Antivirus software firm AhnLab says 16 domestic Web sites are under massive distributed denial-of-service (DDoS) attacks.

As of 9 a.m. Friday, 16 Web sites of 13 local companies came under the DDoS attack, including Daum, Nate, MSN, JoongAng Ilbo, CBS and AhnLab.

The firms believe the attacks were generated by malicious codes found in July and recently discovered variants of the malicious codes.

AhnLab said it has been confirmed more than ten-thousand zombie PCs were infected with malicious codes but no serious damage was done to the sites.

Users of most of the infected PCs are known to have not downloaded vaccines or upgraded antivirus software since July when the malicious codes were detected.

The Korea Internet Security Agency said it is assessing the damage though it doesn't appear to be significant.