CVE-2026-109527: Model Context Protocol Python SDK HTTP and OAuth endpoints allow memory exhaustion through unbounded request bodies
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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Model Context Protocol Python SDKto a version that resolves this vulnerability.Fixed in 1.29.1 - Upgrade
Upgrade
Model Context Protocol Python SDKto a version that resolves this vulnerability.Fixed in 2.1.0 - Compensating control
Enforce an effective upstream request-size limit for HTTP request bodies; applications using StreamableHTTPServerTransport directly must enforce their own maximum request-body size.
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
Servers using affected HTTP-based SDK components are exposed when an unauthenticated remote client can reach the Streamable HTTP session manager, SSE message endpoint, or OAuth authorization-server routes. Stdio-only servers and deployments with an effective upstream request-size limit are not affected.
Does an attacker need credentials or user interaction?
No. An unauthenticated remote client only needs network access to an affected endpoint and can submit a large declared or streamed request body.
Which versions contain the fix?
The standard SDK server applications and session managers are fixed in mcp 1.29.1 and 2.1.0. Affected releases are versions before 1.29.1 and versions from 2.0.0a1 through 2.0.1.
What can be done if upgrading is not immediately possible?
Enforce an effective HTTP request-size limit upstream of the affected endpoint. Applications using StreamableHTTPServerTransport directly must enforce their own request-size limit.