Attackers appear to have compromised tens of thousands of Web sites using a security weakness in sites powered by the forum software vBulletin, security experts warn.
vBulletin is the the most popular forum platform out there. It runs many, or most, of the sites your admins and developers peruse for technical advice and questions. Now tens of thousands of those sites are hosting malware. Hope you have some web filtering capable of detecting and blocking the flood.
This is the very definition of a watering hole attack, as much as I hate that stupid marketing term.