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](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: - `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](https://github.com/modelcontextprotocol/rust-sdk/issues/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](https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-89vp-x53w-74fx) - Upstream CVE: [CVE-2026-42559](https://www.cve.org/CVERecord?id=CVE-2026-42559) - dynoxide release: [v0.9.13](https://github.com/nubo-db/dynoxide/releases/tag/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.