Microsoft Warns about the new Campaign that Delivers FlawedAmmyy RAT via Weaponized MS Excel Documents

Microsoft uncovered a new campaign with a sophisticated infection chain delivering notorious FlawedAmmyy RAT as a final payload. The attack starts with an email that contains .XLS attachments and the contents of the email in the Korean language.

Previous campaigns that involve FlawedAmmyy RAT are carried out by TA505 threat actors, upon successful execution of backdoor let an attacker to control the machine remotely, manages the files, captures the screen.

How the Infection Occurs

Malicious .XLS file delivered through email, when the file executed it automatically runs a macro function that runs Windows Installer msiexec.exe used for download & installing MSI and MSP packages.

The downloaded MSI archive contains a digitally signed executable that is extracted and executed, it decrypts and runs another executable wsus.exe in memory, which in turn decrypts and runs the final payload in the memory.

The final payload that delivered directly in memory is the FlawedAmmyy, according to Microsoft Security Intelligence. The FlawedAmmyy digitally signed using a code signing certificate issued Thawte to the company Dream Body Limited. It appears the rat was signed and timestamped on June 19 and the samples detected on June 22.

The FlawedAmmy RAT functions

• Remote Desktop control
• File system manager
• Proxy support
• Audio Chat

Earlier this year TA505 distributed FlawedAmmyy RAT via weaponized MS Excel documents with malicious Excel 4.0 macro which is hard to detect by standard security controls.

Last October Cybercriminals used IQY Files to deliver FlawedAmmyy malware and executed the backdoor through PowerShell Process.

IoCs (SHA-256):

0e91e6e17f8c8e2f1ae29e13f116c8611cb7679607695eed355025295fb1999a (.xls),
19d8993c742fc1a7c651ab3dba4d8c7f5e142a8421e22dd0c20c2db2d5dccffd (MSI),
cb114123ca1c33071cf6241c3e5054a39b6f735d374491da0b33dfdaa1f7ea22 (digitally signed executable inside MSI)
c2c6f548fe6832c84c8ab45288363b78959d6dda2dd926100c5885de14c4708b (digitally signed wsus.exe),
6860de46fdea393bd48ca000ecff4047920a56728b7945f95a6ca0801c278097 (FlawedAmmyy RAT)

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