Insufficient Session Binding Allows SSE Stream Hijacking via Session ID Replay

A high-severity (CVSS 8.2) session hijacking vulnerability has been disclosed in the MCP Ruby SDK (CVE-2026-33946). The StreamableHTTPTransport implementation in streamable_http_transport.rb stores only one SSE stream per session ID and lacks session-to-user identity binding, ownership validation, and protection against multiple simultaneous connections. An attacker who obtains a valid session ID can hijack the victim's SSE stream and intercept all real-time tool responses, which may include sensitive data. The vulnerability is patched in version 0.9.2 of the ruby-sdk.