CVE-2026-109527 Model Context Protocol Python SDK HTTP and OAuth endpoints allow memory exhaustion through unbounded request bodies
Unauthenticated attackers can exhaust server memory by sending large request bodies to MCP Python SDK HTTP endpoints, causing denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-109527?
CVE-2026-109527 is a high-severity denial of service vulnerability in modelcontextprotocol python-sdk, published on 14 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-770
Which versions are affected by CVE-2026-109527?
How do I fix CVE-2026-109527?
No fix has been published for CVE-2026-109527 yet. Limit exposure of the affected software until one is available.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-109527 being exploited?
There is no known exploitation of CVE-2026-109527 in the wild and no public exploit code.
Sources
Also known as GHSA-fmmv-w9g8-j3gc.