modelcontextprotocol python-sdk up to 1.29.x/2.1.x Streamable HTTP Server.streamable_http_app allocation of resources

Summaryinfo

A vulnerability marked as problematic has been reported in modelcontextprotocol python-sdk up to 1.29.x/2.1.x. Affected by this vulnerability is the function Server.streamable_http_app of the component Streamable HTTP. Performing a manipulation results in allocation of resources. This vulnerability was named CVE-2026-59951. The attack may be initiated remotely. There is no available exploit. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as problematic, was found in modelcontextprotocol python-sdk up to 1.29.x/2.1.x. This affects the function Server.streamable_http_app of the component Streamable HTTP. The manipulation with an unknown input leads to a allocation of resources vulnerability. CWE is classifying the issue as CWE-770. The product allocates a reusable resource or group of resources on behalf of an actor without imposing any restrictions on the size or number of resources that can be allocated, in violation of the intended security policy for that actor. This is going to have an impact on availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-59951 since 07/07/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Technical details of the vulnerability are known, but there is no available exploit. The attack technique deployed by this issue is T1499 according to MITRE ATT&CK.

Upgrading to version 1.30.0 or 2.2.0 eliminates this vulnerability.

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Productinfo

Type

• Programming Language Software

Vendor

• modelcontextprotocol

Name

• python-sdk

Version

• 1.0
• 1.1
• 1.2
• 1.3
• 1.4
• 1.5
• 1.6
• 1.7
• 1.8
• 1.9
• 1.10
• 1.11
• 1.12
• 1.13
• 1.14
• 1.15
• 1.16
• 1.17
• 1.18
• 1.19
• 1.20
• 1.21
• 1.22
• 1.23
• 1.24
• 1.25
• 1.26
• 1.27
• 1.28
• 1.29
• 2.0
• 2.1

Website

• Product: https://github.com/modelcontextprotocol/python-sdk/

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