This type of trojan secretly downloads malicious files from a remote server, then installs and executes the files.
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Trojan-Downloader:W32/Bredolab is a family of trojan-downloaders that are known to download and install rogue antivirus programs - also known as rogueware - onto the infected computer. The installed rogueware generates misleading or downright false alerts, notification messages and/or scanning reports to pressure the user into "purchasing" or "activating" the rogueware in order to disinfect or remove the supposed threats. Even if the user does so however, the program may not function as intended.
During installation, Bredolab variants create the following registry entry so that the trojan-downloader runs every time Windows starts:
HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\ "RunGrpConv" = "1"
It also creates the following mutex so that only one instance of the downloader will be running on the system:
Currently, all the variants analyzed were found to download files from Russian websites such as those listed below:
The files downloaded from those websites are encrypted and will be decrypted by the trojan-downloader before being executed on the infected machine.