Clicking a banner advertisement could be dangerous to your PC's health, if the click takes you to a Web site that attempts to download malicious code.
This happened last week, when some European Web sites were victims of an attack that turned their banner advertising into hyperlinks to sites run by cyber criminals. The attack affected sites in the UK, Sweden and the Netherlands, including The Register computer news site. All were customers of Falk AG, a German ad serving company.
This particular attack is known as the Bofra/IFrame exploit, and it takes advantage of a weakness in Microsoft's Internet Explorer 6.0 browser. The result was that for around six hours -- until the problem was detected -- users of the affected sites were in danger of being redirected to sites that tried to install malicious software such as "porn dialers" and spyware on their PCs.
The exploit does not affect users running alternative browsers, such as Firefox, or Windows XP users who have installed Service Pack 2 (SP2).
Falk AG issued a statement saying: "The cause was a hacker attack on one of our load balancers. This attack made use of a weak point on this specific type of load balancer."
The function of a load balancer is to evenly distribute requests to the multiple servers behind it. This particular system was only used to handle a specific request type to its ad server. Once it became aware of the problem, Falk AG shut down the affected load balancer and removed it. It also carried out checks to ensure all other load balancers were secure.
One of the UK sites affected, The Register, informed readers of the problem and suspended ad serving from Falk for three days. Drew Cullen, the editor of The Register, said: "This is a cause for concern because we remain unsure whether the cause of the problem has been clearly identified. We are also concerned that many sites affected have just kept quiet. When something like this happens, it is imperative that sites make a full disclosure."
Asserta.com, another of Falk's UK customers, would not comment. It remains unclear how many sites were affected, but The Register estimates it could be as high as 30.
On its site, The Register said it had 11,660 unique visitors using Windows and IE6 during the period it was being served malicious ads. But the impact was likely to be limited, because Falk estimates that only one in 30 requests for a banner ad would have been redirected to the site containing the Bofra worm.
Danny Meadows-Klue, the chief executive of the Interactive Advertising Bureau, said the problem was being closely monitored.
"Every community has its vandals and we have to warn users to ensure they are protected with anti-virus software and firewalls.
"Unfortunately, this industry does suffer periodically from this kind of thing," he said.
He agreed that such attacks could increase, but did not think it was likely.
However, analyst firm Gartner is predicting that Bofra/IFrame attacks will increase, especially around systems with what it calls sloppy patching. In an e-mail, it said: "Attacks of this type will become increasingly common, especially around transition points -- systems where multiple versions of software (such as Windows 2000 and XP) are in use without full patches across both platforms."
Meanwhile, Microsoft issued a statement saying it was investigating the attack and confirmed that the vulnerability only existed for IE6. It is encouraging customers to upgrade to SP2.