Analysis by: Robert Tacbad
ALIASES:

Ransom:Win32/Critroni (Windows), Trojan.Win32.Agentb.brnr (Kaspersky)

 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: 794,624 bytes
File Type: EXE
Memory Resident: Yes
Initial Samples Received Date: 29 Apr 2016

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 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.)

It drops the following files:

  • %System Root%\{randomly selected path}\!Decrypt-All-Files-{random extension name}.txt
  • %System Root%\{randomly selected path}\!Decrypt-All-Files-{random extension name}.bmp
  • %All Users Profile%\{random filename 2}.html or %All Users Profile%\Application Data\{random filename 2}.html – list of encrypted files
  • %User Profile%\My Documents\!Decrypt-All-Files-{random extension name}.txt <- Windows XP and below
  • %User Profile%\Documents\!Decrypt-All-Files-{random extension name}.txt <- Windows Vista and above
  • %User Profile%\My Documents\!Decrypt-All-Files-{random extension name}.bmp <- Windows XP and below
  • %User Profile%\Documents\!Decrypt-All-Files-{random extension name}.bmp <- Windows Vista and above
  • C:\Windows\Tasks\{random filename 3} <- Windows XP and below
  • C:\Windows\system32\Tasks\{random filename 3} <- Windows Vista and above

(Note: %System Root% is the Windows root folder, where it usually is C:\ 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.)

Autostart Technique

This Trojan adds the following registry entries to enable its automatic execution at every system startup:

HKEY_LOCAL_MACHINE\Software\Microsoft\
Windows\CurrentVersion\Run
{Default Value} = "%User Temp%\{random filename 1}.exe"

Other System Modifications

This Trojan modifies the following file(s):

  • It encrypts files and appends the extension name .{7 random characters}

It changes the desktop wallpaper by modifying the following registry entries:

HKEY_CURRENT_USER\Control Panel\Desktop
Wallpaper = %My Documents%\!Decrypt-All-Files-{ random extension name}.bmp

Other Details

This Trojan connects to the following possibly malicious URL:

  • http://www.{BLOCKED}bmvfnw4wp4.onion.lt
  • http://www.{BLOCKED}aew3e7ao3q.onion.cab
  • http://www.{BLOCKED}taew3e7ao3q.onion.cab
  • http://www.{BLOCKED}bmvfnw4wp4.onion.cab
  • http://www.{BLOCKED}aew3e7ao3q.tor2web.org
  • http://www.{BLOCKED}taew3e7ao3q.tor2web.org
  • http://www.{BLOCKED}bmvfnw4wp4.tor2web.org

  SOLUTION

Minimum Scan Engine: 9.800
FIRST VSAPI PATTERN FILE: 12.680.05
FIRST VSAPI PATTERN DATE: 28 Jul 2016
VSAPI OPR PATTERN File: 12.681.00
VSAPI OPR PATTERN Date: 29 Jul 2016

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

Delete this registry value

[ 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_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run
    • {Default Value} = "%User Temp%\{random filename 1}.exe"

Step 5

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.
  • %User Profile%\My Documents\!Decrypt-All-Files-{random extension name}.txt <- Windows XP and below
  • %User Profile%\Documents\!Decrypt-All-Files-{random extension name}.txt <- Windows Vista and above
  • %User Profile%\My Documents\!Decrypt-All-Files-{random extension name}.bmp <- Windows XP and below
  • %User Profile%\Documents\!Decrypt-All-Files-{random extension name}.bmp <- Windows Vista and above
  • %System Root%\{randomly selected path}\!Decrypt-All-Files-{random extension name}.txt
  • %System Root%\{randomly selected path}\!Decrypt-All-Files-{random extension name}.bmp
  • %All Users Profile%\{random filename 2}.html  or %All Users Profile%\Application Data\ {random filename 2}.html

Step 6

Deleting Scheduled Tasks

For Windows 2000, Windows XP, and Windows Server 2003:

  1. Open the Windows Scheduled Tasks. Click Start>Programs>Accessories>
    System Tools>Scheduled Tasks.
  2. Locate each task that has the following value in the Schedule column:
    • C:\Windows\Tasks\{random filename 3}  <- Windows XP and below
    • C:\Windows\system32\Tasks\{random filename 3} <- Windows Vista and above
  3. Right-click on the said file(s) with the aforementioned value.
  4. Click on Properties. In the Run field, check for the following string:
    Cmd /c /rd /s /q C:
  5. If the said string is found, delete the task.

For Windows Vista, Windows 7, Windows Server 2008, Windows 8, Windows 8.1, and Windows Server 2012:

  1. Open the Windows Task Scheduler. To do this:
    • On Windows Vista, Windows 7, and Windows Server 2008, click Start, type taskschd.msc in the Search input field, then press Enter.
    • On Windows 8, Windows 8.1, and Windows Server 2012, right-click on the lower left corner of the screen, click Run, type taskschd.msc, then press Enter.
  2. In the left panel, click Task Scheduler Library.
  3. In the upper-middle panel, locate each task that has the value in the Triggers column:
    • C:\Windows\Tasks\{random filename 3}  <- Windows XP and below
    • C:\Windows\system32\Tasks\{random filename 3} <- Windows Vista and above
  4. In the lower-middle panel, click the Actions tab. In the Details column, check for the following string:
    Cmd /c /rd /s /q C:
  5. If the said string is found, delete the task..

Step 7

Restart in normal mode and scan your computer with your Trend Micro product for files detected as RANSOM_CRYPCTB.YUYAKX. 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.

Step 8

Reset your Desktop properties

[ Learn More ]

Step 9

Restore encrypted files from backup.


Did this description help? Tell us how we did.