There was a recently discovered serious vulnerability in a popular WordPress plugin: Spam protection, AntiSpam, FireWall by CleanTalk.
You can rad the full report here:
Due to the severity of the vulnerability and its high potential impact, we proactively upgraded this plugin on all websites on our servers to the latest stable version provided by the vendor to protect them from hacking attacks. The vulnerability is already being exploited in the wild, so it was only a matter of time before a hack attempt was made sites on our servers.
We then contacted the owners of all affected sites individually.