Summary

Vulnerability: Missing Session Ownership Validation in the Ruby MCP SDK's Streamable and SSE HTTP transport implementation. Any attacker with a stolen session ID can execute tools with the victim's session. This is a silent attack - the victim's session is compromised and being used for unauthorized actions, but it is hard to know for the victim.

Impact

• Integrity: HIGH - Attacker can execute unauthorized tools and modify state.

• Availability: LOW - Attacker can disrupt victim's session with injected responses.