MCP Python SDK Resource Exhaustion via Unbounded Session Accumulation

The MCP Python SDK's Streamable HTTP transport in stateful mode (default) failed to reclaim session state, allowing clients to exhaust server memory through unlimited session creation. Sessions did not expire, DELETE requests left entries registered, and refused sessions remained active indefinitely.

Disclosed 15 October 2026 · Record updated 15 October 2026

Impact

Unbounded memory consumption on affected MCP servers accepting HTTP connections from untrusted clients in stateful mode (the default configuration).

Our coverage

A server room fills with stacked filing cabinets growing taller and taller, each drawer labeled with different client names, until the structure topples under its own weight.

Vulnerabilities

MCP Python SDK kept sessions forever, letting memory run out

An advisory published on 15 October 2026 says Streamable HTTP servers in the default stateful mode never reclaimed session state, so any reachable client could grow memory without bound.

15 Oct 2026

Sources

github.comhttps://github.com/advisories/GHSA-84m7-p3x7-pcfv
github.comhttps://github.com/advisories/GHSA-pq7j-f95f-qfcv