A security company has identified a Trojan horse program that replaces Google text advertisements on Web pages with ads from another source, depriving Google of revenue and potentially causing problems for end users.

Google may be powerless to stop the trick since it involves the modification of an internal PC file, called the hosts file, that is used to match domain names of Web sites with IP (Internet protocol) addresses, said Romanian security company BitDefender.

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When a person visits a Web site, the browser checks the hosts file to see if it has an IP address for a particular domain name. If the hosts file is corrupted or hijacked, the browser can be directed to fetch a different Web page than the one the user intended to.

Modifying the hosts file can be done for legitimate reasons. For example, PC users can change the hosts to block banner ads served from known ad networks. When a Web page tries to contact an ad server, the request is diverted by the hosts file and no ad appears.

BitDefender said in an advisory this particular malware directs a browser to download advertisements from a different server than Google's ad server.

BitDefender named the malware Trojan.Qhost.WU and said it is not spreading fast and poses a "medium" risk of damage. It did not say how the Trojan is being circulated, and company representatives did not return a call for comment.

Besides costing Google ad revenue, there is a danger that those replacement advertisements could contain links to sites with malicious software, BitDefender said. Web site owners who buy ads through Google, as well as Google itself, can lose out on both Web traffic and revenue if people are diverted from its ads.

There is not much that Google can do for those who download the malware. However, security products such as BitDefender's can detect and remove it.

Without commenting on the Trojan specifically, Google said it removes Web sites from its search index that contain malware.

"We have canceled customer accounts that display ads re-directing users to malicious sites or that advertise a product violating our software principles," the company said in a statement.