N/A

CVSS Score

-

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GHSA-rcfx-77hg-w2wv

GHSA-rcfx-77hg-w2wv: FastMCP updated to MCP 1.23+ due to CVE-2025-66416

There was a recent CVE report on MCP: https://nvd.nist.gov/vuln/detail/CVE-2025-66416.

FastMCP does not use any of the affected components of the MCP SDK directly. However, FastMCP versions prior to 2.14.0 did allow MCP SDK versions <1.23 that were vulnerable to CVE-2025-66416. Users should upgrade to FastMCP 2.14.0 or later.

(GitHub Advisory)

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GHSA-rcfx-77hg-w2wv

GHSA-rcfx-77hg-w2wv:

N/A

CVSS Score

-

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Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
fastmcp	pip	< 2.14.0	2.14.0

Technical Details

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Root Cause Analysis

The vulnerability, CVE-2025-66416, is not directly in the FastMCP codebase but in one of its dependencies, the mcp (Model Context Protocol) SDK. FastMCP versions prior to 2.14.0 allowed the use of vulnerable versions of the mcp SDK. The fix, implemented in FastMCP version 2.14.0, is to update the dependency requirement for the mcp SDK to version 1.23.1 or later, which contains the security patch.

The analysis of the patch commit for FastMCP reveals changes to accommodate the updated mcp SDK. Specifically, the TokenHandler class in src/fastmcp/server/auth/oauth_proxy.py was modified. A new handle method was introduced, which wraps the handle method of the _SDKTokenHandler from the mcp library. This indicates that TokenHandler.handle is a key function in FastMCP that interacts with the authentication components of the mcp SDK.

Given that the commit message for the patch explicitly mentions 'auth changes', it is highly probable that the vulnerability in the mcp SDK is related to authentication. Therefore, any exploit targeting this vulnerability through a FastMCP application would have the TokenHandler.handle function in its execution path, making it a key runtime indicator for identifying exploitation attempts.

Vulnerable functions

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WAF Protection Rules

WAF Rule

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Reasoning

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