Due to a vulnerability that’s been discovered in the file Author.pm, it’s imperative that any installation of 2.6 be upgraded immediately.
Download the new Author.pm file here:
http://www.movabletype.org/downloads/Author.pm
And replace the version on your server at lib/MT/Author.pm.
Note: If you have yet upgraded to 2.6, you may disregard this message and upgrade to 2.61 at any time.
[from Mena Trott to the MT-Users mailing list]
FYI: It's up to 2.62 now. I have no idea what is critical besides the author.pm file, but the changelog made it seem like upgrading was a "good thing." Everything seems to be working on my site.
I noticed this too. I'm still at 2.51, maybe I'll hold off on upgrading for a little while longer. I also noticed they feel a beta might be prudent for future versions. Another security issue was introduced with 2.61.