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: RubyGems: mcp

## 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…