The Web site for Indian antivirus vendor AvSoft Technologies has been hacked and is being used to install malicious software on visitors' computers, security researchers said Thursday.

The download section of AvSoft's S-cop Web site hosts the malicious code, according to Roger Thompson, chief research officer with security vendor AVG. "They let one of their pages get hit by an iFrame injection," he said. "It shows that anyone can be a victim. ... It's hard to protect Web servers properly."

The technique used on the site has been seen in thousands of similar hacks over the past few months. The attackers open an invisible iFrame Window within the victim's browser, which redirects the client to another server. That server, in turn, launches attack code that attempts to install malicious software on the victim's computer.

The malicious software is a variant of the Virut virus family.

The iFrame pages are commonly used by Web developers to insert content into their Web pages, but because it is possible to create an invisible iFrame window, the technology is often misused by hackers as a way to silently redirect victims to malicious Web sites.

AvSoft, based in New Delhi, sells an antivirus product called SmartCOP and has sold a second antivirus product called Smartdog. The company, which is not well-known in the US, also specializes in recovering data lost due to virus attacks. The company could not be reached for comment Thursday afternoon.

That data recovery service could come in handy for some, as Virut is known as a "parasitic infector" virus that is extremely difficult to remove. "It infects all of your programs on your local hard drives, and then it starts hitting your network drives as well the first time you run," Thompson said.

Fortunately, the malware used to install Virut exploits only well-known bugs, meaning that users who are running antivirus software on fully patched systems will probably not be infected by the attack in its current state, security experts say.

Nobody knows how the malware got onto the Web site in the first place. News of the hack was reported on the Full Disclosure security discussion list on Thursday.

McAfee Security Research Manager Dave Marcus believes that the site was compromised by exploiting a Web programming error, most likely in the site's SQL or PHP code. Security experts say that criminals have written automated programs that scour the Web for these types of flaws and then automatically infect sites, making this an increasingly common problem.