Analysis by: Jimelle Monteser
 Modified by: Alvin Bacani
ALIASES:

W32/Zegost.A (Fortinet), Backdoor:Win32/Zegost.Z (Microsoft)

 PLATFORM:

Windows 2000, Windows Server 2003, Windows XP (32-bit, 64-bit), Windows Vista (32-bit, 64-bit), Windows 7 (32-bit, 64-bit)

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

  • Threat Type: Backdoor

  • Destructiveness: No

  • Encrypted: Yes

  • 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 modifies the Internet Explorer Zone Settings.

It executes the files it drops, prompting the affected system to exhibit the malicious routines they contain.

  TECHNICAL DETAILS

File Size: 1,182,701 bytes
File Type: EXE
Memory Resident: Yes
Initial Samples Received Date: 13 Aug 2013
Payload: Connects to URLs/IPs, Compromises system security, Drops 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 copies of itself into the affected system:

  • %User Temp%\~__UNINST.EXE

(Note: %User Temp% is the current user's Temp folder, which is usually C:\Documents and Settings\{user name}\Local Settings\Temp on Windows 2000, XP, and Server 2003, or C:\Users\{user name}\AppData\Local\Temp on Windows Vista and 7.)

It injects codes into the following process(es):

  • userinit.exe

Autostart Technique

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

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Active Setup\Installed Components\{random numbers}
stubpath = "%system%\in{random letters}.exe"

Other System Modifications

This backdoor adds the following registry keys as part of its installation routine:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\PCRatStact

Backdoor Routine

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

  • Update itself
  • Download files
  • Execute files
  • Terminate Process

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

  • {BLOCKED}346.{BLOCKED}s.com:80

Web Browser Home Page and Search Page Modification

This backdoor modifies the Internet Explorer Zone Settings.

Dropping Routine

This backdoor drops the following files:

  • %User Temp%\{random}_lang.dll - detected as BKDR_INJECT.SMJ
  • %User Temp%\SAinjectecter.exe
  • %User Temp%\server.exe - detected as BKDR_ZEGOST.IND
  • %System%\in{random letters}.exe - detected as BKDR_ZEGOST.IND
  • %System%\in(random letters}.exe_lang.ini

(Note: %User Temp% is the current user's Temp folder, which is usually C:\Documents and Settings\{user name}\Local Settings\Temp on Windows 2000, XP, and Server 2003, or C:\Users\{user name}\AppData\Local\Temp on Windows Vista and 7.. %System% is the Windows system folder, which is usually C:\Windows\System32.)

It executes the files it drops, prompting the affected system to exhibit the malicious routines they contain.

  SOLUTION

Minimum Scan Engine: 9.300
FIRST VSAPI PATTERN FILE: 10.214.02
FIRST VSAPI PATTERN DATE: 13 Aug 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

Restart in Safe Mode

[ Learn More ]

Step 3

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.
  • %User Temp%\{random}_lang.dll
  • %User Temp%\SAinjectecter.exe
  • %User Temp%\server.exe
  • %System%\in(random letters}.exe_lang.ini
  • %User Temp%\~__UNINST.EXE
  • %System%\in(random letters}.exe

Step 4

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. Before you could do this, you must restart in Safe Mode. For instructions on how to do this, you may refer to this page If the preceding step requires you to restart in safe mode, you may proceed to edit the system registry.

  • In HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services
    • PCRatStact

Step 5

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\Active Setup\Installed Components\{737DCD8A-CEF6-48b6-B10F-D62B871DB9A4}
    • stubpath = "%system%\in{random letters}.exe"

Step 6

Remove malware/grayware files dropped/downloaded by BKDR_ZEGOST.IND

     
    • BKDR_INJECT.SMJ

Step 7

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