Security researchers say a malvertising campaign that ran for three weeks before it was discovered highlights the need for businesses to be more vigilant about the kind of companies that participate in online ad networks through programmatic buying.

Several well-known firms, including Answers.com, the Drudge Report and eBay’s U.K. operation, were among the sites that served up ad content that exposed online audiences to the Angler exploit kit, according to an investigation by Malwarebytes. It was a particularly successful example of malvertising, where malicious software is hidden in what look like otherwise legitimate digital ads. Those who clicked on the bogus ads were then subject to attacks from cybercriminals, who could take over their system, demand money or steal personal information. In this particular case, however, clicking on the ads took computer users to sites where they were hit with malware.

As The Register explained, the cybercriminals behind the campaign went to great lengths to try to deceive anyone who might be involved in the online ad-buying process. This includes linking ads back to sites that had been registered with the Better Business Bureau years ago and submitting ads that were completely clean. However, part of the problem involves the use of real-time bidding engines, which essentially gives online advertisers a self-service mechanism for distributing malvertising to unsuspecting publishers.

The onus for staying on top of malvertising may lie less with the publishers than the ad networks, SC Magazine suggested. In this case, those involved included AppNexus, Doubleclick and others. As website owners start to realize that ads can be delivered from the cybercriminals’ own servers via an encrypted HTTPS connection, there may be need for additional checks to ensure the content being served up doesn’t lead to malware. It could also mean some form of testing before ads can go live. For example, Malwarebytes researchers noted that links within the ads were often redirected multiple times before landing on an infected site — something that could have been realized with quick tests before the ads went live.

Of course, malware becomes more effective when it infects computers with old or outdated versions of software such as Adobe’s Flash Player, Dark Reading pointed out. But this case shows that malvertising is becoming so sophisticated that businesses should probably go to greater lengths to make sure the methods they use to monetize online content don’t work so efficiently that attackers can go the better part of a month before they’re found out.