It once was, a user just had to avoid opening suspicious e-mail attachments or downloading dubious software. Now, spyware, malware and viruses can get you through infected online ads, WSJ reports. In May, a hacked banner ad on computer news site TomsHardware.com contained a virus that automatically sent those who clicked on it to a site with malware. Between 50,000 and 100,000 computers were thought to have been affected by the virus. Clicking on ads that appear in the sponsored-link results section of Web-search engines can also entail risks. According to research by anti-virus security firm McAfee in May, about 6.9 percent of sponsored links led to malware-laden sites. Some viruses are so insidious, just visiting a site that contains an infected ad can be enough to affect a computer.

SEE ALSO: Google Buys GreenBorder Technologies

Since marketers often work with a variety of companies when they set up an online ad - one company to sell the ad, one to check it displays correctly, one to save the information of users who click through it - it’s often very easy for a hacker to infiltrate one of those processes. The comments from ad networks in the piece appear to suggest assuredness, while downplaying their concern, lest they appear to invite an attack:

—Online ad exchange operator Right Media, whose purchase by Yahoo was recently completed, said it designed a system called Media Guard that blocks malicious ads. It was first tested in July 2006. Media Guard scanned more than 50,000 ads, and found 17 different viruses.

—Ad network ValueClick combines automated and manual methods to prevent harmful activity, while DoubleClick, whose purchase by Google is still pending, says it provides security technology for its clients but doesn’t control how they use it. And AOL ad network Advertising.com, says it reviews every company it works with and checks ad content.