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CVE-2026-42559 — RMCP: DNS rebinding vulnerability in rmcp Streamable HTTP server transport

Affected: modelcontextprotocol rust-sdk

RMCP is an official Rust SDK for the Model Context Protocol. 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. This vulnerability is fixed in 1.4.0.

GHSA-89vp-x53w-74fx — rmcp Streamable HTTP server transport has a DNS rebinding vulnerability

Affected: crates.io: rmcp

## 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](https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#security-warning).

## 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](https://github.com/modelcontextprotocol/rust-sdk/pull/764) (commit `8e22aa2`), released as v1.4.0 on 2026-04-09:

- `Stream…

RUSTSEC-2026-0140 — DNS rebinding and cross-origin CSRF in dynoxide's MCP HTTP transport

Affected: crates.io: dynoxide-rs

dynoxide's MCP HTTP transport was vulnerable to DNS rebinding via its transitive `rmcp` dependency, plus a related cross-origin CSRF gap.

A malicious web page could make the user's browser send requests to a local `dynoxide mcp --http` or `dynoxide serve --mcp` server with a non-loopback `Host` header, which the server would then process. The Host check alone did not close a related cross-origin CSRF vector: a page could `fetch` the loopback endpoint with `mode: 'no-cors'`, and the Host header would match while the Origin header went unchecked.

Affected MCP write tools include `put_item`, `update_item`, `delete_item`, `create_table`, and `batch_write_item`.

The stdio transport (`dynoxide mcp` without `--http`) is not affected.

## Patches

dynoxide 0.9.13 closes both vectors:

- Upgrades `rmcp` from 1.1.1 to 1.6.0 (which ships a default Host-header allowlist). - Sets explicit `allowed_hosts` and `allowed_origins` on `StreamableHttpServerConfig`.