mziwisky/Oauth2.md

## Oauth2.md

      
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              Oauth2.md
            
          
    OAUTH2

The Problem

I’m a web app that wants to allow other web apps access to my users’ information, but I want to ensure that the user says it’s ok.
The Solution

I can’t trust the other web apps, so I must interact with my users directly.  I’ll let them know that the other app is trying to get their info, and ask whether they want to grant that permission.  Oauth defines a way to initiate that permission verification from the other app’s site so that the user experience is smooth.  If the user grants permission, I issue an AuthToken to the other app which it can use to make requests for that user's info.
Note on encryption

Oauth2 has nothing to do with encryption -- it relies upon SSL to keep things (like the client app’s shared_secret) secure.
Example Scenario:

Gmail wants to allow some 3rd party app, PrettyMail, to do stuff with its users’ information.
Gmail stores server-side:      PrettyMail stores server-side:      Note: client_id tracks the user of the
  oauth_clients: [               client_id: ABC                    oauth privileges, i.e., PrettyMail or 
    pretty_mail: {               shared_secret: XYZ                other web app -- not the end-users
      client_id: ABC
      shared_secret: XYZ
    }, ...]


I visit PrettyMail and click “Login thru GMail”


PrettyMail responds REDIRECT gmail.com/oauth2/auth?client_id=ABC&redirect_uri=prettymail.com/oauth_response
-- note: also common to include ‘scopes’ in query -- i.e., the scope of the information that PrettyMail is asking to access


Gmail makes a session in which it stores provider (PrettyMail, based on client_id -- if client_id doesn’t refer to an authorized oauth_client, render an error) and redirect_uri and then responds:
a. REDIRECT gmail.com/login (for a login form) if the user isn’t logged in, otherwise
b. REDIRECT directly to step (4)


Gmail shows a page saying “PrettyMail (got that from the aforementioned session) wants to access this, that, and the other thing (again, ‘scope’ of access was stored in the session) about your Gmail account.  Do you authorize?”  You click “yup” and Gmail:


generates a one-time-use code that it associates with PrettyMail and the specified user and the requested scope (so it persists it until the next step) and REDIRECTs to the redirect_uri it got in the first place, passing along that code: prettymail.com/oauth_response?code=big_long_thing
Question: why not pass the AuthToken itself along at that step?  Answer: so Gmail can ensure that PrettyMail is indeed the requester of the access -- so far, all the requests Gmail has gotten have come directly from the user, and the only information that identified PrettyMail was its client_id, which isn’t “secret” (i.e. an attacker could guess it).  So Gmail so far is confident that the user is ok with everything, but isn’t yet convinced that PrettyMail is really going to be the one using the Token.  That’s where the next request comes in, in which PrettyMail identifies itself by including the shared secret.


PrettyMail takes the code and directly (i.e., not via a REDIRECT in the user’s browser, but via a server-to-server request) queries Gmail with both code and shared secret, to prove its identity: GET gmail.com/oauth2/token?client_id=ABC&client_secret=XYZ&code=big_long_thing


Gmail verifies and then invalidates the code, and responds with an AccessToken, which PrettyMail can then use (until it expires) to make API requests on the user’s behalf (i.e., to get info about the user, and possibly to perform actions on behalf of the user, if that was in the agreed-upon ‘scope’ of the arrangement)


Security Addendum: the state param

This gist has gotten more attention than I thought it would.  Not an avalanche by any means, but I still feel I should point out an important omission from the original -- there's an XSRF vulnerability in Oauth2 that is easily closed by use of a state param.  This param enables the 3rd party app (PrettyMail) to trust that the request from step (5) above (to the redirect_uri) is legitimately from the user.  Here's a StackOverflow about it, and here's a blog post that goes into greater detail.  In the name of simplicity (and laziness), I have not modified the above explanation to include this param.  You, however, should not be lazy in your implementation -- you should include it.