Defense in Depth

On Thursday, Webmasters around the world noticed unusual spikes in traffic. For some smaller sites the sudden surge of Web traffic toward their sites appeared to be almost a denial-of-service attack.

Turns out it was the free version of AVG Antivirus 8.0 just doing its job.

In a statement on Saturday, Grisoft said "We have actively listened to the Webmasters who have brought this to our attention, and as a company we have reacted quickly to solve them." What it did was issue a new build of the popular free program.

What's different in version 8 from previous versions is the inclusion of Linkscanner, a scanner that stops malware components embedded on compromised Web pages. LinkScanner was created by Exploit Prevention Labs and purchased last summer by Grisoft, maker of AVG products.

One feature of LinkScanner, Secure Shield, works by downloading the home page of each site returned in a common Web search then populates the search result page with colored icons indicating the relative safety of those sites. The feature, which has been previously available, apparently didn't scale to the large numbers of AVG free customers. On Monday, Roger Thompson, who developed LinkScanner and is now chief research officer for Grisoft, confessed, "We knew it would create a spike of some sort, but nothing like what happened."

How dramatic was the surge in traffic? The site AVG-Watch.org provides charts on bandwidth use after the release of AVG 8.0.

In an e-mail to CNET News, Thompson went on to say: "We did not consider the multiplying effect of any given Web site's own marketing within search engine results. In other words, if a Web site, through its marketing, became a common search result, it was scanned much more often than we expected. As soon as we found out, we gathered some data, talked to some Webmasters, and figured out what to do."

However, Thompson disputed a claim by AVG-Watch.org that the updated AVG version now only "pretends to prefetch," and does little more than a DNS (Domain Name System) lookup of the site. Thompson said "it doesn't pretend to pre-scan. It just works off the local blacklist. That involves a DNS lookup, so that we can compare both IPs and URLs."

Making matters worse last week, AVG disguised the scans as coming from Internet Explorer 6 browsers, and not Secure Shield. For a few days it was unclear who was responsible for the surge in Internet traffic. Thompson said they could have made the LinkScanner scans entirely stealth, but they wanted to give Webmasters the option of filtering the scans.

"The real issue is that, like it or not, we're at war on the Web," said Thompson. "Criminals, both organized and opportunistic want our PCs and our money, and they're attacking via the Web. It's no longer like the old days when they wrote this stuff for fun."