Description
The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). From 1.8.0 until 1.30.0 and 2.2.0, stateful Streamable HTTP servers, including FastMCP, MCPServer, Server.streamable_http_app(), and StreamableHTTPSessionManager, retain session entries without an idle expiration or aggregate session-count limit. DELETE, refused opening requests, and completed sessions do not reliably discard their entries, and each entry can retain parsed initialize parameters. A client that can reach the endpoint can therefore create ordinary short requests that grow server memory without bound until restart. Stateless HTTP, stdio, and SSE transports are not affected. This issue is fixed in versions 1.30.0 and 2.2.0.
References7
- github.com/modelcontextprotocol/python-sdk/commit/92120b4b0f503…
- github.com/modelcontextprotocol/python-sdk/commit/d2290ca343473…
- github.com/modelcontextprotocol/python-sdk/pull/3395
- github.com/modelcontextprotocol/python-sdk/pull/3426
- github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0
- github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0
- github.com/modelcontextprotocol/python-sdk/security/advisories/…
Metrics
- CVSS 4.0
-
6.9
MEDIUM
Score source: CNA
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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 - EPSS — exploit probability
- 0% percentile: 40 · as of 11 November 2026
- CWE
- CWE-400CWE-772
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.