Win32/Brontok is a family of mass-mailing e-mail worms. The worm spreads by sending a copy of itself as an e-mail attachment to e-mail addresses that it gathers from files on the infected computer. It can also copy itself to USB and pen drives. Win32/Brontok can disable antivirus and security software, immediately terminate certain applications, and cause Windows to restart immediately when certain applications run. The worm may also conduct denial of service (DoS) attacks against certain Web sites.

What to do now

To detect and remove this threat and other malicious software that may be installed in your computer, run a full-system scan with an up-to-date antivirus product such as the following:
For more information on antivirus software, see

Threat behavior

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/Brontock 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' or ''.
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:
  • Placing a copy of itself in the user's startup folder, i.e. %homepath%\Start Menu\Programs\Startup\Empty.pif
  • Adding a scheduled task to run %homepath%\Templates\ each day at 5:08 p.m.
  • Adding registry value: "Tok-Cirrhatus"
  • with data: <path to Win32/Brontok worm>
    in subkey: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
  • Adding registry value: "Bron-Spizaetus"
  • with data: <path to Win32/Brontok worm>
    in subkey: HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run
  • 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
  • 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:
  • 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
  • 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
  • Prevents the display of Windows system files:
  • Adds value: ShowSuperHidden
    With data: 0
    In subkey: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
  • Prevents the display of executable file extensions:
    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
  • Modifies the Windows HOSTS file to prevent access to certain Internet sites, the majority of which are antivirus or security-related.
  • Attempts ping attacks against certain Web sites, presumably to launch a form of denial of service (DoS) attack.
  • 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.
  • 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.


Symptoms of the presence of Win32/Brontok may differ according to the particular variant. Symptoms may include the following:
  • Immediately after an application starts, the application closes or Windows restarts.
  • The user cannot update certain security-related software or visit certain Web sites.
  • The user cannot modify Windows Explorer folder options.
  • The user cannot run certain Windows tools, such as Task Manager, Registry Editor, or the command shell.


Take the following steps to help prevent infection on your computer:
  • Enable a firewall on your computer.
  • Get the latest computer updates for all your installed software.
  • Use up-to-date antivirus software.
  • Limit user privileges on the computer.
  • Use caution when opening attachments and accepting file transfers.
  • Use caution when clicking on links to Web pages.
  • Avoid downloading pirated software.
  • Protect yourself against social engineering attacks.
  • Use strong passwords.
Enable a firewall on your computer
Use a third-party firewall product or turn on the Microsoft Windows Internet Connection Firewall.
Get the latest computer updates
Updates help protect your computer from viruses, worms, and other threats as they are discovered. It is important to install updates for all the software that is installed in your computer. These are usually available from vendor Web sites.
You can use the Automatic Updates feature in Windows to automatically download future Microsoft security updates while your computer is on and connected to the Internet.
Use up-to-date antivirus software
Most antivirus software can detect and prevent infection by known malicious software. To help protect you from infection, you should always run antivirus software, such as Microsoft Security Essentials, that is updated with the latest signature files. For more information, see
Limit user privileges on the computer
Starting with Windows Vista and Windows 7, Microsoft introduced User Account Control (UAC), which, when enabled, allowed users to run with least user privileges. This scenario limits the possibility of attacks by malware and other threats that require administrative privileges to run.
You can configure UAC in your computer to meet your preferences:
Use caution when opening attachments and accepting file transfers
Exercise caution with e-mail and attachments received from unknown sources, or received unexpectedly from known sources.  Use extreme caution when accepting file transfers from known or unknown sources.
Use caution when clicking on links to Web pages
Exercise caution with links to Web pages that you receive from unknown sources, especially if the links are to a Web page that you are not familiar with, unsure of the destination of, or suspicious of. Malicious software may be installed in your computer simply by visiting a Web page with harmful content.
Avoid downloading pirated software
Threats may also be bundled with software and files that are available for download on various torrent sites. Downloading "cracked" or "pirated" software from these sites carries not only the risk of being infected with malware, but is also illegal. For more information, see 'The risks of obtaining and using pirated software'.
Protect yourself from social engineering attacks
While attackers may attempt to exploit vulnerabilities in hardware or software to compromise a computer, they also attempt to exploit vulnerabilities in human behavior to do the same. When an attacker attempts to take advantage of human behavior to persuade the affected user to perform an action of the attacker's choice, it is known as 'social engineering'. Essentially, social engineering is an attack against the human interface of the targeted computer. For more information, see 'What is social engineering?'.
Use strong passwords
Attackers may try to gain access to your Windows account by guessing your password. It is therefore important that you use a strong password – one that cannot be easily guessed by an attacker. A strong password is one that has at least 8 characters, and combines letters, numbers, and symbols. For more information, see

Alert level: High
This entry was first published on: Oct 31, 2006
This entry was updated on: Apr 17, 2011

This threat is also detected as:
  • W32/Rontokbro.gen@MM (McAfee)