-2026-27124?

Yes. fastmcp (PyPI): upgrade to 3.2.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-27124 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-27124?

Check whether you run fastmcp (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-27124: FastMCP confused deputy vulnerability in OAuthProxy callback

CVE-2026-27124 · Severity: medium · CVSS 6.1 · Published 2026-04-03

FastMCP is a framework used to build Model Context Protocol (MCP) applications, which often integrate with external services like GitHub. A security flaw in its authentication proxy allows an attacker to trick a victim into unintentionally granting the attacker access to the victim's account. If exploited, an attacker could gain unauthorized access to a user's private data or resources on connected platforms like GitHub without the user's knowledge.

Technologies: fastmcp (PyPI). Vendors: PyPI.

Quick answers

Is CVE-2026-27124 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-27124 being exploited in the wild.
How severe is CVE-2026-27124?: Medium severity, with a CVSS score of 6.1, published 3 April 2026.
Which products and versions are affected?: fastmcp (PyPI) ECOSYSTEM:introduced=0,fixed=3.2.0.
Is there a fix for CVE-2026-27124?: Yes. fastmcp (PyPI): upgrade to 3.2.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-27124 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-27124?: Check whether you run fastmcp (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

fastmcp (PyPI): upgrade to 3.2.0 or later
```
pip install "fastmcp>=3.2.0"
```

Technical details

A 'Confused Deputy' vulnerability exists in the FastMCP OAuthProxy component due to insufficient validation of the user's consent state during the Identity Provider (IdP) callback. Specifically, the `OAuthProxy._handle_idp_callback` function does not verify that the browser submitting the authorization code is the same browser that initiated the consent flow. An attacker can initiate an OAuth flow, capture the authorization URL, and lure a victim into clicking it. Because GitHub (and other IdPs) may skip the consent screen for previously authorized applications, the victim's browser automatically completes the callback, providing the attacker's client with a valid authorization code linked to the victim's account. This allows the attacker to exchange the code for an access token and impersonate the victim on the MCP server. The issue is addressed in version 3.2.0 by implementing browser-bound state validation.

Affected products

jlowin fastmcp < 3.2.0

Identifiers

CVE-2026-27124
GHSA-rww4-4w9c-7733
PYSEC-2026-2476

Timeline

2026-03-31: advisory: Vendor advisory published on GitHub
2026-04-03: disclosed: CVE published to NVD
2026-04-03: patched: Fix released in version 3.2.0

References

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

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