I'm lazy. I don't usually enjoy re-reading things I've already read before (Matt Ruff and Neil Stephenson books are an exception). So, as a service to all 3 of my readers, I'll summarize the changes to the OAuth specification for 1.0a (draft 3) as I understand them. Finding changes in a large document is a pain--you have better things to do with your time. Embrace your inner ignorance and leave responsibility to the wind.
As an added bonus, I'll demonstrate how to update consumer and provider code using the Ruby library.
Much has been written about this elsewhere, so I'll be brief. A session fixation attack was discovered a little over a month ago. Essentially, it affixes your sessions to someone else's. Imagine burrs or inverted balls of duct tape with little teeny messages (on grains of rice) attached to them.
Anyway. It was bad. It was one of those things that once you've seen, it's impossible to un-see. There was no straightforward fix that could be deployed, so you started seeing lots of BIG ANGRY warning notices explaining that your car might be repossessed if you were bold enough to continue.
The longer-term solution was to change the specification, get providers on-board to change their code, and finally to reach out to developers to update their client applications. All so the world doesn't implode next time you turn right. DON'T TURN RIGHT. Ok?
Callbacks (to your site or application) have been supported in various
forms (with varying restrictions), but they have always been parameters
passed to the Service Provider's authorization page. Instead, the
oauth_callback parameter is now part of the Request Token phase. If your
client cannot accept callbacks, the value MUST be
oob. SPs use this
to determine whether a client supports 1.0a.
oauth_callback_confirmed will be present (indeed, MUST according to
the spec) when Service Providers supporting 1.0a issue a Request Token.
Clients can use this to determine whether a server supports 1.0a.
oauth_verifier parameter is provided to the client either in the
pre-configured callback URL or through the fingers of your users (the
oob ("out of band") mechanism). This value MUST be
included when exchanging Request Tokens for Access Tokens.
The changes to the spec are limited to the Request Token ➡ Access Token exchange, so once you have an Access Token, everything should behave as it did before. For this reason, clients that only implement 2-legged OAuth are unaffected.
Section 11.14. Cross-Site Request Forgery (CSRF) is also worth reading (and understanding) in order to further secure the use of callback URLs.
At the time of this writing, the only service provider I'm aware of that supports 1.0a is Yahoo!. This doesn't include Fire Eagle (unless you include my laptop), although we intend for this to change in the next couple weeks. Google's OAuth endpoint should support 1.0a soon, but I've yet to see any confirmation.