CVE-2026-33946: MCP Ruby SDK Session Hijacking via SSE Stream Replay — CVSS 8.2

RubySec·high signal

The official MCP Ruby SDK (pre-0.9.2) has a critical session hijacking flaw: StreamableHTTPTransport stores one SSE stream per session ID with no ownership validation or identity binding. An attacker with a valid session ID can hijack the victim's entire SSE stream and intercept all tool responses, which may contain sensitive data. Patched in v0.9.2. Any MCP server built on the Ruby SDK using streamable HTTP transport should update immediately — this is a data exfiltration vector in production agent deployments.