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BKDR_TENGO.A
Malware type: Backdoor
Destructive: No
Platform: Windows 2000, Windows Server 2003, Windows XP (32-bit, 64-bit), Windows Vista (32-bit, 64-bit), Windows 7 (32-bit, 64-bit)
Encrypted: No
In the wild: Yes

Overview


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

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 executes commands from a remote malicious user, effectively compromising the affected system. It connects to a website to send and receive information.

Technical Details


Ports used: TCP Port 443
File size: 65,536 bytes
File type: DLL
Memory resident: No
Initial samples received date: 25 Apr 2013
Payload: Connects to URLs/IPs, Compromises system security, Downloads files, Executes files

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 files:

  • %WINDOWS%\$NtUninstallKB080515$\Usblog_DXM.log

It creates the following folders:

  • %WINDOWS%\$NtUninstallKB080515$

Other System Modifications

This backdoor deletes the following files:

  • {Malware Path}\msvidc161.dll
  • %System%\winmm16.dll

(Note: %System% is the Windows system folder, which is usually C:\Windows\System32.)

It adds the following registry entries:

HKEY_CLASSES_ROOT\exefile
NeverShowExt = "0"

It modifies the following registry entries:

HKEY_CURRENT_USER\Software\Microsoft\
Windows\CurrentVersion\Explorer\
Advanced
Hidden = "0"

(Note: The default value data of the said registry entry is 1.)

HKEY_CURRENT_USER\Software\Microsoft\
Windows\CurrentVersion\Explorer\
Advanced
HideFileExt = "1"

(Note: The default value data of the said registry entry is 0.)

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Explorer\
Advanced\Folder\Hidden\
SHOWALL
CheckValue = "0"

(Note: The default value data of the said registry entry is 1.)

Backdoor Routine

This backdoor executes the following commands from a remote malicious user:

  • Get Malware Information
  • Update Itself
  • Update C&C servers/IP Information
  • Perform sleep command
  • File Manipulation (View added files, Rename, Delete, Check filetype, Modify, Change attiributes, Change filetime, Copy)
  • Directory Manipulation (Change Current, Add, View directories)
  • Reboot and Disconnect
  • Perform shell command
  • View System Informations (Mac Address, Computer Name, Wan/Local IP, Disk Information, Display Mode, CPU Information, Path of System Directory, OS Inofrmation, Drive Information, Malware Uptime)
  • Load other modules
  • Execute a program (with option as Administrator)
  • List IPs
  • List/Disconnect TCP Connections
  • List/Kill Processes
  • Download File
  • Upload File
  • Enumerate/Install Services
  • Load keylogging module (mskb32.dll)

It connects to the following websites to send and receive information:

  • {BLOCKED}.{BLOCKED}.204.62
  • {BLOCKED}.{BLOCKED}.102.244
  • {BLOCKED}.{BLOCKED}.103.42
  • {BLOCKED}.{BLOCKED}.10.239
  • {BLOCKED}.{BLOCKED}.145.118
  • {BLOCKED}i89.{BLOCKED}s.info
  • http://{BLOCKED}02.{BLOCKED}z.info/dxj

NOTES:

It renames the file %System%\winmm.dll to %System%\winmm16.dll. It then drops a copy of itself as %System%\winmm.dll.

It renames the file %System%\winmm.DLL to %System%\winmm16.DLL then drops a copy of itself as %System%\winmm.DLL.

This backdoor copies files with the following format below from removable drives connected through the machine:

  • .doc
  • .pdf
  • .tif
  • .xls
  • .ppt

This will be saved in the folder %SystemRoot%\$NtUninstallKB080515$, which can be retrieved later via remote commands.

Solution


Minimum scan engine: 9.300
First VSAPI Pattern File: 9.876.06
First VSAPI Pattern Release Date: 25 Apr 2013
VSAPI OPR Pattern Version: 9.877.00
VSAPI OPR Pattern Release Date: 25 Apr 2013

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

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_CLASSES_ROOT\exefile
    • NeverShowExt = "0"

Step 3

Restore this modified 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\Explorer\Advanced
    • From: Hidden = "0"
      To: Hidden = "1"
  • In HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
    • From: HideFileExt = "1"
      To: HideFileExt = "0"
  • In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Advanced\Folder\Hidden\SHOWALL
    • From: CheckValue = "0"
      To: CheckValue = "1"

Step 4

Search and delete these folders

[ Learn more ]
Please make sure you check the Search Hidden Files and Folders checkbox in the More advanced options option to include all hidden folders in the search result.
 
  • %WINDOWS%\$NtUninstallKB080515$\

Step 5

Scan your computer with your Trend Micro product to delete files detected as BKDR_TENGO.A. 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 6

Restore this file from backup only Microsoft-related files will be restored. If this malware/grayware also deleted files related to programs that are not from Microsoft, please reinstall those programs on you computer again.

  • {Malware Path}\msvidc161.dll
  • %System%\winmm16.dll
  • %System%\winmm.dll


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Analysis By: Mark Joseph Manahan

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