CVE-2026-59951 MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion
Attackers can exhaust server memory via unbounded session accumulation in stateful Streamable HTTP transports, causing denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-59951?
CVE-2026-59951 is a medium-severity denial of service vulnerability in modelcontextprotocol python-sdk, published on 14 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-59951?
4 open source packages ship an affected version, including mcp.
| Package | Affected | Fixed in |
|---|---|---|
mcp PyPI | From 1.8.0 | 1.30.0 |
mcp PyPI | From 1.8.0 | 1.30.0 |
mcp PyPI | From 2.0.0 | 2.2.0 |
mcp PyPI | From 2.0.0 | 2.2.0 |
How do I fix CVE-2026-59951?
Upgrade mcp to a fixed version for your release line: 1.30.0 or 2.2.0.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-59951 being exploited?
There is no known exploitation of CVE-2026-59951 in the wild and no public exploit code.
Sources
Also known as ECHO-4e93-dae6-642f, GHSA-84m7-p3x7-pcfv, MINI-2352-675j-3jpr, MINI-2m9m-qvmr-jvg6, MINI-37p3-mq97-9qc9, MINI-53r2-p5q2-75gq and 13 more.