Malvertising is an annoying, frustrating phenomenon where ads placed on otherwise legitimate websites are used a a means to infect visitors via the web browser, Flash player, etc. The latest reported instances are for heavily-visited sites like weather.com and drudge.com.
ArsTechnicaMalvertising is a particularly pernicious form of attack because it can infect people who do nothing more than browse to a mainstream site. Depending on the exploit, it can silently hijack computers even when visitors don't click on links. Some browser makers have responded by implementing so-called click-to-play mechanisms that don't render Flash or Java content unless the end user actively permits the plugin to run on a particular site. Some users have resorted to ad blockers, which have the unfortunate side effect of depriving publishers of much-needed advertising revenue.