Analysis by: Christopher Daniel So
 Modified by: Rhena Inocencio
ALIASES:

Trojan:Win32/Yoddos.A (Microsoft); Downloader (Symantec); BackDoor-DKA (McAfee); Backdoor.Win32.Yoddos.a (Kaspersky)

 PLATFORM:

Windows 2000, Windows XP, Windows Server 2003

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

  • Threat Type: Trojan

  • Destructiveness: No

  • Encrypted:

  • In the wild: Yes

  OVERVIEW

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

  TECHNICAL DETAILS

File Size: 31,744 bytes
File Type: EXE
Memory Resident: Yes
Initial Samples Received Date: 06 Feb 2010

Arrival Details

This Trojan 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 Trojan drops the following copies of itself into the affected system:

  • %System%\win32.exe
  • %System%\WinHelp{random number}.exe

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

Autostart Technique

This Trojan 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\win32
Type = "10"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\win32
Start = "2"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\win32
ErrorControl = "0"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\win32
ImagePath = "%System%\win32.exe"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\win32
DisplayName = "win32"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\win32
ObjectName = "LocalSystem"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp{random number}
Type = "10"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp{random number}
Start = "2"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp{random number}
ErrorControl = "0"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp{random number}
ImagePath = "%System%\WinHelp{random number}.exe"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp{random number}
DisplayName = "Windows Help System"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp{random number}
ObjectName = "LocalSystem"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp{random number}
Description = "Windows Help System for X32 windows desktop"

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\win32

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WinHelp{random number}

Backdoor Routine

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

  • v8ddos.{BLOCKED}22.org:9800
  • 5161.{BLOCKED}00.org:5161
  • 5162.{BLOCKED}00.org:5162
  • {BLOCKED}6.{BLOCKED}k.com:2016
  • {BLOCKED}9.{BLOCKED}8.108.238:2016

  SOLUTION

Minimum Scan Engine: 8.900
VSAPI OPR PATTERN File: 6.825.00
VSAPI OPR PATTERN Date: 06 Feb 2010

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

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
    • WinHelp{random number}
  • In HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services
    • win32

Step 4

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