Analysis by: Michael Cabel
ALIASES:

Backdoor:Win32/Numnom.A

 PLATFORM:

Windows 2000, XP, 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 opens a random port to allow a remote user to connect to the affected system. Once a successful connection is established, the remote user executes commands on the affected system.

  TECHNICAL DETAILS

File Size: 104,568 bytes
File Type: PE
Memory Resident: Yes
Initial Samples Received Date: 14 Nov 2010

Installation

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

  • %System%\wausdis.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 registers itself as a system service to ensure its automatic execution at every system startup by adding the following registry entries:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WAUSDIS
Type = 110

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WAUSDIS
Start = 2

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WAUSDIS
ErrorControl = 0

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WAUSDIS
ImagePath = %System%\wausdis.exe

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WAUSDIS
DisplayName = "Windows Automatic Update Secure Download and Installation Service"

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

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\WAUSDIS
Description = "Provides the Background Intelligent Transfer Service (BITS) and Automatic Updates service with a secure interface through which updates can be securely downloaded and installed. Stopping this service may present a security risk. If this service is disa"

Other System Modifications

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

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\SharedAccess\Parameters\
FirewallPolicy\StandardProfile\AuthorizedApplications\
List
%System%\wausdis.exe = "%System%\wausdis.exe:*:Enabled:Windows Automatic Update Secure Download and Installation Service"

Backdoor Routine

This backdoor opens a random port to allow a remote user to connect to the affected system. Once a successful connection is established, the remote user executes commands on the affected system.

It connects to the following URL(s) to send and receive commands from a remote malicious user:

  • {BLOCKED}ndloan.com

  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

Restart in Safe Mode

[ Learn More ]

Step 3

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.

 
  • In HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services
    • WAUSDIS

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_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SharedAccess\Parameters\FirewallPolicy\StandardProfile\AuthorizedApplications\List
    • %System%\wausdis.exe = %System%\wausdis.exe:*:Enabled:Windows Automatic Update Secure Download and Installation Service

Step 5

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