dynoxide: DNS rebinding and cross-origin CSRF via MCP HTTP transport

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.