Threat Descriptons



Category :


Type :


Aliases :

NetSky.N, W32/Netsky.N, I-Worm.Netsky.n


The Netsky.N variant was discovered on March 16th 2004. The worm is similar to its predecessors in the family. The only major difference is that a fake note claiming the message to be virus free is added to the infected messages.


Based on the settings of your F-Secure security product, it will either move the file to the quarantine where it cannot spread or cause harm, or remove it.

A False Positive is when a file is incorrectly detected as harmful, usually because its code or behavior resembles known harmful programs. A False Positive will usually be fixed in a subsequent database update without any action needed on your part. If you wish, you may also:

  • Check for the latest database updates

    First check if your F-Secure security program is using the latest updates, then try scanning the file again.

  • Submit a sample

    After checking, if you still believe the file is incorrectly detected, you can submit a sample of it for re-analysis.

    Note: If the file was moved to quarantine, you need to collect the file from quarantine before you can submit it.

  • Exclude a file from further scanning

    If you are certain that the file is safe and want to continue using it, you can exclude it from further scanning by the F-Secure security product.

    Note: You need administrative rights to change the settings.

Technical Details

System Infection

Upon execution the worm copies itself to the Windows System Directory with the filename 'VisualGuard.exe' which is added to the registry as

[HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run] =

"NetDy" = "%SysDir%\VisualGuard.exe"

The worm removes several registry values that belong to other worms.

Email Propagation

When collecting addresses NetSky.N recursively walks through all hard drives and checks the content of files with the following extensions:


The email messages are randomly composed from different components. For example the subjects are made of the following texts:

"Re: Re:"
"your "
"my "
"approved "
"important "
"word document"
 "excel document"
"read it immediately"

Email bodies are chosen from many different predefined texts:

"Please see the attached file for details."
"Please read the attached file."
"Your document is attached."
"Please read the document."
"Your file is attached."
"Your document is attached."
"Please confirm the document."
"Please read the important document."
"See the file."
"Requested file."
"Authentication required."
"Your document is attached to this mail."
"I have attached your document."
"I have received your document. The corrected document is attached."
"Your document."
"Your details."

The attachment names are hardcoded and can be one of

"doc.txt [lots of spaces].exe"
"document.htm[lots of spaces].scr"
"your_details.doc[lots of spaces].exe"
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