CVE-2025-53366
HIGHEPSS 94th pctlDescription
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 to fix
No published remediation has been found for this vulnerability's affected products yet.
Mitigation guidance may be in the linked vendor advisories in the References section below.
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CVSS v4 Vector
Exploitability
Impact
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/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
Exploit Intelligence
High risk: more likely to be exploited than 94% of all known CVEs.
References
Related Vulnerabilities
Other CWE-248 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2025-59538 | High | 7.5 | 8.9% | - | Fix |
| CVE-2021-37714 | High | 7.5 | 6.9% | - | Fix |
| CVE-2018-11466 | Critical | 9.8 | 4.0% | - | - |
| CVE-2025-59465 | High | 7.5 | 3.8% | - | Fix |
| CVE-2018-7852 | High | 7.5 | 3.6% | - | - |
| CVE-2020-6019 | High | 7.5 | 2.8% | - | Fix |
Common questions
How do I fix CVE-2025-53366?
No published fix for CVE-2025-53366 has been found yet. Check the linked vendor advisories for guidance as it becomes available.
Is CVE-2025-53366 being actively exploited?
Not that we know of. CVE-2025-53366 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 7.3% is the estimated probability that it will be exploited in the next 30 days. That is higher than 94% of all scored CVEs.
How severe is CVE-2025-53366?
CVE-2025-53366 has a CVSS v4 base score of 8.7, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-06-17.