Analysis by: Sabrina Lei Sioting
ALIASES:

Backdoor:Win32/Rbot (Microsoft); PAK:PolyCrypt, PAK:ASPack, Backdoor.Win32.Rbot.aeu (Kaspersky); W32.Spybot.Worm (Symantec); W32/Gaobot.worm.gen.e (McAfee); W32/Rbot-AMB (Sophos); Worm/Rbot.105472.8 (Antivir); W32/RBot.gen!tr.bdr (Fortinet); Backdoor.Rbot (Ikarus); Win32/Rbot trojan (Eset);

 PLATFORM:

Windows 2000, Windows XP, Windows Server 2003

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

  • Threat Type: Backdoor

  • Destructiveness: No

  • Encrypted: Yes

  • 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.

  TECHNICAL DETAILS

File Size: 103,424 bytes
File Type: EXE
Memory Resident: Yes
Initial Samples Received Date: 25 Mar 2011

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:

  • %System%\lfxss.exe

(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.)

Autostart Technique

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

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\RunServices
BnCtest2 = "lfxss.exe"

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Run
BnCtest2 = "lfxss.exe"

Other System Modifications

This backdoor adds the following registry entries as part of its installation routine:

HKEY_CURRENT_USER\Software\Microsoft\
OLE
BnCtest2 = "lfxss.exe"

It creates the following registry entry(ies) to bypass Windows Firewall:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\SharedAccess\Parameters\
FirewallPolicy\StandardProfile\AuthorizedApplications\
List
%System%\lfxss.exe = "%System%\lfxss.exe:*:Enabled:lfxss"

Other Details

This backdoor connects to the following possibly malicious URL:

  • st0ned.{BLOCKED}on.com

  SOLUTION

Minimum Scan Engine: 9.200

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 BKDR_GAOBOT.SL

[ 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

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_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\RunServices
    • BnCtest2 = "lfxss.exe"
  • In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
    • BnCtest2 = "lfxss.exe"
  • In HKEY_CURRENT_USER\Software\Microsoft\OLE
    • BnCtest2 = "lfxss.exe"
  • In HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SharedAccess\Parameters\FirewallPolicy\StandardProfile\AuthorizedApplications\List
    • %System%\lfxss.exe = "%System%\lfxss.exe:*:Enabled:lfxss"

Step 4

Scan your computer with your Trend Micro product to delete files detected as BKDR_GAOBOT.SL. 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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