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Discovered: May 03, 2004
Updated: February 13, 2007 12:22:27 PM
Also Known As: WORM_NETSKY.AC [Trend], W32/Netsky-AC [Sophos], Win32.Netsky.AC [Computer Asso, I-Worm.NetSky.ad [Kaspersky]
Type: Worm
Systems Affected: Windows

W32.Netsky.AC@mm is a worm that scans for the email addresses on all non-CD-ROM drives on an infected computer. The worm then uses its own SMTP engine to send itself to the email addresses that it finds.

The From, Body, and attachment of the email vary. The attachment has a .cpl extension.

This threat is compressed with PECompact.

Antivirus Protection Dates

  • Initial Rapid Release version May 03, 2004
  • Latest Rapid Release version January 19, 2018 revision 022
  • Initial Daily Certified version May 03, 2004
  • Latest Daily Certified version January 20, 2018 revision 002
  • Initial Weekly Certified release date May 03, 2004

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Technical Description

W32.Netsky.AC@mm contains two modules: The .cpl file is the dropper, and the .exe file is the worm.

When the .cpl file is executed, it performs the following actions:

  1. Copies itself as %Windir%\comp.cpl.

    Note: %Windir% is a variable. The worm locates the Windows installation folder (by default, this is C:\Windows or C:\Winnt) and copies itself to that location.
  2. Inserts and executes the worm module as %Windir%\wserver.exe.

When wserver.exe is executed, it performs the following actions:
  1. Creates a mutex named "SkyNet-Sasser," so that only one instance of the worm executes.

  2. Copies itself as %Windir%\wserver.exe.

  3. Adds the value:


    to the registry key:


    so that the worm runs when you start Windows.

  4. Deletes the values:


    from the registry key:


  5. Scans drives C through Z (excluding CD-ROM drives) and retrieves the email addresses from any files that have the following extensions:
    • .eml
    • .txt
    • .php
    • .cfg
    • .mbx
    • .mdx
    • .asp
    • .wab
    • .doc
    • .vbs
    • .rtf
    • .uin
    • .shtm
    • .cgi
    • .dhtm
    • .adb
    • .tbb
    • .dbx
    • .pl
    • .htm
    • .html
    • .sht
    • .oft
    • .msg
    • .ods
    • .stm
    • .xls
    • .jsp
    • .wsh
    • .xml
    • .mht
    • .mmf
    • .nch
    • .ppt

      The worm will ignore the email addresses that contains any of the following strings:
    • icrosoft
    • antivi
    • ymantec
    • spam
    • avp
    • f-secur
    • itdefender
    • orman
    • cafee
    • aspersky
    • f-pro
    • orton
    • fbi
    • abuse
    • messagelabs
    • skynet
    • andasoftwa
    • freeav
    • sophos
    • antivir
    • iruslis

  6. Attempts to use the default DNS server to retrieve the IP address of the email server.

    For example, if the email address is someone@hostname.com, it will attempt to retrieve the IP address of the server, hostname.com. If the worm fails, it will attempt to use one of the following DNS servers:

  7. Uses its own SMTP engine to send itself to gghjj@yahoo.com, as well as all the email addresses that it finds.

    The email has the following characteristics

    Subject: Escalation

    Attachment: Fix_<random.virus.name>_<random.number>.cpl

    <random.virus.name> is a variable. It will be one of the following strings:
  • NetSky.AB
  • Sasser.B
  • Beagle.AB
  • Mydoom.F
  • MSBlast.B

    <random.number> is a decimal number between 0 and 32767.

    For example, the attachment name could be Fix_Beagle.AB_12345.cpl.

    From: (One of the following)
    • support@symantec.com
    • support@nai.com
    • support@norman.com
    • support@sophos.com

    Dear user of <email.server>,

    We have received several abuses:

    - Hundreds of infected e-Mails have been sent
      from your mail account by the new <random.virus.name> worm
    - Spam email has been relayed by the backdoor
      that the virus has created

    The malicious file uses your mail account to distribute
    itself. The backdoor that the worm opens allows remote attackers
    to gain the control of your computer. This new worm
    is spreading rapidly around the world now
    and it is a serios new threat that hits users.

    Due to this, we are providing you to remove the
    infection on your computer and to
    stop the spreading of the malware with a
    .special desinfection tool attached to this mail.

    If you have problems with the virus removal file,
    please contact our support team at <from.address>.
    Note that we do not accept html email messages.


    Attach: <attachment.name>

    <email.server> is a variable. If the worm tries to send itself to the email address "someone@somewhere.com," <email.server> will be somewhere.com.

    <random.virus.name> is a variable. It will be one of the following strings:
  • NetSky.AB
  • Sasser.B
  • Beagle.AB
  • Mydoom.F
  • MSBlast.B

    <from.address> is the from address of this email

    <random.team> is a variable. It will be one of the following strings:
  • Norton AntiVirus Research Team
  • MCAfee AntiVirus Research Team
  • Norman AntiVirus Research Team
  • Sophos AntiVirus Research Team

    <attachment.name> is the attachment name of this email


Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
  • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
  • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.


The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.

  1. Disable System Restore (Windows Me/XP).
  2. Update the virus definitions.
  3. Restart the computer in Safe mode or VGA mode.
  4. Run a full system scan and delete all the files detected as W32.Netsky.AC@mm.
  5. Delete the value that was added to the registry.
For specific details on each of these steps, read the following instructions.

1. To disable System Restore (Windows Me/XP)
If you are running Windows Me or Windows XP, we recommend that you temporarily turn off System Restore. Windows Me/XP uses this feature, which is enabled by default, to restore the files on your computer in case they become damaged. If a virus, worm, or Trojan infects a computer, System Restore may back up the virus, worm, or Trojan on the computer.

Windows prevents outside programs, including antivirus programs, from modifying System Restore. Therefore, antivirus programs or tools cannot remove threats in the System Restore folder. As a result, System Restore has the potential of restoring an infected file on your computer, even after you have cleaned the infected files from all the other locations.

Also, a virus scan may detect a threat in the System Restore folder even though you have removed the threat.

For instructions on how to turn off System Restore, read your Windows documentation, or one of the following articles:
Note: When you are completely finished with the removal procedure and are satisfied that the threat has been removed, re-enable System Restore by following the instructions in the aforementioned documents.

For additional information, and an alternative to disabling Windows Me System Restore, see the Microsoft Knowledge Base article, "Antivirus Tools Cannot Clean Infected Files in the _Restore Folder ," Article ID: Q263455.

2. To update the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:
  • Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate).
  • Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater).

    The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater" for detailed instructions.

3. To restart the computer in Safe mode or VGA mode
Shut down the computer and turn off the power. Wait for at least 30 seconds, and then restart the computer in Safe mode or VGA mode.
  • For Windows 95, 98, Me, 2000, or XP users, restart the computer in Safe mode. For instructions, read the document "How to start the computer in Safe Mode."
  • For Windows NT 4 users, restart the computer in VGA mode.

4. To scan for and delete the infected files
  1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
  2. Run a full system scan.
  3. If any files are detected as infected with W32.Netsky.AC@mm, click Delete.

5. To delete the value from the registry

WARNING: Symantec strongly recommends that you back up the registry before making any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify the specified keys only. Read the document, "How to make a backup of the Windows registry ," for instructions.
  1. Click Start, and then click Run. (The Run dialog box appears.)
  2. Type regedit

    Then click OK. (The Registry Editor opens.)

  3. Navigate to the key:


  4. In the right pane, delete the value:


  5. Exit the Registry Editor.

Writeup By: Robert X Wang