MCP Ruby SDK vulnerabilities

MCP Ruby SDK before version 0.23.0 in StreamableHTTPTransport reads the entire JSON-RPC POST body without size limits, allowing an unauthenticated remote attacker to exhaust process memory.

MCP Ruby SDK before version 0.23.0 does not bind a session ID to its owner, allowing an attacker with a stolen session ID to send tools/call requests in the victim's session.

A vulnerability in MCP Ruby SDK before version 0.23.0 allows process memory exhaustion via unbounded creation of ServerSession objects in StreamableHTTPTransport. Sessions do not expire by default, and repeated initialize requests accumulate objects.

A vulnerability in MCP Ruby SDK before version 0.23.0 allows process memory exhaustion by sending data without a newline in StdioTransport and Stdio client. The IO#gets function is used without a byte limit.

A vulnerability in MCP Ruby SDK before version 0.23.0 allows DNS rebinding attacks due to missing validation of Host and Origin headers in StreamableHTTPTransport. A malicious web page can access a local MCP server and invoke tools.

MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to version 0.9.2, the 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. Version 0.9.2 contains a patch.