In Xymon through 4.3.28, a stack-based buffer overflow vulnerability exists in the alert acknowledgment CGI tool because of expansion in acknowledge.c.
https://lists.xymon.com/archive/2019-July/046570.html
https://lists.debian.org/debian-lts-announce/2019/08/msg00032.html
https://github.com/svn2github/xymon/blob/master/branches/4.3.28/web/acknowledge.c