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GHSA-5p9g-j988-pcwv
No affected components available
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
- Python 3.8+
- Install dependencies:
requests
Running the Demo
- 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
- Terminal 2: Start Victim Client
python3 legitimate_client.py
- Terminal 3 - Attacker Client:
Copy the session ID from Terminal 1 and run:
python3 attacker_client.py abc-123-def-456
- 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
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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