How severe is CVE-2025-53366?

CVE-2025-53366 has a CVSS 4.0 base score of 8.7, rated high severity.

Is CVE-2025-53366 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 6% (92nd percentile), an estimate of the probability of exploitation in the next 30 days.

How do I fix CVE-2025-53366?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2025-53366 have an EU (EUVD) identifier?

Yes. CVE-2025-53366 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-20093.

When was CVE-2025-53366 published?

CVE-2025-53366 was published on 2025-07-04 and last updated on 2026-06-17.

CVE-2025-53366

CVE-2025-53366 is a high-severity vulnerability with a CVSS 4.0 base score of 8.7. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-248.

Key facts

Severity: High (CVSS 4.0 base score 8.7)
EPSS exploit prediction: 6% (92nd percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2025-20093
Weakness: CWE-248
Published: 2025-07-04
Last modified: 2026-06-17

Description

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.

Frequently asked questions

What is CVE-2025-53366?: 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.
How severe is CVE-2025-53366?: CVE-2025-53366 has a CVSS 4.0 base score of 8.7, rated high severity.
Is CVE-2025-53366 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 6% (92nd percentile), an estimate of the probability of exploitation in the next 30 days.
How do I fix CVE-2025-53366?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2025-53366 have an EU (EUVD) identifier?: Yes. CVE-2025-53366 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-20093.
When was CVE-2025-53366 published?: CVE-2025-53366 was published on 2025-07-04 and last updated on 2026-06-17.

References

Other CWE-248 vulnerabilities

CVE-2018-11466 — Critical (CVSS 9.8): A vulnerability has been identified in SINUMERIK 808D V4.7 (All versions), SINUMERIK 808D V4.8 (All versions),…
CVE-2024-42037 — Critical (CVSS 9.3): Vulnerability of uncaught exceptions in the Graphics module Impact: Successful exploitation of this vulnerability may…
CVE-2025-53620 — Critical (CVSS 9.2): @builder.io/qwik-city is the meta-framework for Qwik. When a Qwik Server Action QRL is executed it dynamically load the…
CVE-2026-46689 — High (CVSS 8.7): Kanidm is an identity management platform. Prior to version 1.9.3, a single unauthenticated GET to any /scim/v1/...…
CVE-2026-9509 — High (CVSS 8.7): An unhandled exception in Suprema BioStar 2 (Server), versions 2.9.8, 2.9.10, and 2.9.11, that allows an…
CVE-2025-9124 — High (CVSS 8.7): A denial-of-service security issue in the affected product. The security issue stems from a fault occurring when a…

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