CVE-2026-67431: Modelcontextprotocol Ruby-SDK

High severity, CVSS 8.3. EPSS: 0.5% chance of exploitation in the next 30 days.

MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not bind a session ID to a session owner, allowing an attacker with a stolen session ID to send tools/call requests that execute in the victim's session. This issue is fixed in version 0.23.0.

Affected products

• Modelcontextprotocol Ruby-SDK: before 0.23.0 (fixed in 0.23.0)

Published 2026-07-29. Last modified 2026-07-30.