Unintended Proxy or Intermediary ('Confused Deputy') Affecting fastmcp package, versions [,2.13.0)
Threat Intelligence
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Test your applications- Snyk IDSNYK-PYTHON-FASTMCP-13776148
- published30 Oct 2025
- disclosed29 Oct 2025
- creditDen Delimarsky
Introduced: 29 Oct 2025
New CVE NOT AVAILABLE CWE-441 (opens in a new tab)How to fix?
Upgrade fastmcp to version 2.13.0 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') during the authentication with OAuth providers that don't support Dynamic Client Registration (DCR). An attacker can gain unauthorized access to user accounts by tricking authenticated users into clicking a crafted authorization link for a malicious client, which leverages existing consent and browser state to obtain an authorization code for the attacker's client, allowing them to impersonate the victim.