Win32/Maslan is a family of mass-mailing network worms that targets computers running Microsoft Windows. The worm spreads through e-mail and peer-to-peer file-sharing applications. It can also spread to computers that have not been patched for the Windows vulnerabilities described in Microsoft Security Bulletins MS03-039 and MS04-011. The worm installs two backdoors, one of which is a variant of Win32/Sdbot. The worm has a stealth component that hides certain files and directories.

What to do now

To detect and remove this threat and other malicious software that may be installed on your computer, run a full-system scan with an appropriate, up-to-date, security solution. The following Microsoft products detect and remove this threat:

For more information on antivirus software, see

Threat behavior

Win32/Maslan drops several files in the Windows system folder, including a .dll component, a Trojan-downloader component, a stealth component, a MIME-encoded copy of the worm file, and a variant of Win32/Sdbot. The names of many of the dropped files begin with the string "___" (three successive underscore characters). The worm also modifies several registry keys in order to run automatically each time Windows starts.
The worm injects the .dll component into various processes. The .dll code may bypass a local firewall with full network access because its connections are started from system processes. The .dll code performs several functions, including the following:
  • Scans ports on the network to locate computers with vulnerabilities.
  • Drops and installs a backdoor on the infected computer that opens and monitors local ports.
  • Installs a variant of Win32/Sdbot. This backdoor then connects to an IRC server to receive commands from attackers.
Win32/Maslan may install a stealth component that hides file and directory names that contain the string "___". This component intercepts results returned by certain Windows API calls and replaces any entry in the results that contains "___"  with ".". Most file-viewing applications, such as Windows Explorer, do not display entries named ".". This is because "." merely indicates the current directory. Therefore, in most file viewers the renamed files and directories are hidden from the user. 
At least one variant of Win32/Maslan does not have stealth capabilities. This variant prefixes the names of files it drops with the string "ODBC" (instead of "___"), which may lead the user to believe that the files are legitimate.
Win32/Maslan terminates various processes, primarily processes related to computer security. The worm logs user keystrokes in windows with titles containing strings such as "bank". It also conducts denial of service attacks against certain Web sites.
The worm spreads in the following ways:
  • Sends the MIME-encoded copy of itself as an e-mail attachment to e-mail addresses that it finds on the infected computer. The sender name and e-mail address are spoofed.
  • Copies itself to computers it finds on the network that have not been patched for the Windows RPC DCOM vulnerability described in Microsoft Security Bulletin MS03-039 or the Windows LSASS vulnerability described in MS04-011. 
  • Overwrites files in share directories of peer-to-peer file-sharing applications. Win32/Maslan searches each directory that has a name containing the string "share", "upload", "downlo", "distr", or "setup". The worm searches these directories for files with an .exe, .pif, .rar, or .zip extension that are larger than the original worm-dropper file. After creating the directory C:\___b, it copies files that satisfy these conditions to C:\___b\<original path of copied file>. The files may be damaged during this process. The worm overwrites each file in the original location with a copy of the worm file so that the original file size is not changed. 


There are no readily apparent indications of infection by Win32/Maslan. However, any of the following symptoms may be a result of infection by this worm:
  • Presence of registry value: Microsoft Synchronization Manager
    containing data: ___synmgr.exe
    in any of the following registry keys:
    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\RunServices HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
  • Either the presence of registry value: Microsoft Windows DHCP
    containing data: ___r.exe
    in registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
    or the presence of registry value: Public Microsoft ODBC
    containing data: ODBCr.exe
    in registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
  • Multiple entries having the file name "." are displayed when you enter the command:
    dir %SystemDrive%\%windir%\System32
    at the command line in a command shell. This is because the stealth component dropped by Win32/Maslan replaces any entry that contains "___"  with ".".
  • Files containing the string "___" are not displayed when you attempt to view them in a directory listing. For example, as a test, you can create a file named test___.txt and check whether it is displayed in a file-viewing application such as Windows Explorer.
  • After an automatic or manual recovery from Win32/Maslan, certain files that you expect to find in a particular location may appear instead in the directory C:\___b. This can include any file with extension .exe, .pif, .rar, or .zip that was in a directory with a name containing the string "share", "upload", "downlo", "distr", or "setup". However, files moved to folder C:\___b by Win32/Maslan may be damaged and unusable.


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 webpages.
  • 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 websites.

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 email 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 webpages

Exercise caution with links to webpages that you receive from unknown sources, especially if the links are to a webpage 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 webpage 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 eight characters, and combines letters, numbers, and symbols. For more information, see

Alert level: High
This entry was first published on: Nov 15, 2005
This entry was updated on: Aug 16, 2011

This threat is also detected as:
  • Win32.Maslan (CA)
  • Maslan (F-secure)
  • Net-Worm.Win32.Maslan (Kaspersky)
  • W32/Maslan (McAfee)
  • W32/Maslan (Sophos)
  • W32.Maslan (Symantec)
  • WORM_MASLAN (Trend Micro)