Analysis by: Christopher Daniel So
 Modified by: Erika Bianca Mendoza
ALIASES:

Backdoor.Trojan (Symantec), Backdoor.Win32.Agent.bnnz (Kaspersky)

 PLATFORM:

Windows 2000, Windows XP, Windows Server 2003

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

  • Threat Type: Backdoor

  • Destructiveness: No

  • Encrypted: No

  • In the wild: Yes

  OVERVIEW

This backdoor attempts to connect to certain servers saved in the registry. Once connected, it gathers information from the affected system by executing commands in CMD. The results are saved in the file %System%\TMP2548, before being encrypted and sent to the server. This file is deleted after use. Executing the said commands reveal certain information.

This backdoor may be dropped by other malware.

  TECHNICAL DETAILS

File Size: 40,888 bytes
File Type: EXE
Memory Resident: Yes
Initial Samples Received Date: 23 Aug 2011
Payload: Connects to URLs/IPs, Collects system information

Arrival Details

This backdoor may be dropped by the following malware:

  • TROJ_PIDIEF.SMZX

Installation

This backdoor drops the following files:

  • %System%\wumsvc.dll - also detected by Trend Micro as BKDR_PROXY.ARX

(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 registers its dropped component as a system service to ensure its automatic execution at every system startup. It does this by creating the following registry entries:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WindowsUserManagement
Type = "20"

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

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WindowsUserManagement
ErrorControl = "1"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WindowsUserManagement
ImagePath = "%SystemRoot%\System32\svchost.exe -k netsvcs"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WindowsUserManagement
DisplayName = "Windows User Management"

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

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WindowsUserManagement
Description = "Provides a common management to access information about windows users. If the access is stopped, most Windows-based software will not function properly. If this server is disabled, any services that explicitly depend on it will fail to start."

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WindowsUserManagement\Parameters
ServiceDll = "wumsvc.dll"

It registers its dropped component as a system service to ensure its automatic execution at every system startup. It does this by creating the following registry keys:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WindowsUserManagement

Other System Modifications

This backdoor adds the following registry entries:

HKEY_CLASSES_ROOT\Shell.LocalMachine
CheckCode = "{configuration}"

It modifies the following registry entries:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows NT\CurrentVersion\SvcHost
netsvcs = "{default values} WindowsUserManagement"

(Note: The default value data of the said registry entry is {default values}.)

NOTES:

Backdoor Routines

It attempts to connect to the following servers saved in the registry:

  • {BLOCKED}.94.154
  • {BLOCKED}.153.107

Once connected, it gathers information from the affected system by executing the commands below in CMD:

  • GPRESULT /SCOPE COMPUTER /Z
  • IPCONFIG /ALL
  • NET LOCALGROUP ADMINISTRATORS
  • NET START
  • SYSTEMINFO

The results are saved in the file, %System%\TMP2548, before being encrypted and sent to the server. This file is deleted after use.

Executing the above-mentioned commands reveals the information below:

  • Group policies/computer settings
  • IP/Network configuration
  • Local administrators
  • Started services
  • System information

It may also wait for the server for some commands to control services in the affected system.

Other Details

It checks if the system has a proxy enabled by querying the registries below:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings
ProxyEnable
ProxyServer

  SOLUTION

Minimum Scan Engine: 8.900
FIRST VSAPI PATTERN FILE: 8.374.03
FIRST VSAPI PATTERN DATE: 23 Aug 2011

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 the malware/grayware file that dropped/downloaded BKDR_PROXY.ARX

Step 3

Scan your computer with your Trend Micro product and note files detected as BKDR_PROXY.ARX

Step 4

Restart in Safe Mode

[ Learn More ]

Step 5

Search and delete the file detected as BKDR_PROXY.ARX

[ Learn More ]
Please make sure you check the Search Hidden Files and Folders checkbox in the More advanced options option to include all hidden files in the search result.

Step 6

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. Before you could do this, you must restart in Safe Mode. For instructions on how to do this, you may refer to this page If the preceding step requires you to restart in safe mode, you may proceed to edit the system registry.

  • In HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services
    • WindowsUserManagement

Step 7

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_CLASSES_ROOT\Shell.LocalMachine
    • CheckCode = "{configuration}"

Step 8

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_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SvcHost
    • From: netsvcs = "{default values} WindowsUserManagement"
      To: netsvcs = "{default values}"

Step 9

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