Server-Side Request Forgery (SSRF) in MCP Python SDK - CVE-2026-116686
Published: November 9, 2026
Vulnerability identifier: #VU163091
CSH Severity: Medium
CVSS v4: 6.7 [CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-116686
CWE-ID: CWE-918
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows an unauthenticated remote attacker to force client-side network requests and freeze all sessions in an MCP Python SDK client process.
The vulnerability exists due to unrestricted synchronous external reference resolution during tool result validation after a user adds a malicious MCP server and approves tool calls. An unauthenticated remote attacker can supply an outputSchema referencing a URL that accepts connections without responding, blocking the client's event loop indefinitely.
Affected software
MCP Python SDK
How to mitigate CVE-2026-116686
Install security update from vendor's website.
MCP Python SDK - addressed in versions 1.30.0, 2.2.0