When people ask me what my greatest security concern is these days, I say ads that appear on websites. I don't blame the website itself, of course, as I understand the need for ads and I also believe that the site adminstraters use trusted sources for the ads.
Even so, the ads are too often the source of malware attacks.
That is the case right now with OpenX ad server. According to an article at The H Security, a vulnerability in the free server is being exploited to distribute malware. The article stated:
A server that provides The Pirate Bay with ad banners was hacked, but browsers that use Google's Safe Browsing API to reach the site are warned that it has dangerous content.. . . The problem is the result of a component integrated in OpenX's video plug-in, from a third-party, which allows images to be uploaded. In December 2009, the module "ofc_upload_image.php was" introduced, and it does not check who is uploading what to the server. As a result, executable scripts can be saved and executed on the server.
In addition, an article at Thinq.co.uk reported:
Visitors to the site who aren't running a background virus scanner or who don't use browsers that check Google's list of 'bad' sites are likely to have been exposed to a variety of nasty malware, none of which was directly hosted on The Pirate Bay but instead held on the cracked advertising server.