Malvertising is formed from malicious plus advertising. It’s an online scam in which reputable sites are tricked into distributing bogus advertisements that link to malicious code. It has become a significant issue within the past year, with many well-known sites suffering from the problem, and it’s expected to get worse.
A classic example is an advert that offers a free anti-virus scan, but which instead downloads an application that takes over your browser to harvest credit-card numbers and passwords or send fake e-mails through your account. Since such fake scans work by frightening users, they are also called scareware. The problem for legitimate owners of sites is that it’s hard to detect the fake ads until somebody complains, by which time damage has been done and the publisher’s reputation compromised.