CVE-2026-63118: Modelcontextprotocol Ruby-SDK

Medium severity, CVSS 6.9. EPSS: 0.3% chance of exploitation in the next 30 days.

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 validate the HTTP Host or Origin request headers, which allows a malicious browser page to use DNS rebinding to reach a locally running MCP server and invoke exposed tools. This issue is fixed in version 0.23.0.

Affected products

• Modelcontextprotocol Ruby-SDK: before 0.23.0 (fixed in 0.23.0)

Published 2026-07-29. Last modified 2026-07-30.