A blossoming Web attack, first reported by security researcher Dancho Danchev earlier this month, has expanded to hit more than a million Web pages, including many well-known sites.

"The number and importance of the sites has increased," wrote Danchev in a Friday blog posting where he reported that trusted Web sites such as USAToday.com, Target.com, and Walmart.com have been hit with 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 Web sites' internal search engines.

Here's how an attack would work: The attacker searches for popular keywords, such as "Paris Hilton," on the Web site'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 Web site, which then tries to install fake antispyware or a version of the Zlob Trojan Horse malware on the victim's PC.

In order to boost their Google rankings, Web sites 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 more than 1 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 Web sites 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 Web sites. In recent weeks, security vendors have found hundreds of thousands of Web pages affected by this and other similar attacks.