CVE-2026-67431: 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/streamablehttptransport.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 attackerclient.py legitimateclient.py
Prerequisites
1. Python 3.8+ 2. Install dependencies: requests
Running the Demo
1. Terminal 1: Start the Ruby MCP Server
ruby streamablehttpserver.rb
Makes use of https://github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamablehttpserver.rb This server has a tool call notificationtool which the clients call
2. Terminal 2: Start Victim Client
python3 legitimateclient.py
3. Terminal 3 - Attacker Client:
Copy the session ID from Terminal 1 and run:
bash python3 attackerclient.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
Other sources
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.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
rubygems/mcpto a version that resolves this vulnerability.Fixed in 0.23.0 - Upgrade
Upgrade
mcp gem (MCP Ruby SDK) / MCP::Server::Transports::StreamableHTTPTransportto a version that resolves this vulnerability.Fixed in 0.23.0Patch GHSA-qvqr-5cv7-wh35 - Compensating control
Ensure the MCP server binds session IDs to user-specific information (per MCP security best practice: “MCP servers SHOULD bind session IDs to user-specific information”).
Event History
Frequently Asked Questions
What is the severity of CVE-2026-67431?
The severity of CVE-2026-67431 is rated at 55.
How do I fix CVE-2026-67431?
To fix CVE-2026-67431, update to the MCP Ruby SDK version 0.23.0 or later.
What issue does CVE-2026-67431 present?
CVE-2026-67431 presents a vulnerability where an attacker can exploit session IDs to send unauthorized requests.
What systems are affected by CVE-2026-67431?
CVE-2026-67431 affects all versions of the MCP Ruby SDK prior to 0.23.0.
What component is specifically vulnerable in CVE-2026-67431?
The vulnerability in CVE-2026-67431 is found in MCP::Server::Transports::StreamableHTTPTransport.