it's in advertisements. I don't think SR has any control on it because most sites uses general ad brokers and the site displaying the ads have very little control on them, depending on the ad broker.
This being said, it could happen that you already have the downloader in your PC, and the ads only trigger it with new info. But quite probably is just the advertisements trying to get into your computer via Java or ActiveX. In 99.99% of cases, if an advertisement is based in Java is because it has every possible trojan in the world