SEATTLE – Microsoft Corp. on Thursday filed five first-of-their-kind lawsuits against individuals and companies it suspects of peppering the Web with advertisements laden with malicious code. Each of the suits accuses 20 unknown individuals and the known companies for which they work of distributing “malvertising” and scareware that harms users of Microsoft products.“Our filings in King County Superior Court in Seattle outline how we believe the defendants operated, but in general, malvertising works by camouflaging malicious code as harmless online advertisements,” Microsoft Associate General Counsel Tim Cranton wrote in a post to the software giant’s legal and policy issues blog last week. “These ads then lead to harmful or deceptive content. For example, ads may redirect users to a website that advertises rogue security software, also known as scareware, that falsely claims to detect or prevent threats on the computer. Malvertising may also directly infect a victim’s computer with malicious software like Trojans – programs that can damage data, steal personal information or even bring the users’ computer under the control of a remote operator.”

The companies named in the suits are Soft Solutions, Direct Ad, qiweroqw.com, ITmeter INC. and ote2008.info.

“We hope that today’s filings will help deter malvertising in the future,” Cranton wrote.

Some of the biggest names on the Web have been victimized by malvertising. In mid-August, The New York Times’ website displayed a pop-up malvertisement warning users their computers had been infected by a virus and offering a free “cure.” Both the warning and the cure were bogus.

Cranton called the Microsoft lawsuits “vitally important, because online advertising helps keep the Internet up and running. It’s the fuel that drives search technologies. It pays for free online services like Windows Live, Facebook, Yahoo and MSN. Fraud and malicious abuse of online ad platforms are therefore a serious threat to the industry and for all consumers and businesses that rely on these free services.”

Until the insidious threat is brought under control, Cranton suggested users take common-sense precautions when surfing.

“Make sure you’re using legitimate and up-to-date anti-virus, firewall and anti-malware/spyware tools,” he advised on the Microsoft blog. “Be extra cautious about offers to secure or scan your computer with security software or programs you don’t recognize. Don’t give out personal information or credit card information unless you know the site is secure.”