Allocation of Resources Without Limits or Throttling in MCP Python SDK - CVE-2026-59951
Published: November 9, 2026
Vulnerability identifier: #VU163093
CSH Severity: Medium
CVSS v4: 8.7 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-59951
CWE-ID: CWE-770
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows an unauthenticated remote attacker to exhaust server memory and cause denial of service.
The vulnerability exists due to unlimited session creation and missing reclamation of session state and initialization payloads in the MCP Python SDK's default stateful Streamable HTTP mode. An unauthenticated remote attacker can repeatedly send ordinary short requests to a reachable endpoint to accumulate retained sessions and grow server memory without bound.
Affected software
MCP Python SDK
How to mitigate CVE-2026-59951
Install security update from vendor's website.
MCP Python SDK - addressed in versions 1.30.0, 2.2.0