The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). From 1.8.0 until 1.30.0 and 2.2.0, stateful Streamable HTTP servers, including FastMCP, MCPServer, Server.streamable_http_app(), and StreamableHTTPSessionManager, retain session entries without an idle expiration or aggregate session-count limit. DELETE, refused opening requests, and completed sessions do not reliably discard their entries, and each entry can retain parsed initialize parameters. A client that can reach the endpoint can therefore create ordinary short requests that grow server memory without bound until restart.
Stateless HTTP, stdio, and SSE transports are not affected. This issue is fixed in versions 1.30.0 and 2.2.0.
Attack Parameters
Attack Vector
Network
Can be exploited remotely
Attack Complexity
Low
Easy to exploit
Attack Requirements
None
No additional conditions
Privileges Required
None
No privileges needed
User Interaction
None
No user interaction needed
Impact Assessment
Confidentiality
None
No data leak
Integrity
None
No data modification
Availability
Low
Partial disruption
CVSS Vector v4.0
Weakness Type (CWE)
References 7
https://github.com/modelcontextprotocol/python-sdk/commit/92120b4b0f50385af0965…
security-advisories@github.com
https://github.com/modelcontextprotocol/python-sdk/commit/d2290ca3434731b68ea3e…
security-advisories@github.com
https://github.com/modelcontextprotocol/python-sdk/pull/3395
security-advisories@github.com
https://github.com/modelcontextprotocol/python-sdk/pull/3426
security-advisories@github.com
https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0
security-advisories@github.com
https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0
security-advisories@github.com
https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-84m…
security-advisories@github.com