CVE-2026-67431 — Vulnerability in mcp — CVE Details

### 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. **Terminal 2:** Start Victim Client `python3 legitimate_client.py` 3. **Terminal 3** - Attacker Client: Copy the session ID from Terminal 1 and run: ```bash 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](https://modelcontextprotocol.io/docs/tutorials/security/security_best_practices#mitigation-4)". #### 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