The HTTP MCP server-registry backend factory (atomic agents/mcp registry/http.py, make http mcp server registry backend from url) accepts both http and https schemes. Catalog entries carry command/args that are type-validated but content-unrestricted, and are later spawned as local stdio subprocesses by MCPClientPool. Over a cleartext http:// catalog URL, a network man-in-the-middle can rewrite the catalog response to inject an arbitrary command/args and obtain code execution on the agent host, with no LLM involvement. The Policy MCP allowlist is not a default mitigation (mcp allow fn defaults to None), so absent an operator-authored allowlist every resolved spec connects.

Affected: mcp registry/http.py, all versions through 1.0.0. (The https path is sound: httpx defaults to verify=True, follow redirects=False.)

Fix: require https by default and gate http:// behind a loud explicit opt-in. Defense-in-depth: allowlist the resolved command basename (or require confirmation) before any registry-sourced subprocess spawn. Document the consequence in spec/36.