Trojan:Win32/Vundo.AX is a variant of Win32/Vundo, a multiple-component family of programs that deliver 'out of context' pop-up advertisements. They may also download and execute arbitrary files.
Vundo is often distributed as a DLL file and installed on an affected machine as a Browser Helper Object (BHO) without a user's consent. This family uses advanced defensive and stealth techniques to escape detection and to hinder removal.
Members of the Vundo family may compromise an affected system in a number of different ways. They use diverse methods of installation that often includes multiple components.
Vundo may use a dropper/downloader component that may be detected as one of the following:
(For additional detail on Vundo's downloading functions, please see the Payload section further below.)
If a downloader component is used, it downloads the DLL component, saving it with a filename that may be either randomly generated, or created using any of the following two strings:
For example, 'sysnet.dll'. A unique Class ID registry key may be created to load the newly created DLL. Registry key Class ID values vary among variants. For example:
In some variants, several data files are also created in the same location, using the same name but with the following file extensions (as opposed to '.dll'):
For example, 'sysnet.ini', 'sysnet.tmp'.
In some variants, the trojan may utilize an executable component that may be copied to the any of the following locations:
%windir%\Windows Update Setup Files
Vundo may make several registry modifications in order to load itself when Windows is started, for example:
Adds value: <trojan filename>
With data: <trojan path and filename.dll>
It may also make further modifications to load the program during events such as user logon and logoff, for example:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify\<name of module>
This trojan family may also modify the following registry entry to load itself at startup: HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\ShellExecuteHooks
(Note: this usage is disabled by default in Windows Vista with policy control).
In order to protect itself from being deleted by anti-virus software, the trojan may monitor and possibly modify the following registry entry to rename its file when the system restarts:
Vundo may create a mutex 'SysUpdIsRunningMutex' to monitor thread creation and ensure that the program remains running.
Vundo may also inject its code into the following processes if they are found to be running on the affected machine:
Vundo is designed with delivering 'out of context' advertisements in mind, however, it may also take a number of additional actions on an affected machine in order to satisfy this purpose, including modifying an affected machine's Internet and security settings and sending information from the affected system to a remote server.
Displays Advertisements/Downloads and Executes Arbitrary Files
Vundo has been observed to contact a number of different IP addresses and particular domains in order to access the advertising material that it displays. It may also download and execute arbitrary files, including updates or additional components. Vundo has been observed to connect to the following IP addresses in the wild for this purpose (for example):
Terminates Security Services
Vundo may terminate services associated with the following security-related applications:
Modifies Browser Behavior
Vundo may redirect URLs entered by the user to URLs of the program's choice.
Also, when particular URLs are visited by an affected user, Vundo may disable the display of pop-ups. Presumably this is an anti-competitive measure, as the list of targeted URLs contains a number of popular search engines and domain names associated with ad-servers, for example:
Vundo also disables pop-ups if a targeted URL contains "mil" or "gov" in the domain.
Modifies System Security Settings
Virtuemonde makes the following registry modification in an attempt to bypass firewalls:
Sets value: "ProxyBypass"
With data: "1"
To subkey: HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap\
Sends Information to Remote Server
Virtuemonde may gather and send the following information from the affected machine to a remote server:
Vundo has been observed in the wild being bundled with rogue anti-spyware products, for example, it has been observed being bundled with 'Evidence Eraser Pro'.
Virtuemonde has also been observed using encryption techniques in order to obfuscate its communications with remote sites.
This family may create the following registry entries in which to store data:
Analysis by Jaime Wong