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.
{ "availability": "Available with Ubuntu Pro (Infra-only): https://ubuntu.com/pro", "binaries": [ { "binary_version": "1:2.1-0ubuntu1.7+esm3", "binary_name": "hostapd" }, { "binary_version": "2.1-0ubuntu1.7+esm3", "binary_name": "wpagui" }, { "binary_version": "2.1-0ubuntu1.7+esm3", "binary_name": "wpasupplicant" } ] }
{ "binaries": [ { "binary_version": "1:1.6.19+git20160116-1ubuntu0.1~esm1", "binary_name": "libupnp-dev" }, { "binary_version": "1:1.6.19+git20160116-1ubuntu0.1~esm1", "binary_name": "libupnp6" }, { "binary_version": "1:1.6.19+git20160116-1ubuntu0.1~esm1", "binary_name": "libupnp6-dev" } ] }
{ "availability": "No subscription required", "binaries": [ { "binary_version": "1.2.3-0ubuntu0.20.04.1", "binary_name": "gir1.2-gupnp-1.2" }, { "binary_version": "1.2.3-0ubuntu0.20.04.1", "binary_name": "libgupnp-1.2-0" }, { "binary_version": "1.2.3-0ubuntu0.20.04.1", "binary_name": "libgupnp-1.2-dev" } ] }