CVE-2025-53366: 
Model Context Protocol vulnerability analysis and mitigation

The MCP Python SDK (mcp on PyPI), a Python implementation of the Model Context Protocol (MCP), contains a validation error vulnerability identified as CVE-2025-53366. Prior to version 1.9.4, the SDK fails to properly handle malformed requests, which can result in unhandled exceptions causing service unavailability (500 errors) that requires manual server restart to recover (GitHub Advisory).

The vulnerability stems from insufficient error handling in the request validation process. When processing malformed requests, the server encounters an unhandled exception instead of properly validating and responding to invalid input. The issue has been assigned a CVSS v4.0 score of 8.7 (HIGH) with the vector string 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, indicating it can be exploited remotely with low complexity and requires no privileges or user interaction (GitHub Advisory).

The primary impact of this vulnerability is service unavailability. When exploited, the server becomes unresponsive and returns 500 errors, requiring manual intervention to restore service. The severity of the impact may vary depending on deployment conditions and the presence of infrastructure-level resilience measures (GitHub Advisory).

The vulnerability has been patched in version 1.9.4 of the MCP Python SDK. Users should upgrade to this version or later to address the issue. The fix implements proper error handling for malformed requests, ensuring they return appropriate error responses instead of crashing the server (GitHub Advisory).

The security community has expressed concern about the length of time this vulnerability remained open, with some security researchers noting it as a "huge security issue" that allows attackers to lock up servers by sending malformed payloads (GitHub PR).