Analysis by: Andrei Castillo
ALIASES:

Gen:Trojan.Heur.GM.1004810120(BitDefender), BackDoor.Delf(AVG), W32/Yoddos.AE(Fortinet), W32/RLPacked.B.gen!Eldorado (generic, not disinfectable) (FPROT), Packed.Win32.Klone(Ikarus), Backdoor:Win32/Zegost.AK(Microsoft), a variant of Win32/Yoddos.AE trojan(NOD32), Trojan.Gen.3(Norton), BehavesLike.Win32.Malware.ssc (mx-v)(Sunbelt), TScope.Malware-Cryptor.SB(VBA32), TR/Dropper.Gen(ANTIVIR),

 PLATFORM:

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

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

  • Threat Type: Backdoor

  • Destructiveness: No

  • Encrypted:

  • In the wild: Yes

  OVERVIEW

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 deletes itself after execution.

  TECHNICAL DETAILS

File Size: 19411 bytes
Memory Resident: Yes

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:

  • %System%\WinHelp32.exe

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

Autostart Technique

This backdoor registers its dropped component as a system service to ensure its automatic execution at every system startup. It does this by creating the following registry entries:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp32
DisplayName = "Windows Help System"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp32
ErrorControl = "0"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp32
Type = "16"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp32
ImagePath = "%System%\WinHelp32.exe"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp32
Start = "2"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp32
ObjectName = "LocalSystem"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp32\Security
Security = "{Random Binary Value}"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp32\Enum
Next Instance = "1"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp32\Enum
Count = "1"

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\WinHelp32
SYSTEM\CurrentControlSet\Services\WinHelp32 = "WIndows Help System for X32 windows desktop"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp32\Enum
0 = "Root\LEGACY_WINHELP32\0000"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Enum\Root\LEGACY_WINHELP32
NextInstance = "1"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Enum\Root\LEGACY_WINHELP32\
0000
Class = "LegacyDriver"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Enum\Root\LEGACY_WINHELP32\
0000
ClassGUID = {GUID}

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Enum\Root\LEGACY_WINHELP32\
0000
ConfigFlags = "0"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Enum\Root\LEGACY_WINHELP32\
0000
DeviceDesc = "Windows Help System"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Enum\Root\LEGACY_WINHELP32\
0000
Legacy = "1"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Enum\Root\LEGACY_WINHELP32\
0000
Service = "WinHelp32"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Enum\Root\LEGACY_WINHELP32\
0000\Control
*NewlyCreated* = "0"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Enum\Root\LEGACY_WINHELP32\
0000\Control
ActiveService = "WinHelp32"

Other Details

This backdoor deletes itself after execution.

NOTES:
This malware connects to a possibly malicious IP address {BLOCKED}.173.73.

  SOLUTION

Minimum Scan Engine: 9.300
FIRST VSAPI PATTERN FILE: 10.266.05
FIRST VSAPI PATTERN DATE: 08 Sep 2013
VSAPI OPR PATTERN File: 10.267.00
VSAPI OPR PATTERN Date: 08 Sep 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, and then 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
    • WinHelp32
  • In HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\Root
    • LEGACY_WINHELP32

Step 3

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

NOTES:
In order to remove registry key added as HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\Root\LEGACY_WINHELP32, right-click on the key and click on "Permissions...." Change the permission setting for "Everyone" to allow "Full Control."


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