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Steven Daugherty (@StevenDaugherty) on X
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Steven Daugherty (@StevenDaugherty) on X

Post warning that FastMCP had a critical flaw where a malicious MCP server could execute code on the connecting client through a crafted tool schema. Fixed in FastMCP 3.4.8 and 4.0.11, and there is no patch for the 2.x line. It matters to anyone whose agents connect to MCP servers they don't control. The post gives no CVE ID or technical root cause.

TL;DR FastMCP critical client-side RCE via a malicious server's crafted tool schema; fix in 3.4.8 / 4.0.11, no 2.x patch.
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Organisations named here

  • client-side RCE
  • crafted tool schema
  • malicious MCP server

Technologies named here

  • FastMCP
  • Model Context Protocol
  • Python

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