TROJ_CRYPCTB.WJ

December 19, 2014
 Analysis by: Anthony Joe Melgarejo
 ALIASES:

Ransom:Win32/Critroni.A (Microsoft)

 PLATFORM:

Windows

 OVERALL RISK RATING:
 DAMAGE POTENTIAL:
 DISTRIBUTION POTENTIAL:
 REPORTED INFECTION:
 INFORMATION EXPOSURE:

  • Threat Type: Trojan

  • Destructiveness: No

  • Encrypted:

  • In the wild: Yes

  OVERVIEW

This Trojan arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites.

  TECHNICAL DETAILS

File Size:

823,296 bytes

File Type:

EXE

Initial Samples Received Date:

17 Dec 2014

Arrival Details

This Trojan arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites.

Installation

This Trojan drops the following files:

  • %User Profile%\My Documents\AllFilesAreLocked {7 random characters}.bmp
  • %User Profile%\My Documents\DecryptAllFiles {7 random characters}.txt
  • %User Profile%\My Documents\{Random Filename 3}.html
  • %Windows%\Tasks\{Random Filename 2}.job

(Note: %User Profile% is the current user's profile folder, which is usually C:\Documents and Settings\{user name} on Windows 2000, XP, and Server 2003, or C:\Users\{user name} on Windows Vista and 7.. %Windows% is the Windows folder, where it usually is C:\Windows on all Windows operating system versions.)

It drops the following copies of itself into the affected system:

  • %User Temp%\{Random Filename 1}.exe

(Note: %User Temp% is the user's temporary folder, where it usually is C:\Documents and Settings\{user name}\Local Settings\Temp on Windows 2000, Windows Server 2003, and Windows XP (32- and 64-bit); C:\Users\{user name}\AppData\Local\Temp on Windows Vista (32- and 64-bit), Windows 7 (32- and 64-bit), Windows 8 (32- and 64-bit), Windows 8.1 (32- and 64-bit), Windows Server 2008, and Windows Server 2012.)

Other System Modifications

This Trojan modifies the following registry entries:

HKEY_CURRENT_USER\Control Panel\Desktop
Wallpaper = "%User Profile%\My Documents\AllFilesAreLocked {7 random characters}.bmp"

(Note: The default value data of the said registry entry is "{User Preference}".)

HKEY_CURRENT_USER\Control Panel\Desktop
TileWallpaper = "0"

(Note: The default value data of the said registry entry is {User Preference}.)

Other Details

This Trojan connects to the following possibly malicious URL:

  • {BLOCKED}du43jtxssofz.onion.cab

NOTES:

This Trojan encrypts files with certain extensions then appends .{7 random characters} as the new file extension of the encrypted files.

The registry modifications are for changing the affected system's wallpaper. The new wallpaper informs the user that his files are encrypted and displays the instructions on how to decrypt the files.