RUSTSEC-2026-0189

DNS rebinding vulnerability in rmcp Streamable HTTP server transport

Prior to version 1.4.0, the `rmcp` crate's Streamable HTTP server transport did not validate the incoming `Host` header. This allowed a malicious public website, via a DNS rebinding attack, to send requests to an MCP server running on the victim's loopback or private-network interface. An attacker who convinced a victim to visit a malicious page could enumerate and invoke tools exposed by a locally running rmcp-based MCP server, read resources and prompts, and trigger side effects limited by the tools exposed by that server. Non-HTTP transports such as stdio and child-process transports are not affected. ## Patches The issue was fixed in `rmcp` 1.4.0 by adding default loopback-only host allowlist validation for the Streamable HTTP server transport. Incoming HTTP requests now validate the `Host` header and return HTTP 403 when the host is not allowed. Users should upgrade to `rmcp >= 1.4.0`. ## Workarounds If upgrading is not possible, place the MCP server behind a reverse proxy configured to reject requests whose `Host` header is not one of the expected hostnames. Do not bind the MCP server to `0.0.0.0` without such validation.