CVE-2026-59951

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Source
https://cve.org/CVERecord?id=CVE-2026-59951
Import Source
https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-59951.json
JSON Data
https://api.osv.dev/v1/vulns/CVE-2026-59951
Aliases
  • GHSA-84m7-p3x7-pcfv
Published
2026-10-14T16:22:31Z
Modified
2026-10-15T02:47:43Z
Severity
  • 6.9 (Medium) CVSS_V4 - CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N CVSS Calculator
Summary
MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion
Details

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.

Database specific 
{
    "cna_assigner":  "GitHub_M",
    "cwe_ids":  [
        "CWE-400",
        "CWE-772"
    ],
    "osv_generated_from":  "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/59xxx/CVE-2026-59951.json"
}
References

Affected packages

Git / github.com/modelcontextprotocol/python-sdk

Affected ranges

Type
GIT
Repo
https://github.com/modelcontextprotocol/python-sdk
Events
Introduced
ed25167fa5d715733437996682e20c24470e8177
Fixed
8c2fa6eabacde869d75568e33b9d466b58c2f512
Introduced
6f69a3758ebf2ee55ce050f58b470ce11af71133
Fixed
9972c21aa42054fb1450c5fc614761ed11847ec6
Database specific
Show details 
{
    "extracted_events":  [
        {
            "introduced":  "1.8.0"
        },
        {
            "fixed":  "1.30.0"
        },
        {
            "introduced":  "2.0.0"
        },
        {
            "fixed":  "2.2.0"
        }
    ],
    "source":  "AFFECTED_FIELD"
}

Affected versions

1.*
1.15.0
v1.*
v1.10.0
v1.10.1
v1.11.0
v1.12.0
v1.12.1
v1.12.2
v1.12.3
v1.12.4
v1.13.0
v1.13.1
v1.14.0
v1.14.1
v1.15.0
v1.16.0
v1.17.0
v1.18.0
v1.19.0
v1.20.0
v1.21.0
v1.21.1
v1.22.0
v1.23.0
v1.23.1
v1.23.2
v1.23.3
v1.24.0
v1.25.0
v1.26.0
v1.27.0
v1.27.1
v1.27.2
v1.28.0
v1.28.1
v1.29.0
v1.29.1
v1.8.0
v1.8.1
v1.9.0
v1.9.1
v1.9.2
v1.9.3
v1.9.4
v2.*
v2.0.0
v2.1.0
v2.1.1

Database specific

source 
"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-59951.json"