Security researchers have identified a Trojan that hijacks Google text advertisements, replacing them with "ads" from a different provider that are likely to be laced with spyware.

The Qhost-WU modifies an infected computer's hosts file, thereby poisoning systems with bogus DNS lookup records. The hosts file matches domain names of websites with corresponding IP addresses. By corrupting the file hackers can redirect surfers to domains controlled by hackers even when users visit a trusted location.

In this case, the modified file contains a line redirecting the host "page2.googlesyndication.com" from a server run by Google to an imposter, potentially depriving web masters of revenue while leaving infected punters in a pickle.

"This is a serious situation that damages users and webmasters alike," said Attila-Mihaly Balazs, a virus analyst at Romanian security firm BitDefender. "Users are affected because the advertisements or the linked sites may contain malicious code, which is a very likely situation, given that they are promoted using malware in the first place. Webmasters are affected because the Trojan takes away viewers and thus a possible money source from their websites."

Although damaging, the Qhost-WU Trojan hasn't spread particularly widely. BitDefender rates it as a medium risk pathogen.

Without commenting on Qhost-WU specifically, Google said it purges entries for sites containing malware from its search index. "We have cancelled customer accounts that display ads re-directing users to malicious sites or that advertise a product violating our software principles," the ad trafficking giant told Reuters. ®

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