Fileless Powershell Dropper

Published: 2022-10-17
Last Updated: 2022-10-18 08:04:23 UTC
by Xavier Mertens (Version: 1)
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I found an interesting Powershell script that drops a malware on the victim's computer. The dropped malware is not new (It's kinda old, though) but the dropper has a very low Virustotal score. The script was detected by one of my hunting rules on VT. It is called "autopowershell.ps1" and has only a score of 3/61 (SHA256:3750576978bfd204c5ac42ee70fb5c21841899878bacc37151370d23e750f8c4)[1]. By "fileless", it means that the malware tries to reduce at the minimum interactions with the file system. But, to achieve persistence, it must write something on the disk. Most of the time, it's done through registry keys. That's what happens with this sample:

Suspicious keys are stored in "HKCU\Software\Classes\QDSbIMUygFKR". Note that key names are not randomized, which makes them very interesting IOCs.

In "MSIQ", we find another Powershell script that will be used for persistence. 

In "{08CA0AB0-E83C-4BA2-B013-B6359F962B16}", we find the encrypted payload

In "{62352566-EB1A-4C27-81FD-DDEE4E4CFF50}", we find the key to decrypt the payload

The second Powershell script will extract another Base64-encoded script that provides all the classic functions to perform ReflectivePEInjection from PowerSploit[2]. The injected payload is read from the registry, decrypted and injected. It's a DLL file with a VT score of 30/62 (SHA256:9b3e2e56863fc5a85c5c9f16a82a55c5bc88f5ed049f2e8b21e4e8895e25ec21)[3]. Nothing new, it was first uploaded in 2020!


Xavier Mertens (@xme)
Senior ISC Handler - Freelance Cyber Security Consultant

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