crsud is definitely related to security updates
After reading all of this thread, I went to the System Preferences and unchecked the "Automatically install important security updates" in the general pane of the Security panel. Then I checked this option again and Little Snitch immediately gave the "crsud wants to connect to swscan.apple.com" notification. I've tried this several times, so it is certain.
Of course this doesn't tell us what Apple is really up to...