Summary

I had posted this issue via h1 : 3556146, although my original intention was to reach out to the ruby-sdk maintainers, I thought H1 was the only way to report security issues. When the report was closed, I realized I could also reach out via security tab in the repo, hence this report.

The Ruby SDK's streamable_http_transport.rb implementation contains a session hijacking vulnerability. An attacker who obtains a valid session ID can completely hijack the victim's Server-Sent Events (SSE) stream and intercept all real-time data.

Details

Root Cause
The StreamableHTTPTransport implementation stores only one SSE stream object per session ID and lacks:

• Session-to-user identity binding
• Ownership validation when establishing SSE connections
• Protection against multiple simultaneous connections to the same session

PoC

####Vulnerable Code

File: streamable_http_transport.rb - L336-L339:

def store_stream_for_session(session_id, stream)
  @mutex.synchronize do
    if @sessions[session_id]
      @sessions[session_id][:stream] = stream  # OVERWRITES existing stream
    else
      stream.close
    end
  end
end

Attack Scenario

Step 1: Legitimate Session Establishment

POST / (initialize) → receives session_id: "abc123"
GET / with Mcp-Session-Id: abc123 → SSE stream connected

Step 2: Session ID Compromise

• Attacker obtains the session ID through various means (out of scope for this analysis)

Step 3: Stream Hijacking

GET / with Mcp-Session-Id: abc123 
@sessions["abc123"][:stream] = attacker_stream `# Victim's stream is REPLACED (silently disconnected)

Step 4: Data Interception

• ALL subsequent tool responses/notifications go to attacker
• Legitimate user receives no data and has no indication of the hijacking

Technical Details

The vulnerability happens:

Client 1 connects (GET request)

proc do |stream1|  # ← Rack server provides stream1 for client 1
 @sessions[session_id][:stream] = stream1  # Stored
end

Client 2 connects with SAME session ID (Attack!)

proc do |stream2|  # ← Rack provides stream2 for client 2
 @sessions[session_id][:stream] = stream2  # REPLACES stream1!
end

Now when server sends notifications:

@sessions[session_id][:stream].write(data)  # Goes to stream2 (attacker!)
# stream1 (victim) receives nothing

Comparison: Python SDK Protection

The Python SDK prevents this vulnerability by rejecting duplicate SSE connections:

Refer: https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py#L680-L685

if GET_STREAM_KEY in self._request_streams:  # pragma: no cover
            response = self._create_error_response(
                "Conflict: Only one SSE stream is allowed per session",
                HTTPStatus.CONFLICT,
            )

When a duplicate connection attempt is detected, the Python SDK returns an HTTP 409 Conflict error, protecting the existing connection.

Recommended Mitigations
For SDK Maintainers

• Implement User Binding: All SDKs should bind session IDs to authenticated user identities where possible. Currently only, go-sdk and csharp-sdk do user binding.
• Ruby SDK: Prevent Duplicate Connections: Implement checks to reject or handle multiple simultaneous connections to the same session
• Improve Documentation: Provide clear guidance on secure session management implementation for SDK consumers

Steps To Reproduce:

Please find attached two python client files demonstrating the attack

Terminal 1:
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:

python3 legitimate_client_ruby_server.py

What happens:

• Client connects and prints session ID
• Press Enter to start SSE stream
• Notifications start appearing every 3 seconds as the client makes tool call

Terminal 3 (while legitimate client is running):

python3 attacker_client_ruby_server.py <SESSION_ID>

Replace <SESSION_ID> with the ID from Terminal 2.

What happens immediately:

• Terminal 2 (Legitimate): Stops receiving notifications, shows disconnect message
• Terminal 3 (Attacker): Starts receiving ALL the tool call response

Impact

While the absence of user binding may not pose immediate risks if session IDs are not used to store sensitive data or state, the fundamental purpose of session IDs is to maintain stateful connections. If the SDK or its consumers utilize session IDs for sensitive operations without proper user binding controls, this creates a potential security vulnerability. For example: In the case of Ruby SDK, the attacker was able hijack the stream and receiving all the tool response belonging to the victim. The tool response can be sensitive confidential data.

Additional Details

Session Hijacking Protection in MCP Implementations

The MCP specification recommends - "MCP servers SHOULD bind session IDs to user-specific information".

Current Implementation Status Across SDKs

Of the 10 official MCP SDKs, only the following implementations bind session IDs to user-specific information:

1. csharp-sdk - https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L93-L97
2. Go-sdk - https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L281C1-L288C2

attacker_client_ruby_server.py
legitimate_client_ruby_server.py
The remaining SDKs do not implement session-to-user binding. Most implementations only verify that a session ID exists, without validating ownership. Additionally, SDK documentation does not provide clear guidance on implementing secure session management, leaving security responsibilities unclear for SDK consumers.