Tumblr seems to be suffering from a viral hack at the moment, as several blogs appeared to have been compromised and are now displaying a message from the notorious troll organization GNAA.The problem seems to be with Tumblr, which has acknowledged it, so account credentials probably haven't been compromised.
"There is a viral post circulating on Tumblr which begins 'Dearest 'Tumblr' users.' If you have viewed this post, please log out of all browsers that may be using Tumblr immediately. Our engineers are working to resolve the issue as swiftly as possible," Tumblr explained.
There's no way to know how many blogs have been affected so far and the only way to avoid your blog being taken over is to not use Tumblr and log out of your account. Not the greatest of fixes, but it's all that works for now.
The exploit through which all of this was accomplished is unknown for now, the speculation is that the hackers were able to use a bug in Tumblr's embedding system and get their scripts to run from there.