Unintended Proxy or Intermediary ('Confused Deputy') Affecting fastmcp package, versions [,3.2.0)
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-PYTHON-FASTMCP-15871030
- published1 Apr 2026
- disclosed31 Mar 2026
- creditan7y
Introduced: 31 Mar 2026
CVE-2026-27124 (opens in a new tab)How to fix?
Upgrade fastmcp to version 3.2.0 or higher.
Overview
fastmcp is a The fast, Pythonic way to build MCP servers and clients.
Affected versions of this package are vulnerable to Unintended Proxy or Intermediary ('Confused Deputy') in the OAuthProxy._handle_idp_callback function. An attacker can gain unauthorized access to resources associated with another user's account by luring a victim to visit a crafted authorization URL, allowing the attacker to obtain a valid authorization code and access token for the victim's account.