Online ads can be annoying, distracting and occasionally intriguing, but they should never be something Web surfers have to fear. Late last week, however, Jerome Segura, senior security researcher for Malwarebytes, published a blog post reporting that DoubleClick and Zedo, two of the world’s largest ad networks, were serving online ads that could install Zemot, a piece of malware that connects to remote servers and installs even more malicious software.

The attack works by directing unsuspecting users to websites that host an exploit kit called Nuclear, which runs on older versions of Microsoft’s Internet Explorer browser or Adobe Systems’ Flash. If it detects them, Zemot is used to target and infect those devices.

In its report on the online ad attack, Ars Technica described Zemot as a “beachhead” of cybercriminal activity, potentially unleashing a Pandora’s box of Trojans that steal personal information or spambots that render computers nearly useless.

DoubleClick, which is owned by Google, immediately severed its connection to Zedo, effectively stopping the malware from spreading, it told Ars Technica. However, Segura estimated the attack could have affected millions of computers.

‘Malvertising’ Hits Malware Scene With DoubleClick

As an article on Tom’s Guide pointed out, so-called “malvertising” is on the rise. Besides Doubleclick, other examples include a network called Kyle and Stan, which managed to use online ads on nearly 6,500 domains, including well-known sites such as YouTube and Amazon.

The online ad attack will be of concern to any chief information security officer who works for a company whose website uses online ad networks (which is nearly everyone). Ad networks such as DoubleClick act as a sort of middleman between advertisers that want to reach specific targets and online properties that have excess or unsold inventory on their domains.

Malvertising is scary because it goes largely undetected by the sites serving the ads — in this case, the domains distributing the malware included streaming radio service Last.fm and newspapers such as the Jerusalem Post and the Times of Israel — and potentially the ad networks themselves, such as DoubleClick. If more consumers start installing ad blockers in response to these sorts of attacks, it could threaten what could be an important incremental revenue stream.

In fact, a report from Blue Coat Systems published in March said malvertising has overtaken pornography as the No. 1 malware threat on mobile devices, with activity tripling since it began tracking the problem two years ago.

Of course, the best recourse is for consumers to ensure their machines are running nothing but patched, up-to-date software. If nothing else, the attention this attack has received will hopefully act as a sort of massive online ad campaign to convince them to do so.