TROJ_CRYPCTB.YWC

 Analysis by: Pearl Charlaine Espejo

 ALIASES:

Trojan.Cryptolocker.E (Symantec); RDN/Ransom-O!a (McAfee); Ransom:Win32/Critroni.B (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

Infection Channel:

Downloaded from the Internet


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:

870,912 bytes

File Type:

EXE

Memory Resident:

Yes

Initial Samples Received Date:

28 Apr 2015

Payload:

Connects to URLs/IPs, Encrypts files, Displays graphics/image, Displays message/message boxes

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 copies of itself into the affected system and executes them:

  • %User Temp%\{random filename}.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.)

It drops the following files:

  • %ProgramData%\{randomly selected folder}\{random filename} (for Windows Vista and above)
  • %All Users Profile%\Application Data\{randomly selected folder}\{random filename} (for Windows XP and below)
  • %All Users profile%\{random filename}.html (for Windows Vista and above)
  • %ProgramData%\{random filename}.html (for Windows Vista and above)
  • %All Users Profile%\Application Data\{random filename}.html (for Windows XP and below)
  • %User Profile%\Documents\!Decrypt-All-Files-{random letters}.bmp (for Windows Vista and above)
  • %User Profile%\Documents\!Decrypt-All-Files-{random letters}.txt (for Windows Vista and above)
  • %User Profile%\My Documents\!Decrypt-All-Files-{random letters}.bmp (for Windows XP and below)
  • %User Profile%\My Documents\!Decrypt-All-Files-{random letters}.txt (for Windows XP and below)

(Note: %ProgramData% is the Program Data folder, where it usually is C:\Program Files in Windows 2000, Windows Server 2003, and Windows XP (32- and 64-bit); C:\ProgramData in 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.. %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.)

Autostart Technique

This Trojan drops the following files:

  • %System%\Tasks\{random filename} (for Windows Vista and above)
  • %Windows%\Tasks\{random filename}.job (for Windows XP and below)

(Note: %System% is the Windows system folder, where it usually is C:\Windows\System32 on all Windows operating system versions.. %Windows% is the Windows folder, where it usually is C:\Windows on all Windows operating system versions.)

Other System Modifications

This Trojan adds the following registry keys:

HKEY_CURRENT_USER\{no name} (for Windows Vista and above)

HKEY_CURRENT_USER\{symbol} (for Windows XP and below)

It modifies the following registry entries:

HKEY_CURRENT_USER\Control Panel\Desktop
WallpaperStyle = "0"

(Note: The default value data of the said registry entry is {user-defined}.)

HKEY_CURRENT_USER\Control Panel\Desktop
Wallpaper = "%User Profile%\{Documents or My Documents}\!Decrypt-All-Files-{random letters}.bmp"

(Note: The default value data of the said registry entry is {user-defined}.)

Other Details

This Trojan connects to the following possibly malicious URL:

  • http://{BLOCKED}4qk625n6.onion.gq
  • http://{BLOCKED}4qk625n6.onion.lt
  • http://{BLOCKED}4qk625n6.tor2web.{BLOCKED}gie.de
  • http://{BLOCKED}cad3kn7v.tor2web.org
  • http://{BLOCKED}4qk625n6.onion.cab
  • http://{BLOCKED}cad3kn7v.onion.cab

NOTES:
It appends the extension .{seven random characters} to the encrypted files. (ex: sample.docx.{seven random characters})

  SOLUTION

Minimum Scan Engine:

9.750

FIRST VSAPI PATTERN FILE:

11.634.04

FIRST VSAPI PATTERN DATE:

28 Apr 2015

VSAPI OPR PATTERN File:

11.635.00

VSAPI OPR PATTERN Date:

29 Apr 2015

Step 1

Before doing any scans, Windows XP, Windows Vista, and Windows 7 users must disable System Restore to allow full scanning of their computers.

Step 2

Note that not all files, folders, and registry keys and entries are installed on your computer during this malware's/spyware's/grayware's execution. This may be due to incomplete installation or other operating system conditions. If you do not find the same files/folders/registry information, please proceed to the next step.

Step 3

Restart in Safe Mode

[ Learn More ]

Step 4

Search and delete these files

[ Learn More ]
There may be some files that are hidden. Please make sure you check the Search Hidden Files and Folders checkbox in the "More advanced options" option to include all hidden files and folders in the search result.

  • %ProgramData%\{randomly selected folder}\{random filename} (for Windows Vista and above)
  • %All Users Profile%\Application Data\{randomly selected folder}\{random filename} (for Windows XP and below)
  • %All Users profile%\{random filename}.html (for Windows Vista and above)
  • %ProgramData%\{random filename}.html (for Windows Vista and above)
  • %All Users Profile%\Application Data\{random filename}.html (for Windows XP and below)
  • %User Profile%\Documents\!Decrypt-All-Files-{random letters}.bmp (for Windows Vista and above)
  • %User Profile%\Documents\!Decrypt-All-Files-{random letters}.txt (for Windows Vista and above)
  • %User Profile%\My Documents\!Decrypt-All-Files-{random letters}.bmp (for Windows XP and below)
  • %User Profile%\My Documents\!Decrypt-All-Files-{random letters}.txt (for Windows XP and below)
  • %System%\Tasks\{random filename} (for Windows Vista and above)
  • %Windows%\Tasks\{random filename}.job (for Windows XP and below)

Step 5

Restore this modified registry value

[ Learn More ]

Important: Editing the Windows Registry incorrectly can lead to irreversible system malfunction. Please do this only if you know how to or you can seek your system administrator’s help. You may also check out this Microsoft article first before modifying your computer's registry.

  • In HKEY_CURRENT_USER\Control Panel\Desktop
    • From: WallpaperStyle = "0"
      To: WallpaperStyle = "{User-defined}"
  • In HKEY_CURRENT_USER\Control Panel\Desktop
    • From: Wallpaper = "%User Profile%\{Documents or My Documents}\!Decrypt-All-Files-{random letters}.bmp"
      To: Wallpaper = "{User-defined}"

Step 6

Delete this registry key

[ Learn More ]

Important: Editing the Windows Registry incorrectly can lead to irreversible system malfunction. Please do this step only if you know how or you can ask assistance from your system administrator. Else, check this Microsoft article first before modifying your computer's registry.

  • In HKEY_CURRENT_USER
    • {no name}   (for Windows Vista and above)
  • In HKEY_CURRENT_USER
    • {symbol}   (for Windows XP and below)

Step 7

Scan your computer with your Trend Micro product to delete files detected as TROJ_CRYPCTB.YWC. If the detected files have already been cleaned, deleted, or quarantined by your Trend Micro product, no further step is required. You may opt to simply delete the quarantined files. Please check this Knowledge Base page for more information.


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