I would say "...a malicious templates owner/catalog may conduct an XSS attack once..." (also removing the comma after catalog). Would also remove the inner parenthesis "(session cookies/CSRF tokens)" and s/informartion/information/. Finally I would say "setups using Heat together with Horizon" so that it's clear it's the combo that is affected.
I would say "...a malicious templates owner/catalog may conduct an XSS attack once..." (also removing the comma after catalog). Would also remove the inner parenthesis "(session cookies/CSRF tokens)" and s/informartion/ information/ . Finally I would say "setups using Heat together with Horizon" so that it's clear it's the combo that is affected.