An Attack Using Only System Tools Is Not Invisible – You Just Have to Look Harder

Microsoft's team behind Windows Defender ATP (the commercial version of the company's Windows Defender free antivirus) has identifieda new "fileless" malware campaign that can bypass standard antivirus tools. Andrea Lelli wrote up a blog post about how they did it.

Everything started when telemetry to WD ATP showed a sharp increase at the end of May/beginning of June in the use of the Windows Management Instrumentation Command-line (WMIC) tool to run a script (a technique that MITRE refers to XSL Script Processing), indicating a fileless attack. This was their first tip-off that something was going on.

They then found the object of all this was to install the backdoor called "Astaroth" directly into memory. The backdoor was first found in 2018. This info-stealing malware is known for stealing sensitive information such as credentials, keystrokes and other data, which it then exfiltrates and can send to a remote attacker. It was previously seen by Cybereason in a campaign against Brazilian and European users in February of this year.

Microsoft identified the path of attack thusly: "A malicious link in a spear-phishing email leads to an LNK file" they said. "When double-clicked, the LNK file causes the execution of the WMIC tool with the '/Format' parameter, which allows the download and execution of a JavaScript code. The JavaScript code in turn downloads payloads by abusing the Bitsadmin tool."

After the payload is decrypted into DLLs, the Regsvr32 tool is used to load one of the decoded DLLs, which in turn decrypts and loads other files until the final payload, Astaroth, is injected into the Userinit process.

The attack uses no external tools. It uses legitimate tools that are already present on the target system to obscure what is going on as regular activity. The results of this kind of attack leaves no disk files that traditional AV products can scan.

Microsoft strongly makes the point that being fileless doesn't mean being invisible; and it certainly doesn't mean being undetectable. They think that some of the fileless techniques can be so unusual and anomalous that they will draw immediate attention to the malware, in the same way that a bag of money moving by itself would.

In some ways, Microsoft's alert is a way to show off their Defender ATP product. By being able to detect a "fileless" attack they are trying to show what they perceive as a technological superiority compared to other products.

— 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.