Malvertising works by camouflaging malicious code as harmless online advertisements. These ads then lead to harmful or deceptive content. For example, ads may redirect users to a Web site that advertises rogue security software, also known as scareware, that falsely claims to detect or prevent threats on a computer. Malvertising may also directly infect a victim's computer with malicious software like Trojans.

Escalating the Malvertising Battle

Although Microsoft actively engages with other leaders of online ad platforms to mitigate malvertising threats, Tim Cranton, associate general counsel at Microsoft, said the company is taking the next step in the battle.

The lawsuits allege that individuals using the business names Soft Solutions, Direct Ad, qiweroqw.com, ITmeter INC. and ote2008.info used malvertisements to distribute malicious software or present deceptive Web sites that peddled scareware to unsuspecting Internet users.

"Although we don't yet know the names of the specific individuals behind these acts, we are filing these cases to help uncover the people responsible and prevent them from continuing their exploits," Cranton said. "We hope that [Thursday's] filings will help deter malvertising in the future, but meanwhile, adopting a few good habits can help you avoid online scams and ensure the safest computing experience possible."

Avoiding Online Scams

Microsoft offers three key points to consumers hoping to avoid malvertising. First, the software giant suggested, make sure you're using legitimate and up-to-date antivirus, firewall and anti-malware and spyware tools. Second, be extra cautious about offers to secure or scan your computer with security software or programs you don't recognize. Finally, don't give out personal information or credit-card information unless you know the site is secure.

"This work is vitally important because online advertising helps keep the Internet up and running," Cranton said. "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."

Thursday's filings build on other recent actions Microsoft has taken against click fraud and instant-messaging spam (aka "spim"). The malware suits also call to mind actions Microsoft has taken against software bootleggers.

But Greg Sterling, principal analyst at Sterling Market Intelligence, said Microsoft is acting in self-interest with the malware suits.

"If the ad platform becomes a launching pad for malware or phishing or whatever malicious advertising, it hurts Microsoft's ad programs and the brand more generally," Sterling said. "With software, Microsoft is trying to shut down a market that may directly impact profits and sales. With the malware stuff, the impact on Microsoft is indirect and less immediate."