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TROJ_CRYPCTB.YVG

March 16, 2015
 Analysis by: David John Agni
 ALIASES:

Ransom:Win32/Critroni.B (Microsoft), Trojan.Win32.Filecoder (Ikarus), W32/Filecoder.DA!tr (Fortinet), Win32/Filecoder.DA (ESET-NOD32)

 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:

712,686 bytes

File Type:

EXE

Memory Resident:

Yes

Initial Samples Received Date:

03 Mar 2015

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:

  • %Windows%\Tasks\{7 Random Character 2}.job
  • %All Users Profile%\Application Data\{hosts folder}\{7 Random Character 3}
  • %User Profile%\My Documents\!Decrypt-All-Files-{7 Random Characters 4}.bmp
  • %User Profile%\My Documents\!Decrypt-All-Files-{7 Random Characters 4}.txt

(Note: %Windows% is the Windows folder, where it usually is C:\Windows on all Windows operating system versions.. %All Users Profile% is the All Users folder, where it usually is C:\Documents and Settings\All Users on Windows 2000, Windows Server 2003, and Windows XP (32- and 64-bit); C:\ProgramData 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.. %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.)

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

  • %User Temp%\{7 Random Character}.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
Wallpaper = "%User Profile%\My Documents\!Decrypt-All-Files-{7 Random Characters 4}.bmp"

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

HKEY_CURRENT_USER\Control Panel\Desktop
WallpaperStyle = "0"

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

NOTES:

This Trojan drops the following files in various folders:

  • !Decrypt-All-Files-{7 Random Characters 4}.txt
  • !Decrypt-All-Files-{7 Random Characters 4}.bmp

It encrypts files with certain extensions in all physical and removable drives. It appends .{7 Random Characters 4} 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.