MCP Ruby SDK: Ruby SSE Session Poisoning

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/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L36
• https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L95-L98

Go-sdk

• https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L66
• https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L316-L320

Comparison with the Stream Replacement vulnerability

Stream Replacement https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35

• Target: SSE stream connection
• Attack Vector: GET /mcp
• What happens to Victim: The connection is disconnected and doesn't receive reponses

Session Poisoning

• Target: Tool execution
• Attack Vector: POST /mcp
• What happens to Victim: The connection stays connected and receives responses of tool calls by attacker