Analysis by: Sabrina Lei Sioting
 PLATFORM:

Windows 2000, Wndows XP, Windows Server 2003

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

  • Threat Type: Trojan

  • Destructiveness: No

  • Encrypted: No

  • In the wild: Yes

  TECHNICAL DETAILS

File Size: 100,616 bytes
File Type: PE
Memory Resident: Yes
Initial Samples Received Date: 09 Mar 2011
Payload: Hides files with certain attributes

Installation

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

  • %User Profile%\Local Settings\Application Data\Start\update.exe

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

It creates the following folders:

  • %User Profile%\Local Settings\Application Data\S-1-5-31-1286970278978-5713669491-166975984-320
  • %User Profile%\Local Settings\Application Data\S-1-5-31-1286970278978-5713669491-166975984-320\dmc
  • %User Profile%\Local Settings\Application Data\S-1-5-31-1286970278978-5713669491-166975984-320\Rotinom
  • %User Profile%\Local Settings\Application Data\S-1-5-31-1286970278978-5713669491-166975984-320\tlsr
  • %User Profile%\Local Settings\Application Data\Start

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

Autostart Technique

This Trojan modifies the following registry entries to ensure it automatic execution at every system startup:

HKEY_CURRENT_USER\Software\Microsoft\
Windows\CurrentVersion\Explorer\
Shell Folders
Startup = "%User Profile%\Local Settings\Application Data\Start"

(Note: The default value data of the said registry entry is %User Profile%\Start Menu\Programs\Startup.)

HKEY_CURRENT_USER\Software\Microsoft\
Windows\CurrentVersion\Explorer\
User Shell Folders
Startup = "%User Profile%\Local Settings\Application Data\Start"

(Note: The default value data of the said registry entry is %User Profile%\Start Menu\Programs\Startup.)

Other System Modifications

This Trojan modifies the following registry entries to hide files with Hidden attributes:

HKEY_CURRENT_USER\Software\Microsoft\
Windows\CurrentVersion\Explorer\
Advanced
Hidden = "2"

(Note: The default value data of the said registry entry is 1.)

HKEY_CURRENT_USER\Software\Microsoft\
Windows\CurrentVersion\Explorer\
Advanced
HideFileExt = "1"

(Note: The default value data of the said registry entry is 0.)

HKEY_CURRENT_USER\Software\Microsoft\
Windows\CurrentVersion\Explorer\
Advanced
ShowSuperHidden = "0"

(Note: The default value data of the said registry entry is 1.)

  SOLUTION

Minimum Scan Engine: 8.900

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

Identify and terminate files detected as TROJ_FOLSTART.AA

[ 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 3

Restore this modified 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\Explorer\Shell Folders
    • From: Startup = %User Profile%\Local Settings\Application Data\Start
      To: Startup = %User Profile%\Start Menu\Programs\Startup
  • In HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders
    • From: Startup = %User Profile%\Local Settings\Application Data\Start
      To: Startup = %User Profile%\Start Menu\Programs\Startup
  • In HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
    • From: Hidden = 2
      To: Hidden = 1
  • In HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
    • From: HideFileExt = 1
      To: HideFileExt = 0
  • In HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
    • From: ShowSuperHidden = 0
      To: ShowSuperHidden = 1

Step 4

Search and delete these folders

[ Learn More ]
Please make sure you check the Search Hidden Files and Folders checkbox in the More advanced options option to include all hidden folders in the search result.
  • %User Profile%\Local Settings\Application Data\S-1-5-31-1286970278978-5713669491-166975984-320
  • %User Profile%\Local Settings\Application Data\S-1-5-31-1286970278978-5713669491-166975984-320\dmc
  • %User Profile%\Local Settings\Application Data\S-1-5-31-1286970278978-5713669491-166975984-320\Rotinom
  • %User Profile%\Local Settings\Application Data\S-1-5-31-1286970278978-5713669491-166975984-320\tlsr
  • %User Profile%\Local Settings\Application Data\Start

Step 5

Scan your computer with your Trend Micro product to delete files detected as TROJ_FOLSTART.AA. 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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