Hackers are using banner ads on websites to hide malicious software, or "malware," that hijacks computers without the user even clicking on them.

The malware has been spotted on several legitimate websites, including ones run by The Economist magazine and Major League Baseball, as well as Canada.com, Wired magazine reported on Thursday.

Hackers are hiding the harmful code in the DART platform used by DoubleClick, which is a major online ad services company. Many web publishers, including the CBC, use DoubleClick to manage their ad inventory.

The malicious code causes the web browser window to close and re-open, then redirects the user to an antivirus site. A dialog box then appears and tells the user their computer is infected and their hard drive is being scanned.

Because the malware is hidden in an ad, there is little a user can do to detect it before it is too late.

Matt Doris, manager of advertising operations for AOL Canada, which services ads for CBC.ca, said that while the site is potentially vulnerable, there have not been any reports of compromised ads related to this malware.

"Any site serving ads is vulnerable," he said.

Doris said all the ads on the CBC website are being reviewed for the malware. He added that at this point, there is not much a user can do to avoid it.

DoubleClick has acknowledged the problem and said it has implemented additional security measures, which have captured and disabled a hundred ads.

"Unfortunately, there are bad actors who misrepresent themselves and purchase advertising as an avenue to distribute malware," Sean Harvey, senior product manager at DoubleClick DART, told Wired. "This has the potential to affect all businesses and consumers in the online environment."

DoubleClick urged web publishers to pay close attention to the agencies and advertisers they work with, particularly any new ones.

Security firms have suggested the malware is the work of AdTraff, an online marketing company that poses as a legitimate advertiser.

AdTraff could not be reached for comment by Wired.

Internet search leader Google Inc. is in the midst of a $1.3-billion U.S. acquisition of DoubleClick, but the deal has hit a snag with European regulators expressing antitrust concerns.