Threat description




Trojan:W32/Vundo is a large family of trojans closely associated with adware program that downloads and displays pop-up advertisements.


Automatic action

Depending on the settings of your F-Secure security product, it will either automatically delete, quarantine or rename the suspect file, or ask you for a desired action.

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Technical Details

The adware is normally downloaded by users from the websites; these malicious sites are normally promoted by spam e-mails. The Vundo family is often distributed as DLL files. It is known to be bundled with adware and/or spyware programs.

For representative examples of Vundo variants and associated programs:


Malware in the Vundo family are installed without the user's consent as a Browser Helper Objects (BHOs). During installation, the DLL file is dropped in Windows system directory. The file name is randomly generated using 8 alphabetic characters:

  • C:\WINDOWS\system32\{8_random_alpha_char}.dll

Upon execution, the DLL is injected into 2 running processes:

  • Explorer.exe
  • Winlogon.exe

In some variants, several data files are created in the same location as the DLL file's execution. The data files' attributes are set as system file and hidden. The files use random names and the following extensions:

  • ini
  • ini2

Once installed, the trojan modifies Microsoft Internet Explorer's security setting to trust cookie content created by the trojan:

  • [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Browser Helper Objects\ {unique_CLSID}]
  • [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\3] {unique_CLSID}="malware_hex_value"

Some variants may also attempt to download and execute arbitrary files from remote websites such as:


The program runs hidden from the user.


During installation, malware in the Vundo family create a registry launch point with a unique Class ID. This launch point automatically starts the DLL when system is restarted.

To find this launch point, navigate to the following registry and look for the malicious file pointed by "InprocServer32" registry key:

  • [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\CLSID\{unique_CLSID}\InprocServer32] @="C:\WINDOWS\system32\{8_random_alpha_char}.dll"

The program will also make further modifications to load itself at startup:

  • [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\ShellExecuteHooks] {unique_CLSID}

For some variants, the following registry keys are also created to load the program whenever the user logs on or off. The keys will use the same DLL file name as its key:

  • [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify\{DLL_file_name}] Asynchronous=dword:00000001 DllName="{DLL_file_name}" Impersonate=dword:00000000 Logon="o" Logoff="f"

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