CVE-2026-109527

7.5High

Published 14 Oct 2026 · NVD status: Awaiting Analysis · AI triage by gemini-3.8-flash

7.5High

Attack vector: Network
Complexity: Low
Privileges needed: None
User interaction: None
Impact: High impact on availability
Weakness: CWE-770

Impact

Unauthenticated remote attackers can crash or freeze the server via memory exhaustion. They only need network access to reach HTTP, SSE, or OAuth endpoints with large or streamed request bodies.

Root cause

HTTP request bodies are buffered entirely into memory without enforcing a maximum size limit.

Affected systems

mcp before 1.29.1, and 2.0.0a1 through 2.0.1 (stdio-only servers not affected)

CVSS v3.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Fix & remediation

Upgrade mcp to version 1.29.1 or 2.1.0. If patching immediately is not possible, enforce an upstream request body size limit at your reverse proxy or load balancer.

shell

$ pip install --upgrade 'mcp>=1.29.1,<2.0.0' || pip install --upgrade 'mcp>=2.1.0'

AI-suggested. Run it in staging first and check it against the vendor advisory. Why?

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NVD description

The Model Context Protocol Python SDK is the official Python SDK for Model Context Protocol servers and clients. In mcp versions before 1.29.1 and from 2.0.0a1 through 2.0.1, the Streamable HTTP session manager, SSE message endpoint, and OAuth authorization-server routes buffer complete HTTP request bodies without enforcing a maximum size. An unauthenticated remote client able to reach an affected endpoint can send a large declared or streamed request body that consumes multiple times the body size in process memory and can repeatedly exhaust memory or terminate the server. Stdio-only servers and deployments with an effective upstream request-size limit are not affected, and applications that use StreamableHTTPServerTransport directly remain responsible for enforcing their own limit. The standard SDK server applications and session managers are fixed in versions 1.29.1 and 2.1.0.

References

Source: nvd.nist.gov/vuln/detail/CVE-2026-109527