Squid before 4.4 has XSS via a crafted X.509 certificate during HTTP(S) error page generation for certificate errors.
Squid before 4.9, when certain web browsers are used, mishandles HTML in the host (aka hostname) parameter to cachemgr.cgi.
The cachemgr.cgi web module of Squid through 4.7 has XSS via the user_name or auth parameter.