GHSA-rcfx-77hg-w2wv: Fastmcp FastMCP Updated To MCP 1.23+ Vulnerability

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Advisory Summary

GHSA-rcfx-77hg-w2wv is a HIGH fastmcp updated to mcp 1.23+ vulnerability (General Security Weakness) affecting Fastmcp libraries in the PyPI ecosystem. The issue involves fastmcp updated to mcp 1.23+ due to cve-2025-66416. Remediation is available by upgrading affected packages to version 2.14.0.

GHSA-rcfx-77hg-w2wv at a Glance

CVE Identifier N/A
GHSA Identifier GHSA-rcfx-77hg-w2wv
Severity HIGH
CVSS v3.1 Base Score N/A
CWE Classification General Security Weakness
Affected Product Fastmcp
Package Ecosystem PyPI
Published Date
Last Updated

How to Fix GHSA-rcfx-77hg-w2wv

To remediate this vulnerability, upgrade all affected Fastmcp dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

fastmcp fixed: 2.14.0 
pip install fastmcp>=2.14.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show fastmcp

How to Check if You Are Affected

To determine whether your Python environment is affected by GHSA-rcfx-77hg-w2wv, inspect your installed packages using pip:

pip show fastmcp

Analyze the terminal output to locate any version of Fastmcp in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 2.14.0), your application is affected.

Note: The pip show fastmcp command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under General Security Weakness. In Fastmcp, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

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.

GHSA-rcfx-77hg-w2wv Affected Fastmcp Versions

1 component(s)
Fixed Versions:
2.14.0
Product / Package Ecosystem Affected Versions Fixed Version
fastmcp
pypi/fastmcp
PyPI
< 2.14.0
2.14.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/jlowin/fastmcp/security/advisories/GHSA-rcfx-77hg-w2wv
GitHub Advisory / Wiki ADVISORY
https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-9h52-p55h-vw2f
NVD — GHSA-rcfx-77hg-w2wv NVD
https://nvd.nist.gov/vuln/detail/CVE-2025-66416
Fastmcp Source Repository REPOSITORY
https://github.com/jlowin/fastmcp

Frequently Asked Questions

What is GHSA-rcfx-77hg-w2wv?
GHSA-rcfx-77hg-w2wv is a security vulnerability in Fastmcp involving FastMCP updated to MCP 1.23+ due to CVE-2025-66416 under General Security Weakness.
Which Fastmcp versions are affected?
Affected versions include releases prior to 2.14.0 across affected packages: fastmcp.
What version fixes GHSA-rcfx-77hg-w2wv?
Upgrading to 2.14.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is fastmcp affected?
Yes, fastmcp is affected in releases prior to 2.14.0. Upgrade your dependency to 2.14.0 or later.
What CWE is associated with GHSA-rcfx-77hg-w2wv?
GHSA-rcfx-77hg-w2wv is classified under General Security Weakness.
How can I check PyPI dependencies for GHSA-rcfx-77hg-w2wv?
Run the dependency tree command: 'pip show fastmcp'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-rcfx-77hg-w2wv",
  "modified" : "2025-12-26T23:20:50Z",
  "published" : "2025-12-26T23:20:50Z",
  "aliases" : [ ],
  "summary" : "FastMCP updated to MCP 1.23+ due to CVE-2025-66416",
  "details" : "There was a recent CVE report on MCP: https://nvd.nist.gov/vuln/detail/CVE-2025-66416. \n\nFastMCP 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.",
  "severity" : [ ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "fastmcp"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.14.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/jlowin/fastmcp/security/advisories/GHSA-rcfx-77hg-w2wv"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-9h52-p55h-vw2f"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2025-66416"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/jlowin/fastmcp"
  } ],
  "database_specific" : {
    "cwe_ids" : [ ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2025-12-26T23:20:50Z",
    "nvd_published_at" : null
  }
}