Threat description




NetSky.AC worm was found on May 3rd, 2004. Nearly 95% of the code in NetSky.AB is present in NetSky.AC.


Automatic action

Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.

More scanning & removal options

More information on the scanning and removal options available in your F-Secure product can be found in the Help Center.

You may also refer to the Knowledge Base on the F-Secure Community site for further assistance.

Eliminating a Local Network Outbreak

If the infection is in a local network, please follow the instructions on this webpage:

Technical Details

The worm's file is a packed PE executable 36864 bytes long.

Installation to system

Upon execution NetSky.AC copies itself as 'wserver.exe' file to Windows folder and adds a startup key for this file into System Registry:

[HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run]  "wserver" = "%WinDir%\wserver.exe"  

where %WinDir% represents Windows folder name.

And created a mutex name "SkyNet-Sasser" ro ensure only one instance of the worm is running.

Email Spreading

The worm scans all hard drives from C: to Z: to harvest e-mail addresses. The worm looks for e-mail addresses in files with 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  

Netsky.AC worm ignores e-mail addresses that contain 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  

The worm composes different e-mail message. The sender of the message will appear to be any of the following:  

The subject is fixed, always containing the text:


The body will look like:

Dear user of (name)   We have received several abuses:   - Hundreds of infected e-Mails have been sent     from your mail account by the new (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  (Anti-Virus Vendor e-mail)  Note that we do not accept html email messages.  (Anti-Virus Team)  

(Virus name) can be any of the following:

NetSky.AB  Sasser.B  Bagle.AB  Mydoom.F  MSBlast.B  

(Anti-Virus Vendor e-mail) any of:  

And (Anti-Virus Team) any of:

Sophos AntiVirus Research Team  Norman AntiVirus Research Team  MCAfee AntiVirus Research Team  Norton AntiVirus Research Team  

Netsky.AC attaches its executable file to e-mails that it sends out. The attachment name has the following format:

Fix_(Virus name)_(number).cpl  

where (number) will be a decimal number not greater then 32767.

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