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Search engines from Microsoft and Yahoo! Have once again been caught displaying ads that direct users to malicious content, some that infects them with malware that's hard to detect and get rid of, researchers said.

Queries such as “FireFox Download,” “Download Skype,” and “Download Adobe Player” typed into the sites returned links promising to deliver the software requested but instead attempted to hijack people's computers, GFI Labs researcher Christopher Boyd said in a blog post published Friday. Clicking on the links takes users to pages that look like the software maker's official site, except for the URL.

“As an example, the fake Firefox file installs a rootkit, runs IE silently in the background attempting clickfraud and also performs Google redirects,” Boyd wrote. Microsoft and Yahoo were in the process of removing the malicious ads, he said.

It's not the first time widely used search engines have been caught displaying ads intended to harm their millions of users. Ad services used by Google and Yahoo have repeatedly been duped into serving content that punts malware and other threats.

Criminals often go to elaborate lengths to pose as legitimate marketers in an attempt to get links to their toxic wares in front of as many eyeballs as possible.

"Microsoft's Security Team has identified the source of this malware attack and is blocking those sites from loading additional malware," the company said in a statement. "We are continuously monitoring our sites to protect customers; and also working with law enforcement authorities to find and prosecute the people responsible for these types of attacks."

A Yahoo representative didn't respond to an email seeking comment. ®

This article was updated to add comment from Microsoft.

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