Tumblr confirmed the ongoing issue to us this morning, and warned users who've seen the message to "please log out of all browsers that may be using Tumblr, " as that's one way the "viral post" is being spread.
The "two-factor" verification system, which will be optional, asks users to register a phone number, e-mail account and six-digit code that would have to be entered, via text message, each time they log in to the site.