Microsoft Is Waking Up to 'Fileless' Malware Threats

It took a while, but Microsoft's security engineers are starting to address concerns about 'fileless' malware. Redmond is looking to build additional defenses into Windows Defender ATP.

In a blog post this week, Microsoft acknowledged the rise of so-called "fileless" malware. This sort of malware does not use a physical file on the victim machine to run the malicious code.

Since this type of malware typically loads in the context of some legitimate process, usually triggered by some type of social engineering to get the victim to activate the malicious script -- an example is wscript.exe -- it does not require the creation of a disk file.

With no creation means, the antivirus is not get triggered when the malware strikes.

Not only that, but it will usually leave no trace on the disk after the "run and done," such that forensic analysis will find limited evidence of the malware, if any.

Great for the malware, bad for the security team.

Redmond realizes it will take some new tools to deal with this, and engineers there think Windows Defender ATP will have to go beyond file scanning as it grows. The company believes that new areas to look at will include behavior monitoring, memory scanning, as well as boot sector protection.

The Microsoft blog describes one example called Sharpshooter. The Sharpshooter technique was documented and published by MDSec in 2017.

It describes the process of Microsoft's efforts needing to go wide:

When the Sharpshooter technique became public, we knew it was only a matter time before it would be used it in attacks. We implemented a detection algorithm based on runtime activity rather than on the static script. In other words, the detection is effective against the Sharpshooter technique itself, thus against new and unknown threats that implement the technique.

The ability to generalize malware behavior of a script without having to do the static script analysis is a big win.

The engineers add their response, "targets a generic malicious behavior (a fingerprint of the malicious fileless technique). Script engines have the capability to log the APIs called by a script at runtime. This API logging is dynamic and is therefore not hindered by obfuscation: a script can hide its code, but it cannot hide its behavior."

When they tried it out, it worked better than they thought it would.

Besides nabbing the behavior that they thought would be there, they also found "a very stealthy .NET executable. The malware payload downloads data from its command-and-control (C&C) server via the TXT records of DNS queries. In particular, it downloads the initialization vector and decryption key necessary to decode the core of the malware."

While they think they found a penetration exercise here, the ability to find all these instantiations shows that behavior-based API approaches definitely have their place in the security struggle. It shows you things that could be otherwise missed.

But memory scanning and boot sector routines will also be necessary, according to Microsoft.

Windows Defender ATP will be faced with a host of challenges. By opening up its toolbox so it can respond to ever-changing malware can only be beneficial.

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— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.