I don't have CIS installed anymore but I had recently and the firewall never blocked Avast. Avast is first in the list of trusted vendors so no issue with Def+, all avast processes are allowed to connect (including avast.setup), either following an alert or automatically, depending on the firewall mode, so...the only (minor) issue that I know of is that avast.setup being generated temporarily, if you purge the firewall and def+ rules, its permissions will be removed.But it's not blocked.You're prompted again when needed.