"An ad company that Yahoo owns, Right Media, served up some particular advertisements several million times that ended up being loaded with Trojans. These ads, while all over the Internet, were most prominently featured on MySpace and PhotoBucket – not shady warez sites.
The issues began last month, and according to ScanSafe the articles were seen many, many times:
The banner ads, which were brokered by Right Media, were served an estimated 12 million times over a three-week period starting in early August, according to ScanSafe, a managed security provider. Earlier this year, Yahoo paid $650m to acquire the 80 percent of the company it didn't already own. "