Polycom VVX 500 and 601 devices 5.8.0.12848 and earlier allows man-in-the-middle attackers to obtain sensitive credential information by leveraging failure to validate X.509 certificates when used with an on-premise installation with Skype for Business.
https://www.syss.de/fileadmin/dokumente/Publikationen/Advisories/SYSS-2018-027.txt