Threat description



Naco.F worm was found on June 12th, 2003. It can spread via email and peer-to-peer networks. It also tries to steal or delete user's data. Additionally the worm has backdoor capabilities. The worm arrives in emails which subject, body text and attachment name vary.


Automatic action

Depending on the settings of your F-Secure security product, it will either automatically delete, quarantine or rename the suspect file, or ask you for a desired action.

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 more information.

Contact Support

F-Secure customers can request support online via the Request support or the Chat forms on our Home - Global site.

Technical Details

This worm's variant is close to the previous one - Naco.E but it has much more bugs that can render an infected system inoperable shortly after infection. Out test workstation and Exchange server were jammed by a huge number of e-mails that the worm sent. Also on a test workstation there appeared numerous Registry Editor's import failure messageboxes..

The differences comparing to the previous version are as follows:

1. The worm drops itself to Windows System directory as CSRSS32.EXE file. There can be more than one copy of the worm in memory.

2. The worm's file is compressed with TELOCK file compressor. The compressed file's size is 45568 bytes.

3. Most of the worm's text strings are encrypted with a simple cryptoalgorithm. The worm dynamically decrypts its strings when it uses them.

4. The worm displays a different messageboxes:

You are the most pretty girl I ever saw!
Anacon 6 W0rm
THanX f0r SupPoRted:
Dincracker, Foot-Art, PakBrain, Fady911x, Anacon, Axam, Sh4m_Skru, AjeedNASA,
Invisibleman, Zied666 and all my frenz...

5. The worm puts a different message on a defaced webserver:

Melhacker WhAcKeRs
Melhacker + Anacon Gotcha! New Version Of Anacon Worm!
You Are Hacked By WhAcKeRs Team!

6. The worm copies itself many times to Startup folder with random name that consists of four numbers. On our test system the worm created more than 250 files in Startup folder.

7. The worm sends itself in e-mail usually as CSRSS32.EXE. It can also use a four-digit randomly generated name for its attachment, for example 5131.EXE. It should be noted that a recipient of an infected message will see a different attachment's name - CLIMBING.JPG with some e-mail clients, for example with Microsoft Outlook while Netscape shows the attachment name correctly.

8. The worm can infect EXE files in Windows System directory. Due to bugs in the worm's code it can infect files multiple times.


F-Secure Anti-Virus detects Naco.F worm with the updates published on June 12th, 2003:

Database: 2003-06-12_03

Submit a Sample

Suspect a file or URL was wrongly detected? Send it to our Labs for further analysis

Submit a Sample

Give And Get Advice

Give advice. Get advice. Share the knowledge on our free discussion forum.

More Info