Analysis by: Nikko Tamana
ALIASES:

VirTool:Win32/Obfuscator.XY, Backdoor:Win32/Hupigon.CK (Microsoft); BackDoor-AWQ (McAfee); Backdoor.Graybird!Gen (Symantec); Trojan.Win32.Generic.pak!cobra (Sunbelt)

 PLATFORM:

Windows 2000, Windows XP, Windows Server 2003

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

  • Threat Type: Worm

  • Destructiveness: No

  • Encrypted: Yes

  • In the wild: Yes

  OVERVIEW

Infection Channel: Downloaded from the Internet, Propagates via removable drives

This worm arrives via removable drives. It arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites.

It drops an AUTORUN.INF file to automatically execute the copies it drops when a user accesses the drives of an affected system.

It connects to certain URLs. It may do this to remotely inform a malicious user of its installation. It may also do this to download possibly malicious files onto the computer, which puts the computer at a greater risk of infection by other threats. As of this writing, the said sites are inaccessible.

It deletes itself after execution.

  TECHNICAL DETAILS

File Size: 655,872 bytes
File Type: EXE
Memory Resident: No
Initial Samples Received Date: 17 Oct 2012
Payload: Connects to URLs/IPs

Arrival Details

This worm arrives via removable drives.

It 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 worm drops the following copies of itself into the affected system:

  • %System%\_twunk_64.aaa
  • %Program Files%\Common Files\Microsoft Shared\MSINFO\twunk_64.aaa
  • %System Root%\twunk_64.aaa

(Note: %System% is the Windows system folder, which is usually C:\Windows\System on Windows 98 and ME, C:\WINNT\System32 on Windows NT and 2000, or C:\Windows\System32 on Windows XP and Server 2003.. %Program Files% is the default Program Files folder, usually C:\Program Files.. %System Root% is the root folder, which is usually C:\. It is also where the operating system is located.)

Propagation

This worm drops an AUTORUN.INF file to automatically execute the copies it drops when a user accesses the drives of an affected system.

The said .INF file contains the following strings:

[AutoRun]
open=twunk_64.aaa
shellexecute=twunk_64.aaa
shell\Auto\command=twunk_64.aaa

Dropping Routine

This worm drops the following files:

  • %Program Files%\Common Files\Microsoft Shared\MSINFO\ReDelBat.bat - used to delete the initially executed copy

(Note: %Program Files% is the default Program Files folder, usually C:\Program Files.)

Download Routine

This worm connects to the following malicious URLs:

  • {BLOCKED}.{BLOCKED}.59.118

As of this writing, the said sites are inaccessible.

Other Details

This worm deletes itself after execution.

NOTES:

This worm also executes %System%\calc.exe and %Program Files%\Internet Explorer\IEXPLORE.EXE. Both running processes are hidden.

  SOLUTION

Minimum Scan Engine: 9.300
FIRST VSAPI PATTERN FILE: 9.466.01
FIRST VSAPI PATTERN DATE: 17 Oct 2012

Step 1

For Windows XP and Windows Server 2003 users, before doing any scans, please make sure you disable System Restore to allow full scanning of your computer.

Step 2

Search and delete these files

[ Learn More ]
There may be some component 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%\Common Files\Microsoft Shared\MSINFO\ReDelBat.bat

Step 3

Search and delete AUTORUN.INF files created by WORM_AUTORUN.ZS that contain these strings

[ Learn More ]
    [AutoRun]
    open=twunk_64.aaa
    shellexecute=twunk_64.aaa
    shell\Auto\command=twunk_64.aaa

Step 4

Scan your computer with your Trend Micro product to delete files detected as WORM_AUTORUN.ZS. 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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