Summary

MCP::Server::Transports::StreamableHTTPTransport (the Rack-mountable Streamable HTTP transport in the mcp gem) processes every incoming JSON-RPC request without ever inspecting the HTTP Host or Origin request headers. There is no AllowedHosts/AllowedOrigins allowlist and no DNS-rebinding guard anywhere in the transport. A local MCP server that binds a loopback or LAN HTTP port is therefore reachable by any web origin a victim's browser visits, via a DNS-rebinding attack: a malicious page rebinds its own hostname to 127.0.0.1, then drives the local MCP server cross-origin to enumerate and invoke its tools and exfiltrate their output. This is the standard browser-driven local-service attack that the MCP Streamable HTTP guidance exists to prevent.

Impact

An attacker who can get a victim to open a web page can reach any MCP server the victim runs locally over the Streamable HTTP transport (e.g. a developer-tools or filesystem MCP server on localhost). Because the transport issues a session and dispatches tools/list/ tools/call from a foreign Host/Origin with no rejection, the attacker can drive arbitrary server-exposed tools and read their results, exfiltrating local data (files, secrets, command output) to the attacker's origin. The blast radius is whatever the locally-running MCP server exposes. For MCP servers wired to filesystem, shell, or credential tools, this is sensitive-data disclosure and, depending on the tool set, local action execution.