The Open Connectivity Foundation UPnP specification before 2020-04-17 does not forbid the acceptance of a subscription request with a delivery URL on a different network segment than the fully qualified event-subscription URL, aka the CallStranger issue.
{
"binaries": [
{
"binary_name": "hostapd",
"binary_version": "1:2.1-0ubuntu1.7+esm3"
},
{
"binary_name": "wpagui",
"binary_version": "2.1-0ubuntu1.7+esm3"
},
{
"binary_name": "wpasupplicant",
"binary_version": "2.1-0ubuntu1.7+esm3"
}
],
"availability": "Available with Ubuntu Pro (Infra-only): https://ubuntu.com/pro"
}