What’s Vulnerable › Vulnerabilities

CVE-2026-109527: Model Context Protocol Python SDK HTTP and OAuth endpoints allow memory exhaustion through unbounded request bodies

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.7% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-14 · updated 2026-10-20
• Weakness: CWE-770

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.

Affected products

• modelcontextprotocol python-sdk: before 1.29.1; from 2.0.0a1, < 2.1.0

References

• https://github.com/modelcontextprotocol/python-sdk/commit/03aaebd3aadb2567878dd811cb1f141454e8a080
• https://github.com/modelcontextprotocol/python-sdk/commit/4dc224fab201bdef2fd5a065976b7ca31420187c
• https://github.com/modelcontextprotocol/python-sdk/commit/57394b0548d1e2dc2dce8d67d84985769df3b8bb
• https://github.com/modelcontextprotocol/python-sdk/commit/5f0b6af9f2b0aad3be3ba5cbfe80a9e4ea32e189
• https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.29.1
• https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.1.0
• https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-fmmv-w9g8-j3gc

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