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: RubyGems: mcp

### 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](https://github.com/user-attachments/files/26044817/attacker_client.py) [legitimate_client.py](https://github.com/user-attachments/files/26044818/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.…