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Comment by EGreg | Hacker News Reader

No, that's not what I am asking. And I am very familiar with how to combat XSS and CSRF, having written a PHP framework that does it for you out of the box, among other things (http://phponpie.com)

I am asking another question.

If I am operating an OpenID provider, say Auth.com, and CoolStuff.com uses me to authenticate a user, then they can use that session. Great. But now let's say I operate an OAuth provider, and it releases this user's uid, first_name and last_name. What measures does facebook and other OAuth providers take to prevent the uid, first_name, last_name and other data from just being changed by the user in javascript, before being posted to the CoolStuff.com servers, after they have been obtained using Javascript ... such as FB.api('fql.query', ...)

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If I understand the question correctly, you are asking what's to stop someone from changing the UID, etc when the user is redirected from auth.com to coolstuff.com?

From my understanding of OAuth, when the uid, first_name and last_name are sent by auth.com it also sends a cryptographic hash of everything. So if you change the uid, you would also have to change the hash and you can't change the hash without knowing the shared secret that the auth.com and coolstuff.com have decided on prior to your request.

so what you are saying is that coolstuff.com's servers must always look for a signature from auth.com when they are accepting user data, they cannot trust the user input. That makes sense. So does facebook connect, google, etc. have this? I assume not.

Well if you protect against CSRF you should be checking a nonce for all form submissions anyway. This doesn't appear to be all that different to me.

I'm intrigued now, I hope someone can give details!

OAuth 2.0 login providers use an access token that is user specific for all subsequent interaction.

For example, with Facebook, once a user has logged in and given Application X permission to access their details, FB will send the user's id, name, etc. (whatever data the user has granted access to) along with a unique access token. The next time, and every subsequent time Application X wants to access the FB API on the user's behalf it is required to send that access token. From javascript you might be able to change the token, however, Application X's next interaction with the FB API will fail if the token is invalid and there is no way to derive a token value from a FB user's id.

That's it? That sucks.

I don't have to change the token. I just have to change the data given by facebook (including the uid) before the website's dumb javascript uses it in a post back to the server. Since it's not signed by Facebook, how can the website's server trust the uid? Never trust your user input.

The user logs in on Facebook's server, there is no opportunity to change a uid. Facebook might return the logged in user's id, however that's not useful, the only way to interact with their API will be with the access token (which only grants you access to the logged in user's scope).

I am not talking about interacting with their API.

Facebook returns a uid. When the user takes an action, this uid is sent to the server. The server trusts the uid, and saves this action as taken by the user identified by this uid.

And let's say it's not the uid. Let's say it's the user's name.

It trusts the user input basically. But it should probably be getting it directly from facebook, or in a signed structure, right?

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