CVE-2026-59951 · GHSA-84m7-p3x7-pcfv
MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion
- Risk score
- 59/100
- CVSS
- 6.9 v4.0
- Exploit chance (30 days)
- 0.5%
- Published
- 14 Oct 2026
Open in the live forecast Today's forecast
What to know
- What it is
- 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…
- Affected
- mcp (pip) >= 1.8.0, < 1.30.0; mcp (pip) >= 2.0.0, < 2.2.0
- Fix
- Upgrade mcp to 1.30.0 or 2.2.0 (whichever matches your release line) or later.
- Exploitation
- No known exploitation
Why it is Yellow
- CVSS 6.9 medium
- On your watchlist: python, PyPI
Affected packages
| Package | Ecosystem | Vulnerable | Fixed in |
|---|---|---|---|
| mcp | pip | >= 1.8.0, < 1.30.0 | 1.30.0 |
| mcp | pip | >= 2.0.0, < 2.2.0 | 2.2.0 |
References
- https://github.com/advisories/GHSA-84m7-p3x7-pcfv
- https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-84m7-p3x7-pcfv
- https://nvd.nist.gov/vuln/detail/CVE-2026-59951
- https://github.com/modelcontextprotocol/python-sdk/pull/3395
- https://github.com/modelcontextprotocol/python-sdk/pull/3426
- https://github.com/modelcontextprotocol/python-sdk/commit/92120b4b0f50385af096592d7cee932fc1d994e7
- https://github.com/modelcontextprotocol/python-sdk/commit/d2290ca3434731b68ea3e2270bc06a6e6575931b
- https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0
- https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0
- https://github.com/advisories/GHSA-84m7-p3x7-pcfv
Original 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.
Last activity 14 Oct 2026. Patch Forecast updates every hour.