rmcp CVE-2026-42559

Summary

Prior to version 1.4.0, the rmcp crate's Streamable HTTP server transport (crates/rmcp/src/transport/streamable_http_server/) did not validate the incoming Host header. This allowed a malicious public website, via a DNS rebinding attack, to send authenticated requests to an MCP server running on the victim's loopback or private-network interface - violating the MCP specification's transport security guidance.

Impact

An attacker who convinces a victim to visit a malicious page can:

• Enumerate and invoke any tool exposed by a locally-running rmcp-based MCP server.
• Read resources, prompts, and any state accessible via the MCP session.
• Trigger side effects (file writes, shell execution, API calls, etc.) limited only by what tools the victim's server exposes.

Because MCP servers frequently run with the user's privileges and expose developer tooling (filesystems, shells, browser control, language servers, etc.), the practical impact can extend to arbitrary code execution on the victim's machine.

Affected Versions

rmcp < 1.4.0 - all prior releases of the Streamable HTTP server transport. Non-HTTP transports (stdio, child-process) are not affected.

Patched Versions

rmcp >= 1.4.0 (current: 1.5.1).

Patch

Fixed in PR #764 (commit 8e22aa2), released as v1.4.0 on 2026-04-09:

• StreamableHttpServerConfig::allowed_hosts now defaults to a loopback-only allowlist: ["localhost", "127.0.0.1", "::1"].
• All incoming HTTP requests pass through validate_dns_rebinding_headers(), which parses the Host header and returns HTTP 403 if the host is not on the allowlist.
• Public deployments can configure an explicit allowlist via StreamableHttpService::with_allowed_hosts(...), or opt out (not recommended without an upstream reverse proxy that validates Host) via disable_allowed_hosts().

This fix validates the Host header only. Origin header validation is tracked as a defense-in-depth follow-up in #822 and is not required to block the DNS rebinding attack described here - the browser cannot forge the Host header sent to the rebound server.

Workarounds for Unpatched Users

• Upgrade to rmcp >= 1.4.0.
• If upgrade is not possible, place the MCP server behind a reverse proxy (e.g. nginx, Caddy) configured to reject requests whose Host header is not one of your expected hostnames.
• Do not bind the MCP server to 0.0.0.0 without such a proxy.

Resources

• PR: https://github.com/modelcontextprotocol/rust-sdk/pull/764
• Issue: https://github.com/modelcontextprotocol/rust-sdk/issues/815
• Follow-up (Origin validation): https://github.com/modelcontextprotocol/rust-sdk/issues/822
• MCP transport security guidance: https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#security-warning

Related advisories (same class of vulnerability)

• TypeScript SDK: GHSA-w48q-cv73-mx4w
• Python SDK: GHSA-9h52-p55h-vw2f
• Go SDK: GHSA-xw59-hvm2-8pj6
• Java SDK: GHSA-8jxr-pr72-r468