VulnSocial

CVE-2026-59951

LOWCVSS 6.9 · MEDIUM

Signal is active with 1 mentions in latest observed window

Immediate actions

  • Track advisory updates for patch or workaround availability

Recommended action window: Monitor and triage in normal cycle

NVD 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.

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0.0/ 10 priority

Sources & remediation

Other references
Weakness type (CWE)
CWE-400CWE-772

Priority

LOW

Exploitation

NONE

PoC

NONE

Patch

NONE

Momentum

NONE

Threat summary

  • 1 mentions across 1 observed day

What's happening

  • 1 total mentions across 1 day

Deep dive

Activity timeline1 mentions / 1d
00111Mentions · 2026-10-17: 110-17
Full discourse1 post
  • Jace Mercer@JustinJohn95674

    MCP Python SDK advisory (CVE-2026-59951, 15 Oct): stateful Streamable HTTP hosts on mcp 1.8.0–1.29.1 or 2.0.0–2.1.1 can accumulate sessions into unbounded memory growth, per the advisory. The failure mode is boring: session accumulation with no ceiling. In production that can look like an agent host getting flaky rather than a security incident. Two mitigations listed in the advisory: 1. Upgrade to mcp 1.30.0 or 2.2.0. 2. Set stateless_http=True. Scope check: if you only run outbound client MCP connections, the advisory calls it low drama. But if any Hermes or MCP host exposes Streamable HTTP, the advisory's advice is to patch the SDK. Per the advisory, session accumulation on stateful Streamable HTTP is the mechanism — worth checking what your hosts are actually running. Who's still running a Streamable HTTP MCP host on an unpatched SDK — and have you checked your version today?

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