I had a conversation recently, which started out being about endpoint technologies. At one point in the conversation, the topic of AV came up. The question was, is there still value in AV?
I believe there is; I believe that AV, when managed properly, can be a valuable tool. However, what I've very often seen, through targeted threat response and DFIR analysis, is that AV isn't maintained or updated, and when it does detect something, that detection is ignored.
MS systems have had the Malicious Software Removal Tool (MSRT) installed for some time. This is a micro-scanner, designed to target specific instances of malware. Throughout many of the investigations I've done, I've seen where systems were infected with malware variants that should have been prevented by MSRT; however, in those instances, I've found that MSRT hasn't been kept up to date, and was last updated well prior to the infection.
Not long ago, I was assisting with some analysis work, and found that the customer was using WebRoot as their AV product. I found entries for AV detections in the Registry, and based on the timing, it was clear that while the product had detected and quarantined an instance of Dridex, the customer was still infected with ransomware. That was due to no one being aware of the detection, and as such, no one took action. The threat actor was able to find something else they could install that wasn't detected by the installed AV product, and proceed with their attack.
Over the years, I've had more than a few opportunities to observe threat actor behavior, through a combination of EDR telemetry and DFIR analysis. As such, I've seen more than a few methods for circumventing AV, and in particular, Windows Defender. Windows Defender is actually a pretty decent AV product; I've had my research interrupted, as when I would download a web shell or "malicious" LNK file for testing, and Windows Defender would wake up and quarantine the file. Very recently, I was conducting some analysis as part of an interview questionnaire, and wrote a Perl script to deobfuscate some code. I ran the script and redirected the output to a file, and Windows Defender pounced on the resulting file. Apparently, I did things correctly.
Again, I've seen threat actors disable Windows Defender through a variety of means, from stopping the service, to uninstalling the product. I've also seen more subtle "tweaks", such as adding path exclusions to the product, or just disabling the AV component via a Registry value. However, the attacks have proceeded, because the infrastructure lacked the necessary visibility to detect these system modifications. Further, there was no proactive threat hunting activity, not even an automated 'sweep' of the infrastructure, checking against various settings.
So, no...AV isn't dead. It's simply not being maintained, and no one is listening for alerts. AV can be very valuable. Not only can checking AV status be an effective threat hunting tool (in both proactive scanning and DFIR threat hunting), but I've also been using "check AV logs" as part of my malware detection process. This is because AV has always been a great place to look for indications of attacks.