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CVE-2025-53366

MCP SDK Vulnerable to FastMCP Server Validation Error, Leading to Denial of Service

8.7 HighFix available

The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.9.4, 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. Version 1.9.4 contains a patch for the issue.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
None
Integrity
None
Availability
High

Exploit likelihood

9.75%Up 9.7 points since Jul 5, 2025
90%50%10%1%

What each source reports

CVE List Score used 8.7

MCP SDK Vulnerable to FastMCP Server Validation Error, Leading to Denial of Service

The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.9.4, 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. Version 1.9.4 contains a patch for the issue.

Record
CVE-2025-53366
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Weakness
CWE-248
Updated
2026-11-02
EUVDEUVD-2025-20093 8.7

The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.9.4, 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. Version 1.9.4 contains a patch for the issue.

Record
EUVD-2025-20093
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-3qhf-m339-9g5v 8.7

MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS

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.

Record
GHSA-3qhf-m339-9g5v
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Weakness
CWE-248
Updated
2026-11-02
NVD 8.7

The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.9.4, 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. Version 1.9.4 contains a patch for the issue.

Record
CVE-2025-53366
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Weakness
CWE-248
Updated
2026-11-02
OSV 8.7

MCP SDK Vulnerable to FastMCP Server Validation Error, Leading to Denial of Service

The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.9.4, 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. Version 1.9.4 contains a patch for the issue.

Record
CVE-2025-53366
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Weakness
CWE-248
Updated
2026-11-02
OSVGHSA-3qhf-m339-9g5v 8.7

MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS

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.

Record
GHSA-3qhf-m339-9g5v
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Weakness
CWE-248
Updated
2026-11-02
OSVOESA-2026-1151 5.5

python-mcp security update

The official Python SDK for Model Context Protocol servers and clients

Security Fix(es):

The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.10.0, if a client deliberately triggers an exception after establishing a streamable HTTP session, this can lead to an uncaught ClosedResourceError on the server side, causing the server to crash and requiring a restart to restore service. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures. Version 1.10.0 contains a patch for the issue.(CVE-2025-53365)

The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.9.4, 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. Version 1.9.4 contains a patch for the issue.(CVE-2025-53366)

Record
OESA-2026-1151
CVSS 3.1
5.5 MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:C/C:L/I:L/A:L
Updated
2026-11-02
OSVPYSEC-2026-1616 8.7

MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS

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.

Record
PYSEC-2026-1616
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Updated
2026-11-02
OSVROOT-APP-PYPI-CVE-2025-53366 5.3

CVE-2025-53366 in mcp - Patched by Root

Root has patched CVE-2025-53366 in the mcp package for Root:PyPI. Multiple fixed versions available.

Record
ROOT-APP-PYPI-CVE-2025-53366
CVSS 3.1
5.3 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
mcp
PyPI
 before 1.9.4
1.9.4
mcp
PyPI
 before 1.9.4, plus listed versions
0.9.11.0.01.1.01.1.11.1.2
Show 18 moreShow less
1.1.31.2.01.2.0rc11.2.11.3.01.3.0rc11.4.01.4.11.5.01.6.01.7.01.7.11.8.01.8.11.9.01.9.11.9.21.9.3
1.9.4
python-mcp
openEuler:24.03-LTS-SP3
From an advisory covering 2 CVEs
 before 1.12.3-1.oe2403sp3
1.12.3-1.oe2403sp3
mcp
Root:PyPI
 before 1.1.2+aikido.4
1.1.2+aikido.41.0.0+aikido.31.1.0+aikido.21.2.0+aikido.41.8.1+aikido.21.6.0+aikido.41.9.1+aikido.21.8.1+aikido.3

History

History
RecordedChangedSources
affected, facts
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2025-53366 is a high vulnerability (CVSS 8.7) in Modelcontextprotocol Python-sdk, published July 4, 2025. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 9.75%, the 95th percentile. Fixed in mcp 1.9.4.

References

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