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BKDR_ZEGOST.TIDAFAT

July 24, 2018
 Analysis by: John Anthony Banes
 ALIASES:

Backdoor:Win32/Zegost (Microsoft)

 PLATFORM:

Windows

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

  • Threat Type: Backdoor

  • Destructiveness: No

  • Encrypted: No

  • In the wild: Yes

  OVERVIEW

Infection Channel:

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 may be dropped by other malware.

As of this writing, the said sites are inaccessible.

It deletes the initially executed copy of itself.

  TECHNICAL DETAILS

File Size:

9,728 bytes

File Type:

EXE

Memory Resident:

Yes

Initial Samples Received Date:

23 Jul 2018

Payload:

Connects to URLs/IPs, Downloads 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.

It may be dropped by the following malware:

  • TROJ_DLOADR.AUSUML

Installation

This Backdoor drops the following files:

  • %User Temp%\{random numbers}.vbs - deletes malware copy

(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 copies of itself into the affected system:

  • %Program Files%\DFnsa.exe

(Note: %Program Files% is the Program Files folder, where it usually is C:\Program Files on all Windows operating system versions; C:\Program Files (x86) for 32-bit applications running on Windows 64-bit operating systems.)

Autostart Technique

This Backdoor registers itself as a system service to ensure its automatic execution at every system startup by adding the following registry entries:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\{random name}
Description = {random}

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\{random name}
MarkTime = {yyyy-mm-dd hh:mm}

It adds and runs the following services:

  • Service path: %Program Files%\DFnsa.exe
  • Service name: {random}

(Note: %Program Files% is the Program Files folder, where it usually is C:\Program Files on all Windows operating system versions; C:\Program Files (x86) for 32-bit applications running on Windows 64-bit operating systems.)

It registers as a system service to ensure its automatic execution at every system startup by adding the following registry keys:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\{random name}

Download Routine

This Backdoor connects to the following URL(s) to download its component file(s):

  • http://ca.{BLOCKED}sh.org:443/NetSyst96.dll

It saves the files it downloads using the following names:

  • %Program Files%\AppPatch\mysqld.dll (or {drive letter}\Program Files\AppPatch\mysqld.dll)

(Note: %Program Files% is the Program Files folder, where it usually is C:\Program Files on all Windows operating system versions; C:\Program Files (x86) for 32-bit applications running on Windows 64-bit operating systems.)

As of this writing, the said sites are inaccessible.

Other Details

This Backdoor deletes the initially executed copy of itself

  SOLUTION

Minimum Scan Engine:

9.850

FIRST VSAPI PATTERN FILE:

14.400.02

FIRST VSAPI PATTERN DATE:

24 Jul 2018

VSAPI OPR PATTERN File:

14.401.00

VSAPI OPR PATTERN Date:

25 Jul 2018

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

Remove the malware/grayware file that dropped/downloaded BKDR_ZEGOST.TIDAFAT. (Note: Please skip this step if the threat(s) listed below have already been removed.)

     
    • TROJ_DLOADR.AUSUML

Step 4

Restart in Safe Mode

[ Learn More ]

Step 5

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_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services
    • {random name}

Step 6

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.
  • %Program Files%\AppPatch\mysqld.dll
  • {drive letter}\Program Files\AppPatch\mysqld.dll
  • %User Temp%\{random numbers}.vbs

Step 7

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