Looks like the cutoff for 10.9 getting security updates has now happened. Today's very important update is only being made available for 10.10 and 10.11.

https://support.apple.com/en-us/HT207130

This one is what was being exploited on the iPhone by some government malware to spy on specific targets. With the fixes out now, odds are this will transition to the more widely used malware quickly to target those not up to date or intentionally holding back on the major OS upgrades.