Login
What happened to XProtect this week?
This week’s security data updates were quite a surprise. We’ve grown accustomed to Apple tweaking XProtect’s data most weeks, but this week was a bit different, and came with an update to XProtect Remediator as well, the first in four months. This article explores what they have brought.
Although this security data all goes under the name of XProtect, there are three different protection systems involved.
The traditional XProtect contains a set of ‘Yara’ rules used when performing Gatekeeper scans of new executable code, most notably when a quarantined app is first run, although recent macOS also runs XProtect checks on other occasions. Those rules are used to determine whether the code being scanned is known to be malicious, and if it’s found to be positive, macOS refuses to run that code and you’re told to trash the app.
XProtect Remediator only runs in Catalina and later, where it performs daily background scans to detect and remove software it believes to be malicious. It currently contains 24 separate scanning modules, each designed to detect and ‘remediate’ a different family of malware. Some of its modules also use the detection rules in traditional XProtect, so are improved by regular XProtect data updates. Surprisingly, if XProtect Remediator detects and removes malware, you aren’t notified, although that is recorded in the log and reported as an Endpoint Security event that can be detected by some third-party security software.
Inside the XProtect Remediator app are two files used by the third XProtect, which detects potentially malicious activity such as tampering with parts of a browser’s files. This is therefore referred to as XProtect Behavioural, or by the name it gives to the detection rules it uses, Bastion. Unlike the other two XProtects, this doesn’t rely on performing static checks, but is watching constantly for malicious activity. Although it records that in its local database, at present it doesn’t inform the user, but reports the activity to Apple, to help it acquire intelligence to improve the battle against malware.
XProtect
XProtect version 5304, provided by Apple on 8 July, makes substantial changes to its Yara detection rules to add what appears to be a new family of malware, code-named Bonzai. New rules refer to five different forms, which are most likely to be different components in the same malware, or separate variants, named Bonanza, Barricade, Blaster, Bonder and Banana. It’s likely that independent security researchers will identify these in the coming days, but for the moment the public name of this malware isn’t known.
Looking through these new Yara rules, they look most likely to be for a ‘stealer’, a type of malware that’s currently prevalent, and steals your secrets to send them to a remote server. There are references to Chrome, Brave, Edge and Firefox extensions, and most interestingly some of the malware has been compiled from code written in the Go language, which is becoming popular in cross-platform malicious code.
The last times that Apple added detection rules as substantial as these were in XProtect version 5284 for Adload and Bundlore, and in 5269 for Dolittle, each being major threats.
Bastion
Until now, the behavioural rules used by Bastion have evolved steadily, and the most rules added in one release has only been two, when XProtect Remediator version 123 came with rules 8 and 9, and changes to rule 7, back in January 2023. This update brings four new rules:
- Rule 14 detects sending AppleEvents to Safari, Firefox or Chrome.
- Rule 15 detects sending AppleEvents to the Finder or Terminal.
- Rule 16 detects Mach lookups for com.apple.pasteboard.1.
- Rule 17 detects writing shell files hidden in ~/ or /etc, such as ~/.zlogin, or /etc/zlogin.
The first two may be intended to detect AppleScript being used to control those browsers, the Finder or to run scripts in Terminal. Rule 16 may also be related to Apple’s recent announcement on controlling access to the pasteboard in macOS 26. Rule 17 concerns settings files commonly used by command shells, readily seen if you reveal hidden files for your Home folder.
These may well be related to Bonzai, and enable Apple to get a better idea of what is going on out here in the wild, and focus its efforts in improving its detection.
XProtect Remediator
Once samples of malware have been obtained, developing and testing new Yara rules to detect it is relatively quick, and often uses AI to accelerate the process. Writing a new scanning module for XProtect Remediator is more complicated, and takes more time. It may well be that an additional Bonzai scanner is already on its way, and might be delivered in a further update in the next couple of weeks, perhaps with some fine-tuning of the new Bastion rules. I’ll be keeping a lookout for those.
Above all, it will be interesting to see what changes are made in third-party security software, and how well those tackle what appears to be novel malware for macOS.
