CVE-2026-67431

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 Details https://github.com/modelcontextprotocol/ruby-sdk/blob/main/lib/mcp/server/transports/streamable_http_transport.rb#L260-L278 Victim starts a legitimate MCP session and receives session ID abc-123 Attacker obtains the session ID (various means - network sniffing, logs, etc. out of scope for this analysis) Attacker sends POST to /messages/abc-123 with a tool call Server accepts the request (no ownership validation!) Server executes the tool and sends response to victim's SSE stream Victim receives attacker's response, thinking it's legitimate PoC attacker_client.py legitimate_client.py Prerequisites 1. Python 3.8+ 2. Install dependencies: requests Running the Demo 1. Terminal 1: Start the Ruby MCP Server ruby streamable_http_server.rb Makes use of https://github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb This server has a tool call notification_tool which the clients call 2. Terminal 2: Start Victim Client python3 legitimate_client.py 3. Terminal 3 - Attacker Client: Copy the session ID from Terminal 1 and run: python3 attacker_client.py abc-123-def-456 4. Back to Terminal 2 - Victim sees the injected response: Impact - Integrity: HIGH - Attacker can execute unauthorized tools and modify state - Availability: LOW - Attacker can disrupt victim's session with injected responses Additional Details Session Hijacking Protection in MCP Implementations The MCP specification recommends - "MCP servers SHOULD bind session IDs to user-specific information". User Binding - Comparison other SDKs csharp-sdk - https://github.com/modelcontextprotocol/csharp-sdk/blob/main/

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