Analysis by: Mc Justine De Guzman
ALIASES:

Backdoor:Win32/LimeRat.YA!MTB (Microsoft) ;

 PLATFORM:

Windows

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

  • Threat Type: Backdoor

  • Destructiveness: No

  • Encrypted:

  • In the wild: Yes

  OVERVIEW

Infection Channel: Dropped by other malware, Downloaded from the Internet

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

It runs certain commands that it receives remotely from a malicious user. Doing this puts the affected computer and information found on the computer at greater risk.

It terminates itself if it detects it is being run in a virtual environment.

  TECHNICAL DETAILS

File Size: 5,242,880 bytes
File Type: EXE
Memory Resident: Yes
Initial Samples Received Date: 11 Jan 2020
Payload: Deletes files, Connects to URLs/IPs, Steals information

Arrival Details

This Backdoor 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 Backdoor drops the following copies of itself into the affected system and executes them:

  • %Application Data%\Wservices.exe

(Note: %Application Data% is the current user's Application Data folder, which is usually C:\Documents and Settings\{user name}\Application Data on Windows 2000(32-bit), XP, and Server 2003(32-bit), or C:\Users\{user name}\AppData\Roaming on Windows Vista, 7, 8, 8.1, 2008(64-bit), 2012(64-bit) and 10(64-bit).)

It adds the following mutexes to ensure that only one of its copies runs at any one time:

  • {Derived from Processor ID, BIOS Serial Number, Base Board Serial Number, Video Controller Name}

Autostart Technique

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

HKCU\Software\Microsoft\
Windows\CurrentVersion\Run
Wservices.exe = %Application Data%\Wservices.exe
(Created if the user has no administrative privileges)

Other System Modifications

This Backdoor deletes the following files:

  • %Application Data%\Wservices.exe:Zone.Identifier

(Note: %Application Data% is the current user's Application Data folder, which is usually C:\Documents and Settings\{user name}\Application Data on Windows 2000(32-bit), XP, and Server 2003(32-bit), or C:\Users\{user name}\AppData\Roaming on Windows Vista, 7, 8, 8.1, 2008(64-bit), 2012(64-bit) and 10(64-bit).)

It adds the following registry entries:

HKCU\Software\{Mutex created}
Rans-Status =

HKCU\Software\{Mutex created}
USB =

HKCU\Software\{Mutex created}
Flood =

Backdoor Routine

This Backdoor executes the following command(s) from a remote malicious user:

  • Create Registry Key
  • Modify Registry Value
  • Get Registry Value
  • Creates a bitmap image
  • Loads an assembly file
  • Checks for a presence of a Removable Drive
  • Download a file
  • Execute the downloaded file

It connects to the following URL(s) to send and receive commands from a remote malicious user:

  • https://{BLOCKED}in.com/raw/DDTVwwbu

Information Theft

This Backdoor gathers the following data:

  • Processor ID
  • Win32 BIOS Serial Number
  • Win32 Baseboard Serial Number
  • Video/Graphics Card Name
  • Computer's Username
  • Computer's Operating System
  • Processor Architecture
  • Last edited time of the Executable Path
  • Installed Antivirus products
  • Processor Name (Intel or not)
  • Microsoft .NET Framework Version

Other Details

This Backdoor adds the following registry keys:

HKCU\Software\{Mutex created}

It terminates itself if it detects it is being run in a virtual environment.

It does not proceed to its malicious routine if it detects that it is being debugged.

It adds the following scheduled tasks:

  • Name: LimeRAT-Admin
    • Trigger: On log on of any user
    • Action: %Application Data%\Wservices.exe
    (Created if the user has administrative privileges)

(Note: %Application Data% is the current user's Application Data folder, which is usually C:\Documents and Settings\{user name}\Application Data on Windows 2000(32-bit), XP, and Server 2003(32-bit), or C:\Users\{user name}\AppData\Roaming on Windows Vista, 7, 8, 8.1, 2008(64-bit), 2012(64-bit) and 10(64-bit).)

  SOLUTION

Minimum Scan Engine: 9.850
FIRST VSAPI PATTERN FILE: 15.650.03
FIRST VSAPI PATTERN DATE: 29 Jan 2020
VSAPI OPR PATTERN File: 15.651.00
VSAPI OPR PATTERN Date: 30 Jan 2020

Step 1

Trend Micro Predictive Machine Learning detects and blocks malware at the first sign of its existence, before it executes on your system. When enabled, your Trend Micro product detects this malware under the following machine learning name:

     
    • Troj.Win32.TRX.XXPE50FFF034

Step 2

Before doing any scans, Windows 7, Windows 8, Windows 8.1, and Windows 10 users must disable System Restore to allow full scanning of their computers.

Step 3

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 4

Restart in Safe Mode

[ Learn More ]

Step 5

Deleting Scheduled Tasks

The following {Task Name} - {Task to be run} listed should be used in the steps identified below:

  • LimeRAT-Admin - %Application Data%\Wservices.exe

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 Name} values listed above in the Name column.
  3. Right-click on the said file(s) with the aforementioned value.
  4. Click on Properties. In the Run field, check for the listed {Task to be run}.
  5. If the strings match the list above, 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 Name} values listed above in the Name column.
  4. In the lower-middle panel, click the Actions tab. In the Details column, check for the {Task to be run} string.
  5. If the said string is found, delete the task.

Step 6

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_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
    • Wservices.exe = %Application Data%\Wservices.exe
  • In HKEY_CURRENT_USER\Software\{Mutex created}
    • Rans-status = ""
  • In HKEY_CURRENT_USER\Software\{Mutex created}
    • USB = ""
  • In HKEY_CURRENT_USER\Software\{Mutex created}
    • Flood = ""

Step 7

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\Software\{Mutex created}

Step 8

Search and delete this file

[ 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.
  • %Application Data%\Wservices.exe

Step 9

Restart in normal mode and scan your computer with your Trend Micro product for files detected as Backdoor.MSIL.LIMERAT.AC. 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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