An attack on websites, first reported by security researcher Dancho Danchev, has expanded to hit over a million web pages, including many well-known sites.
"The number and importance of the sites has increased," wrote Danchev in a blog post, where he reported that the websites for USAToday and Walmart were among those hit by the attack.
The criminals behind this have not actually hacked into servers, but they are taking advantage of web programming errors to inject malicious code into search results pages created by the websites' internal search engines.
Here's how an attack would work: the attacker searches for popular keywords, such as "Paris Hilton," on the website's internal search engine. But instead of conducting a normal search, the bad guy tacks an HTML command to the end of his search. This command that opens up an invisible iFrame window in the victim's browser that then redirects it to a malicious site, which then tries to install fake anti-spyware or a version of the Zlob Trojan Horse malware on the victim's PC.
In order to boost their Google rankings, websites often save a copy of these search results and submit them to Google. When a victim searches Google for the keyword, these cached search results then pop up, with the malicious code now inside them.
"Malicious parties are actively poisoning these sites search query caching feature to position the keywords among the top ten search results, thereby infecting anyone coming across them," said Danchev, in an instant-message interview.
He believes that over one million web pages have been infected using this technique.
"The more keywords they submit with [malicious] script, the more pages with popular keywords the high page ranked sites would cache," he said. This increases the chance that someone will see the search results hosted on the reputable site and click on the malicious page.
The websites that have been hit with this attack could fix the problem by doing a better job of checking the search queries on their internal search engines to make sure that there is no malicious code in them, Danchev said.
Hackers are increasingly looking for ways to install their code on trusted websites. In recent weeks, security vendors have found hundreds of thousands of web pages affected by this and other similar attacks.