CVE-2025-53366: Mcp Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-3qhf-m339-9g5v
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CWE-248
Published
Advisory Summary

CVE-2025-53366 is a HIGH denial of service vulnerability (CWE-248: Uncaught Exception) affecting Mcp libraries in the PyPI ecosystem. The issue involves mcp python sdk vulnerability in the fastmcp server causes validation error, leading to dos. Remediation is available by upgrading affected packages to version 1.9.4.

CVE-2025-53366 at a Glance

CVE Identifier CVE-2025-53366
GHSA Identifier GHSA-3qhf-m339-9g5v
Severity HIGH
CVSS v3.1 Base Score N/A
CWE Classification CWE-248: Uncaught Exception
Affected Product Mcp
Package Ecosystem PyPI
Published Date
Last Updated

How to Fix CVE-2025-53366

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

mcp fixed: 1.9.4 
pip install mcp>=1.9.4
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 mcp

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2025-53366, inspect your installed packages using pip:

pip show mcp

Analyze the terminal output to locate any version of Mcp in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.9.4), your application is affected.

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

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-248: Uncaught Exception, where an exception is thrown from a function, but it is not caught. In Mcp, 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

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.

CVE-2025-53366 Affected Mcp Versions

1 component(s)
Fixed Versions:
1.9.4
Product / Package Ecosystem Affected Versions Fixed Version
mcp
pypi/mcp
PyPI
< 1.9.4
1.9.4

References

GitHub Advisory / Wiki ADVISORY
https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-3qhf-m339-9g5v
NVD — CVE-2025-53366 NVD
https://nvd.nist.gov/vuln/detail/CVE-2025-53366
Mcp Source Repository REPOSITORY
https://github.com/modelcontextprotocol/python-sdk/pull/822
Security Fix Commit FIX COMMIT
https://github.com/modelcontextprotocol/python-sdk/commit/29c69e6a47d0104d0afcea6ac35e7ab02fde809a
GitHub Advisory / Wiki ADVISORY
https://github.com/advisories/GHSA-3qhf-m339-9g5v
Mcp Source Repository REPOSITORY
https://github.com/modelcontextprotocol/python-sdk
Mcp Release RELEASE
https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.9.4
Mcp Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/mcp/PYSEC-2026-1616.yaml
pypi.org/project/mcp WEB
https://pypi.org/project/mcp

Frequently Asked Questions

What is CVE-2025-53366?
CVE-2025-53366 is a security vulnerability in Mcp involving MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS under CWE-248: Uncaught Exception.
Which Mcp versions are affected?
Affected versions include releases prior to 1.9.4 across affected packages: mcp.
What version fixes CVE-2025-53366?
Upgrading to 1.9.4 (or the latest patched release for your respective branch) resolves the vulnerability.
Is mcp affected?
Yes, mcp is affected in releases prior to 1.9.4. Upgrade your dependency to 1.9.4 or later.
What CWE is associated with CVE-2025-53366?
CVE-2025-53366 is classified under CWE-248: Uncaught Exception (An exception is thrown from a function, but it is not caught.).
How can I check PyPI dependencies for CVE-2025-53366?
Run the dependency tree command: 'pip show mcp'. 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-3qhf-m339-9g5v",
  "modified" : "2026-07-16T18:30:56Z",
  "published" : "2025-07-04T22:06:49Z",
  "aliases" : [ "CVE-2025-53366" ],
  "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.\n\nThank you to Rich Harang for reporting this issue.",
  "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "mcp"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.9.4"
      } ]
    } ]
  } ],
  "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" : "ADVISORY",
    "url" : "https://github.com/advisories/GHSA-3qhf-m339-9g5v"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/modelcontextprotocol/python-sdk"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.9.4"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/mcp/PYSEC-2026-1616.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://pypi.org/project/mcp"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-248" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2025-07-04T22:06:49Z",
    "nvd_published_at" : "2025-07-04T22:15:22Z"
  }
}