TROJ_SMALL.WZ

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.

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:

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.