Google’s AdSense system became very popular recently, with many web sites and blogs making fortunes from advertisement. But it seems that hackers asked themselves why not hijacking the ads Google places on web pages and thus, depriving the huge Internet company of revenue and causing potential problems for end users.

Recently, the Romanian security company BitDefender warned about the existence of a new Trojan virus that replaces Google’s text ads with ads from a different provider. Practically, the Trojan horse modifies end users’ infected computers’ hosts file to replace Google’s ads with their own ones; Google may be reportedly powerless to stop this attack, as it involves the modification of an the internal hosts file, which is used to match domain names of Web sites with IP addresses.

This is the second security warning related to Google’s products being a target for worms and Trojans this week. Only a few days ago, Aviv Raff, a security expert, claimed that an attacker could download malicious files or launch a phising attack by simply spoofing the origin of Google Toolbar’s button.

“This is a serious situation that damages users and Webmasters alike. Users are affected because the advertisements and/or the linked sites may contain malicious code. Webmasters are affected because the Trojan takes away viewers and thus a possible money source from their Web sites,” BitDefender’s virus analyst Attila Balazs said about the recently discovered Google text ads Trojan.

So, although BitDefender described this Trojan as a low risk worm, it could really harm Google’s highly popular AdSense business.