Win32/Brontok is a family of mass-mailing e-mail worms that also copies itself to USB and pen drives. When the worm runs, it creates a folder and downloads a text file from a remote website to that folder. The worm also makes several copies of itself in various other folders, using different file names with one of the following extensions: .com, .exe, .scr, or .pif. File names used by the worm may be identical to certain Windows system file names, such as csrss.exe, lsass.exe, services.exe, smss.exe, or winlogon.exe.
In most cases, Win32/Brontok use the Windows 'new folder' icon for the worm files. By default, Windows suppresses the extension on executable files. Even if this feature has been disabled, Win32/Brontok variants disable executable file extension viewing. This, in conjunction with the use of the 'new folder' icon, can cause the file to appear as if it were a new folder rather than an executable file. An unsuspecting user clicking on the "folder" to view its contents thereby inadvertently runs the worm file. To further promote the impression that the worm file is merely a folder, a new Explorer window is opened when the worm is run.
The Win32/Brontok worm spreads by sending a copy of itself as an e-mail attachment to e-mail addresses that it gathers from various files on the infected computer. The worm spoofs the sender 'From' address. The e-mail characteristics vary. For example, some variants compose an e-mail with a blank subject line and empty message body, and the infected attachment is named 'kangen.exe'. Still other variants compose an e-mail in which either the subject line or message body may contain a political message or advertise some pictures, written in a mix of Indonesian and English. In such cases, the attachment is often named 'Sample Picture.zip' or 'photo.zip'.
When the infected e-mail attachment is opened, the worm may use various methods to run automatically each time Windows starts. Automatic startup methods that the worm employs may include:
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Placing a copy of itself in the user's startup folder, i.e. %homepath%\Start Menu\Programs\Startup\Empty.pif
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Adding a scheduled task to run %homepath%\Templates\A.kotnorB.com each day at 5:08 p.m.
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Adding registry value: "Tok-Cirrhatus"
with data: <path to Win32/Brontok worm>
in subkey: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
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Adding registry value: "Bron-Spizaetus"
with data: <path to Win32/Brontok worm>
in subkey: HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run
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Adding registry value: Shell
with data: "explorer.exe " <path to Win32/Brontok worm>
in registry subkey: HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\WinLogon
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Modifies registry value: AlternateShell
with data: <Win32/Brontok file name>
in registry subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SafeBoot
Note: the default setting for this key is "AlternateShell"="cmd.exe"
Win32/Brontok may attempt to lower security settings by making the following changes:
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Prevents the user from accessing the Registry Editor by making the following registry edit:
Adds value: DisableRegistryTools
With data: 1
In subkey: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System
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Prevents the display of files and folders with the 'hidden' attribute set:
Adds value: Hidden
With data: 0
In subkey: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
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Prevents the display of Windows system files:
Adds value: ShowSuperHidden
With data: 0
In subkey: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
Adds value: HideFileExt
With data: 1
In subkey: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
- Prevents access to the Folder Options menu:
Adds value: NoFolderOptions
With data: 1
In subkey: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
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Modifies the Windows HOSTS file to prevent access to certain Internet sites, the majority of which are antivirus or security-related.
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Attempts ping attacks against certain Web sites, presumably to launch a form of denial of service (DoS) attack.
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Terminates applications or restarts Windows when the title of the active window contains certain strings, many of which may be representative of antivirus or system tools that might ordinarily be used to detect or remove the worm.
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Overwrites the autoexec.bat file with the word "pause", causing systems that employ the autoexec.bat file to pause on bootup. Some variants of Win32/Brontok may modify the autoexec.bat in order to display a message during bootup.
Some Win32/Brontok variants are written in C; others are written in Visual Basic.
