Backup software seems well positioned to detect cryptowall infections as they happen. If I could see “possible cryptowall infection” in my urBackup computer list I could go out and fix it before users start complaining and before the good files cycle off the backup server.
UrBackup is backup software, not antivirus. The best way to mitigate this is to run actual antivirus that will detect the infection and then have a retention policy on UrBackup that is long enough to keep your files to recover if you get hit by it.