Security researchers have noticed a malicious campaign that used Windows event logs to store malware, a technique that has not been previously documented publicly for attacks in the wild.
The method enabled the threat actor behind the attack to plant fileless malware in the file system in an attack filled with techniques and modules designed to keep the activity as stealthy as possible.
The dropper copies the legitimate OS error handling file WerFault.exe to ‘C:\Windows\Tasks’ and then drops an encrypted binary resource to the ‘wer.dll’ (Windows Error Reporting) in the same location, for DLL search order hijacking to load malicious code.
Legezo says that the dropper’s purpose is to loader on the disk for the side-loading process and to look for particular records in the event logs (category 0x4142 – ‘AB’ in ASCII. If no such record is found, it writes 8KB chunks of encrypted shellcode, which are later combined to form the code for the next stager.
“The dropped wer.dll is a loader and wouldn’t do any harm without the shellcode hidden in Windows event logs” – Denis Legezo, lead security researcher at Kaspersky
The new technique analyzed by Kaspersky is likely on its way to becoming more popular as source code for injecting payloads into Windows event logs has been available in the public space for a brief period.