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": "No subscription required",
    "binaries": [
        {
            "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",
            "binary_version": "1.2.3-0ubuntu0.20.04.1"
        }
    ]
}{
    "availability": "Available with Ubuntu Pro (Infra-only): https://ubuntu.com/pro",
    "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"
        }
    ]
}{
    "binaries": [
        {
            "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",
            "binary_version": "1:1.6.19+git20160116-1ubuntu0.1~esm1"
        }
    ]
}