id: PYSEC-2026-1616
published: "2026-07-07T16:02:56.172263Z"
modified: "2026-07-07T17:24:40.207478Z"
aliases:
- CVE-2025-53366
- GHSA-3qhf-m339-9g5v
summary: MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS
details: |-
  A validation error in the MCP SDK can cause an unhandled exception when processing malformed requests, resulting in service unavailability (500 errors) until manually restarted. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures.
  
  Thank you to Rich Harang for reporting this issue.
affected:
- package:
    name: mcp
    ecosystem: PyPI
    purl: pkg:pypi/mcp
  ranges:
  - type: ECOSYSTEM
    events:
    - introduced: "0"
    - fixed: 1.9.4
  versions:
  - 0.9.1
  - 1.0.0
  - 1.1.0
  - 1.1.1
  - 1.1.2
  - 1.1.3
  - 1.2.0
  - 1.2.0rc1
  - 1.2.1
  - 1.3.0
  - 1.3.0rc1
  - 1.4.0
  - 1.4.1
  - 1.5.0
  - 1.6.0
  - 1.7.0
  - 1.7.1
  - 1.8.0
  - 1.8.1
  - 1.9.0
  - 1.9.1
  - 1.9.2
  - 1.9.3
references:
- type: WEB
  url: https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-3qhf-m339-9g5v
- type: ADVISORY
  url: https://nvd.nist.gov/vuln/detail/CVE-2025-53366
- type: WEB
  url: https://github.com/modelcontextprotocol/python-sdk/pull/822
- type: WEB
  url: https://github.com/modelcontextprotocol/python-sdk/commit/29c69e6a47d0104d0afcea6ac35e7ab02fde809a
- type: PACKAGE
  url: https://github.com/modelcontextprotocol/python-sdk
- type: WEB
  url: https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.9.4
- type: PACKAGE
  url: https://pypi.org/project/mcp
- type: ADVISORY
  url: https://github.com/advisories/GHSA-3qhf-m339-9g5v
severity:
- type: CVSS_V4
  score: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N