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.
https://www.tenable.com/cyber-exposure/2020-threat-landscape-retrospective
https://www.kb.cert.org/vuls/id/339275
https://www.debian.org/security/2021/dsa-4898
https://www.debian.org/security/2020/dsa-4806
https://usn.ubuntu.com/4494-1/
https://lists.debian.org/debian-lts-announce/2020/12/msg00017.html
https://lists.debian.org/debian-lts-announce/2020/08/msg00013.html
https://lists.debian.org/debian-lts-announce/2020/08/msg00011.html
https://github.com/yunuscadirci/CallStranger
https://github.com/corelight/callstranger-detector
https://corelight.blog/2020/06/10/detecting-the-new-callstranger-upnp-vulnerability-with-zeek/
http://www.openwall.com/lists/oss-security/2020/06/08/2
http://packetstormsecurity.com/files/158051/CallStranger-UPnP-Vulnerability-Checker.html