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

Summary

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. Affects 0.9.3 to 0.9.12. The stdio transport (dynoxide mcp without --http, which is the default) is not affected.

Impact

If a user is running dynoxide mcp --http (or dynoxide serve --mcp) on their machine and then visits a malicious web page, the attacker's JavaScript can call any MCP tool exposed by the running dynoxide instance.

Reachable tools include reads (get_item, query, scan, batch_get_item, describe_table, list_tables) and writes (put_item, update_item, delete_item, create_table, batch_write_item).

Any data in tables that the local dynoxide instance has access to can be read, modified, or destroyed.

Patches

dynoxide 0.9.13 closes both the named CVE and a related cross-origin CSRF gap:

1. DNS rebinding (the named CVE). rmcp is upgraded from 1.1.1 to 1.6.0. rmcp 1.4+ ships a default Host-header allowlist (["localhost", "127.0.0.1", "::1"]) which rejects requests carrying any other Host header with a 403.

2. Defence in depth. Explicit allowed_hosts and allowed_origins lists are now set on StreamableHttpServerConfig directly. The Host allowlist protects against a future rmcp default flip. The Origin allowlist closes a related cross-origin CSRF gap that the Host check alone does not address: a malicious page could fetch the loopback endpoint with mode: 'no-cors', the Host header would match (it's the literal loopback address the browser is connecting to), but the Origin header would otherwise have been unchecked.

Native MCP clients that don't send an Origin header (Claude Code, Cursor, the dynoxide CLI) are unaffected by the Origin check and continue to work.

Workarounds

• Upgrade to dynoxide 0.9.13.
• If upgrade is not immediately possible: do not run the MCP HTTP transport. Run dynoxide mcp (stdio, the default) instead of dynoxide mcp --http, and don't pass --mcp to dynoxide serve.

Resources

• Upstream rmcp advisory: GHSA-89vp-x53w-74fx
• Upstream CVE: CVE-2026-42559
• dynoxide release: v0.9.13
• MCP transport security guidance: https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#security-warning

Credits

Vulnerability identified via GitHub Dependabot alert on the transitive rmcp dependency.

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.

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.