TROJ_SMALL.WZ

 Analysis by: Karl Dominguez

 PLATFORM:

Windows 2000, Windows XP, Windows 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 arrives as an attachment to email messages spammed by other malware/grayware or malicious users.

It arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites.

This Trojan drops a another malicious file in the affected system. The location it is saved depends on the operating system version and user priviledges. The dropped DLL is detected by Trend Micro as TROJ_SMALL.WIE.

It drops files onto the affected system. As a result, routines of the dropped files are exhibited in the system.

It also adds registry entries on the affected system.

The kind of dropped file and the added registry entries depend on the OS version of the malware.

This Trojan arrives as an attachment to email messages spammed by other malware/grayware or malicious users. It 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:

29,440 bytes

File Type:

EXE

Memory Resident:

Yes

Initial Samples Received Date:

08 Feb 2011

Arrival Details

This Trojan arrives as an attachment to email messages spammed by other malware/grayware or malicious users.

It arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites.

NOTES:

This Trojan drops a another malicious file in the affected system. The location it is saved depends on the operating system version and user priviledges. The dropped DLL is detected by Trend Micro as TROJ_SMALL.WIE.

On systems with OS Major Version 5 (2000, XP, 2003):

If the user has administrative priviledges:

The DLL is dropped in %System% folder as ms{6 random characters}.DLL.

It creates the following registry entries to bypass Windows Firewall:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SharedAccess\Parameters\
FirewallPolicy\StandardProfile\AuthorizedApplications\List
%System%\rundll32.exe = %System%\rundll32.exe:*:Enabled:Run a DLL as an App

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SharedAccess\Parameters\
FirewallPolicy\StandardProfile\GloballyOpenPorts\List
80:TCP = 80:TCP:*:Enabled:System Service

It modifies the following registry entry to enable the dropped .DLL to execute every system startup. It appends the file name of the .DLL to the list of legitimate Security Provider files:


HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders
SecurityProviders = {default values} {file name of the dropped DLL}
SecurityProviders = {default values}

If the user has no administrative priviledges:

The DLL is dropped in %User Profile%\Application Data folder as ms{6 random characters}.DLL. The file time of kernel32.LL is then copied to the dropped DLL.

It creates the following registry entry to enable the dropped DLL to execute every system startup.
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
Windows Explorer = rundll32.exe "%User Profile%\Application Data\{file name of the dropped DLL}",DllRegisterServer

It creates the following registry entries to bypass Windows Firewall:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SharedAccess\Parameters\FirewallPolicy\
StandardProfile\AuthorizedApplications\List
%System%\rundll32.exe = %System%\rundll32.exe:*:Enabled:Run a DLL as an App

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SharedAccess\Parameters\FirewallPolicy\
StandardProfile\GloballyOpenPorts\List
80:TCP = 80:TCP:*:Enabled:System Service

On systems with OS Major Version 6 (Vista, 2008, 7):

If the user has administrative priviledges:

The DLL is dropped in %System% folder as ms{random characters}.DLL.

It modifies the following registry entry to enable the dropped .DLL to execute every system startup. It appends the file name of the .DLL to the list of legitimate Security Provider files:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders
SecurityProviders = {default values} {file name of the dropped DLL}
SecurityProviders = {default values}

It creates Firewall rules to add rundll32.EXE as an authorized application. The rule created is named Security Update.

If the user has no administrative priviledges:

To elevate its permission and bypass UAC, the malware will inject codes into Explorer.EXE via the System Tray. It drops the malicous DLL as %System%\sysprep\cryptbase.DLL then execute the legitimate file %System%\sysprep\sysprep.EXE.

The DLL is executed because of a flaw in sysprep.exe that loads cryptbase.dll if it exists in the same location as itself. The DLL is then also dropped in %User Profile%\Appdata\Roaming\ms{random characters}.DLL.

It also creates the following registry entry to enable the dropped DLL to execute every system startup:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
Windows Explorer = rundll32.exe "%User Profile%\Application Data\{file name of the dropped DLL}",DllRegisterServer

It creates Firewall rules to add rundll32.EXE as an authorized application. The rule created is named Security Update.

  SOLUTION

Minimum Scan Engine:

8.900

FIRST VSAPI PATTERN FILE:

7.822.13

FIRST VSAPI PATTERN DATE:

08 Feb 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 malware/grayware files dropped/downloaded by TROJ_SMALL.WZ

    TROJ_SMALL.WIE

Step 3

Restart in Safe Mode

[ Learn More ]

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
    • Windows Explorer = rundll32.exe "%User Profile%\Application Data\{file name of the dropped DLL}",DllRegisterServer
  • In HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SharedAccess\Parameters\FirewallPolicy\StandardProfile\AuthorizedApplications\List
    • %System%\rundll32.exe = %System%\rundll32.exe:*:Enabled:Run a DLL as an App
  • In HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SharedAccess\Parameters\FirewallPolicy\StandardProfile\GloballyOpenPorts\List
    • 80:TCP = 80:TCP:*:Enabled:System Service

Step 5

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\SYSTEM\CurrentControlSet\Control\SecurityProviders
    • From: SecurityProviders = {default values} {file name of the dropped DLL}
      To: SecurityProviders = {default values}

Step 6

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

NOTES:

Solution notes:

To delete the added Firewall Rules for OS Version 6 (Vista, 2008, 7):

  1. Open Windows Firewall. Click Start, type Windows Firewall with Advanced Security in the Search input field, and press Enter.
  2. In the left panel select Inbound Rules.
  3. Select the rule(s) named "System Update" in the Inbound Rules list then press Delete.
  4. Click yes on the confirmation box that appears.
  5. Close Windows Firewall with Advanced Security.


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