What is the purpose of the implicit grant authorization type in OAuth 2?

 

I don't know if I just have some kind of blind spot or what, but I've read the OAuth 2 spec many times over and perused the mailing list archives, and I have yet to find a good explanation of why the Implicit Grant flow for obtaining access tokens has been developed. Compared to the Authorization Code Grant, it seems to just give up on client authentication for no very compelling reason. How is this "optimized for clients implemented in a browser using a scripting language" (to quote the specification)?

Both flows start out the same (source: http://tools.ietf.org/html/draft-ietf-oauth-v2-22):

  • The client initiates the flow by directing the resource owner's user-agent to the authorization endpoint.
  • The authorization server authenticates the resource owner (via the user-agent) and establishes whether the resource owner grants or denies the client's access request.
  • Assuming the resource owner grants access, the authorization server redirects the user-agent back to the client using the redirection URI provided earlier (in the request or during client registration).
  • The redirection URI includes an authorization code (Authorization code flow)
  • The redirection URI includes the access token in the URI fragment (Implicit flow)

    • Here's where the flows split. In both cases the redirection URI at this point is to some endpoint hosted by the client:

  • In the Authorization code flow, when the user agent hits that endpoint with the Authorization code in the URI, code at that endpoint exchanges the authorization code along with its client credentials for an access token which it can then use as needed. It could, for example, write it into a web page that a script on the page could access.
  • The Implicit flow skips this client authentication step altogether and just loads up a web page with client script. There's a cute trick here with the URL fragment that keeps the access token from being passed around too much, but the end result is essentially the same: the client-hosted site serves up a page with some script in it that can grab the access token.

    • Hence my question: what has been gained here by skipping the client authentication step?

    Here are my thoughts: The purpose of auth code + token in authorization code flow is that token and client secret will never be exposed to resource owner because they travel server-to-server. On the other side, implicit grant flow is for clients that implemented entirely using javascript and running in resource owner's browser. You do not need any server side code to use this flow. Then, if everything happens in resource owner's browser it makes no sense to issue auth code & client secret anymore, because token & client secret will still be shared with resource owner. Including auth code & client secret just makes the flow more complex without adding any more real security.

    So the answer on "what has been gained?" is "simplicity".

    It's there for security reasons, not for simplicity.

    You should consider the difference between the user agent and the client :

    The user-agent is the software whereby the user ("resource owner") communicates with other parts of the system (authentication server and resource server).

    The client is the software which wants to access the resources of the user on the resource server.

    In the case of decoupled user-agent and client the Authorization Code Grant makes sense. Eg the user uses a web-browser (user-agent) to login with his Facebook account on Kickstarter. In this case the client is one of the Kickstarter's servers, which handles the user logins. This server gets the access token and the refresh token from Facebook. Thus this type of client considered to be "secure", due to restricted access, the tokens can be saved and Kickstarter can access the users' resources and even refresh the access tokens without user interaction.

    If the user-agent and the client are coupled (eg native mobile application, javascript application), the Implicit Authorization Workflow may be applied. It relies on the presence of the resource owner (for entering the credentials) and does not support refresh tokens. If this client stores the access token for later use, it will be a security issue, because the token can be easily extracted by other applications or users of the client. The absence of the refresh token is an additional hint, that this method is not designed for accessing the user resources in the absence of the user.

    The usual explanation is that the Implicit grant is easier to implement when you're using a JavaScript client. But I think this is the wrong way to look at it. If you're using a JavaScript client that requests protected resources directly via XMLHttpRequest, the Implicit grant is your only option, although it's less secure.

    The Authorization Code grant provides additional security, but it only works when you have a web server requesting the protected resources. Since the web server can store the access token, you run less risk of the access token being exposed to the Internet, and you can issue a token that lasts a long time. And since the web server is trusted, it can be given a "refresh token", so it can get a new access token when the old one expires.

    But -- and this is a point that's easy to miss -- the security of the Authorization code flow works only if the web server is protected with a session, which is established with user authentication (login). Without a session, an untrusted user could just make requests to the web server, using the client_id, and it would be the same as if the user had the access token. Adding a session means that only an authenticated user can access the protected resources. The client_id is just the "identity" of the JS webapp, not authentication of said webapp.

    It also means that you can end the session before the OAuth token expires. There's no standard way to invalidate an access token. But if your session expires, the access token is useless, since nobody knows it but the web server. If an untrusted user gained access to your session key, they would only be able to access the protected resources for as long as the session was valid.

    If there's no web server, you have to use the Implicit grant. But this means that the access token is exposed to the Internet. If an untrusted user gains access to it, they can use it until it expires. This means they'll have access to it for longer than with an Authorization Code grant. So you may want to consider making the token expire sooner, and avoid giving access to more sensitive resources.

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