Squid before 4.9, when certain web browsers are used, mishandles HTML in the host (aka hostname) parameter to cachemgr.cgi.
https://www.debian.org/security/2020/dsa-4732
https://usn.ubuntu.com/4356-1/
https://lists.debian.org/debian-lts-announce/2020/07/msg00009.html
https://github.com/squid-cache/squid/pull/505
https://github.com/squid-cache/squid/pull/504
http://lists.opensuse.org/opensuse-security-announce/2020-05/msg00018.html