HSTS.plist - anyone know what this is about/from? HSTS.plist. Anyone know what this file is about? I thought it was a mal file slipped in, but it does seem to come from Apple. I have accounts at none of the sites listed in the file and visited twitter (it's listed in the file) only once recently to see what William Shatner had to say.

It is found in ~/Library/Cookies/HSTS.plist

Delete it, it comes back.

Found in both Mavericks and Yosemite.
HSTS.plist is a Safari settings file related to HTTP Strict Transport Security. It is a mandatory setting for Mavericks and newer OS releases. For more information about this security policy mechanism, you can read up about it at Wikipedia:

http://en.wikipedia.org/wiki/HTTP_Strict_Transport_Security