CVE-2025-53366
The MCP Python SDK, called `mcp` on PyPI, is a Python implementation of the Model Context Protocol (MCP).
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (9.75%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
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.
- 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/M
- EPSS
- 9.75% probability · 95th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Not listed
- Weakness
- CWE-248
- Source
- security-advisories@github.com
EPSS moved from 9.3% on 5 October 2026 to 9.8% on 26 October 2026. The dashed line is the ten per cent mark our analysts treat as “patch before the next change window”.
References
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.