Analysis by: Kathleen Notario
ALIASES:

VirTool:Win32/Obfuscator.LC (Microsoft); Trojan-Downloader.Win32.Agent.flsr (Kaspersky);

 PLATFORM:

Windows 2000, XP, Server 2003

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

  • Threat Type: Trojan

  • Destructiveness: No

  • Encrypted: No

  • In the wild: Yes

  OVERVIEW

This Trojan executes the downloaded files. As a result, malicious routines of the downloaded files are exhibited on the affected system.

  TECHNICAL DETAILS

File Size: 393,216 bytes
File Type: EXE
Memory Resident: Yes
Initial Samples Received Date: 17 Feb 2011
Payload: Downloads files

Installation

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

  • %Application Data%\loader.exe

(Note: %Application Data% is the current user's Application Data folder, which is usually C:\Windows\Profiles\{user name}\Application Data on Windows 98 and ME, C:\WINNT\Profiles\{user name}\Application Data on Windows NT, and C:\Documents and Settings\{user name}\Local Settings\Application Data on Windows 2000, XP, and Server 2003.)

It drops the following non-malicious file:

  • %Application Data%\config.txt

(Note: %Application Data% is the current user's Application Data folder, which is usually C:\Windows\Profiles\{user name}\Application Data on Windows 98 and ME, C:\WINNT\Profiles\{user name}\Application Data on Windows NT, and C:\Documents and Settings\{user name}\Local Settings\Application Data on Windows 2000, XP, and Server 2003.)

Autostart Technique

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

HKEY_CURRENT_USER\Software\Microsoft\
Windows\CurrentVersion\Run
loader = %Application Data%\loader.exe

Download Routine

This Trojan downloads an updated copy of itself from the following website(s):

  • http://{BLOCKED}fernao.sites.uol.com.br/puxador.html

It connects to the following website(s) to download and execute a malicious file:

  • http://{BLOCKED}fernao.sites.uol.com.br/modulo.html - detected as TROJ_BHO.YW

It saves the files it downloads using the following names:

  • %Application Data%\md_67.dll

(Note: %Application Data% is the current user's Application Data folder, which is usually C:\Windows\Profiles\{user name}\Application Data on Windows 98 and ME, C:\WINNT\Profiles\{user name}\Application Data on Windows NT, and C:\Documents and Settings\{user name}\Local Settings\Application Data on Windows 2000, XP, and Server 2003.)

It then executes the downloaded files. As a result, malicious routines of the downloaded files are exhibited on the affected system.

  SOLUTION

Minimum Scan Engine: 8.900
VSAPI PATTERN File: 7.842.01
VSAPI PATTERN Date: 17 Feb 2011
VSAPI PATTERN Date: 2/17/2011 12:00:00 AM

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

Remove malware files dropped/downloaded by TROJ_DLOADR.WOO

     TROJ_BHO.YW

Step 3

Identify and terminate files detected as TROJ_DLOADR.WOO

[ Learn More ]
  1. If the detected file is displayed in either Windows Task Manager or Process Explorer but you cannot delete it, restart your computer in safe mode. To do this, refer to this link for the complete steps.
  2. If the detected file is not displayed in either Windows Task Manager or Process Explorer, continue doing the next steps.

Step 4

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_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
    • loader = %Application Data%\loader.exe

Step 5

Search and delete this file

[ 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. %Application Data%\config.txt

Step 6

Scan your computer with your Trend Micro product to delete files detected as TROJ_DLOADR.WOO. 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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