Hundreds of thousands of websites appear to have been compromised by hi-tech criminals using a well-known attack vector that exploits security loopholes on other sites to insert a link to their website.

Those visiting the criminals' webpage were told that their machines were infected with many different viruses, said a report in our sister publication, the Gulf Daily News.

Swift action by security researchers has managed to get the sites offering the sham software shut down.

Security firm Websense has been tracking the attack since it started on 29 March. The initial count of compromised sites was 28,000 sites but this has grown to encompass many times this number as the attack has rolled on.

Websense dubbed it the Lizamoon attack because that was the name of the first domain to which victims were redirected. The fake software is called the Windows Stability Centre.

The redirections were carried out by what is known as an SQL injection attack. This succeeded because many servers keeping websites running do not filter the text being sent to them by web applications. By formating the text correctly it is possible to conceal instructions in it that are then injected into the databases these servers are running. In this case the injection meant a particular domain appeared as a redirection link on webpages served up to visitors.

Early reports suggested that the attackers were hitting sites using Microsoft SQL Server 2003 and 2005 and it is thought that weaknesses in associated web application software are proving vulnerable.

Ongoing analysis of the attack reveals that the attackers managed to inject code to display links to 21 separate domains. The exact numbers of sites hit by the attack is hard to judge but a Google search for the attackers' domains shows more than three million weblinks are displaying them.

Security experts said it was the most successful SQL injection attack ever seen. – TradeArabia News Service