Design for Facebook authentication in an iOS app that also accesses a secured web service

I just dealt with this myself, and here's the part that bit me:

In your step 5... It's possible for a user to register for an account with you entirely separate from their Facebook ID, right? Then some other time they log in with Facebook.... And you just created them a second account and lost their first one.

There needs to be a way to be logged in to your web service, then log in to facebook, and capture the association between the facebook ID and the local account.

Apart from that, your plan sounds solid.

Update: Facebook has added a doc outlining such a scenario HERE

Use https to transmit the auth token to your server, as stated by Facebook

Sharing of Access Tokens

Our Data Policies explicitly prohibit any sharing of an Access Token for your app with any other app. However, we do allow developers to share Tokens between a native implementation and a server implementation of the same App (ie. using the same App ID) as long as the transfer takes place using HTTPS.

One problem I can see with this strategy, is that somebody can give you an access token obtained for a different facebook app. As far as I know, there's no way to verify that the access token is for your application, so you'll just go on and use it.

It doesn't sound very harmful, though. Generally people/apps try to protect the access tokens, rather than sharing them.

One possible exploit of this would be, for somebody to create their own site or mobile app, obtain access tokens for their users and try to authenticate them, using your API. If this succeeds (the user is has a facebook account in your site), the malicious site will be able to use your API impersonating the user.

It's a bit of a long shot, but I think it could work.

Edit: It looks like there is a way to validate the access token after all. See the answer by @Daaniel on question Get application id from user access token (or verify the source application for a token).