The python API made a restrictive-looking configuration unsafe by default. A caller could configure only reverse- proxy credential routes, put the child in CapabilitySet.proxy only, and reasonably expect network access to be limited to those routes. Instead, because empty allowed hosts meant allow-all inside nono-proxy, the child could use the local proxy as a transparent CONNECT tunnel to non-route nominated hosts (not including metadata endpoints).

That is an authorization bypass / policy confusion issue:

This should be classified as medium severity by default, potentially high if users rely on route-only configs for strict egress control around untrusted code or sensitive credentials. The fix is security-relevant because it changes the default from implicit allow-all to explicit opt-in.