When the apiserver is proxying a request to a node though one of its addresses, it performs a filter validation. If the address type is a DNS record (Hostname, ExternalDNS, InternalDNS), the apiserver performs two DNS queries, one for filter validation, another for proxying the request. If the attacker sets the hostname to a custom DNS server, that is able return different values with zero TTL, it is possible to bypass that filter.