Patch that fixes the issue
If a privileged user diffs a hidden revision against another revision (hidden or not; or maybe even against emptiness), that diff maybe cached in Squid if an &smaxage parameter is passed, and subsequently served to non-privileged users.
I've attached a patch that fixes this by never exposing hidden content (the rest of the module does this too). I'm filing this in BZ because I'm unsure whether this warrants a security release or a hidden deployment or whatever.
attachment proprevisionscachepollution.patch ignored as obsolete