Why is there an “Authorization Code” flow in OAuth

Question

With the "Implicit" flow the client (likely a browser) will get a access token, after the Resource Owner (i.e. the user) gave access.

With the "Authorization Code" flow however, the client (usually a web server) does only get an authorization code after the Resource Owner (i.e. the user) gave access. With that authorization code the client then makes another call to the API passing client_id and client_secret together with the authorization code to obtain the access token. All well described here.

Both flows have the exact same result: an access token. However, the "Implicit" flow is much simpler.

The question: Why bother with "Authorization Code" flow, when "Implicit" flow seams to be fine? Why not also using "Implicit" for webserver?

It's more work both for the provider and the client.

Answer 1

From paragraph 4.2 of OAuth spec:

The implicit grant type is used to obtain access tokens (it does not support the issuance of refresh tokens) and is optimized for public clients known to operate a particular redirection URI. These clients are typically implemented in a browser using a scripting language such as JavaScript.

As a redirection-based flow, the client must be capable of interacting with the resource owner's user-agent (typically a web browser) and capable of receiving incoming requests (via redirection) from the authorization server.

Unlike the authorization code grant type in which the client makes separate requests for authorization and access token, the client receives the access token as the result of the authorization request.

The implicit grant type does not include client authentication, and relies on the presence of the resource owner and the registration of the redirection URI. Because the access token is encoded into the redirection URI, it may be exposed to the resource owner and other applications residing on the same device.

So what we can concider:

1. This is for public OAuth i.e. when client not needed to be registered and doesn’t have it’s own client secrets. But what auth server checks redirect url and this is actually enough for security.

2. The Access token occurs in browser’s address bar so user can copy the url and send to someone else and it also becomes logged as the user i.e. it’s something like Session fixation. But the browser make an additional redirect with replacing history to remove hash fragment from the url. It also possible to a hacker to stole the access token by sniffing a HTTP trafic but this can be easily protected by HTTPS. Some malicious browser extensions can have an access to urls from address bar but this is ultimately bad situation like broken HTTPS cert. And even Auth code flow can’t help here ether. So what I can see is that passing access token via hash fragment of url is absolutely safe.

3. The separation of ephemeral access token and refresh token are useless when using a HTTPS and to be honest not so useful even on raw HTTP. But the fact that client via implicit flow can’t receive the refresh token is also nonsense.

Thus I think we should introduce a new grant flow “safe implicit” which works strictly over https, allows refresh token (or we should get rid off them at all), and is preferable than Auth Cose grant flow

Answer 2

tl;dr: This is all because of security reasons.

OAuth 2.0 wanted to meet these two criteria:

1. You want to allow developers to use non-HTTPS redirect URI because not all developers have an SSL enabled server and if they do it's not always properly configured (non-self signed, trusted SSL certificates, synchronised server clock...).
2. You don't want hackers to be able to steal access/refresh tokens by intercepting requests.

Details below:

The implicit flow is only possible in a browser environment because of security reasons:

In the implicit flow the access token is passed directly as a hash fragment (not as a URL parameter). One important thing about hash fragment is that, once you follow a link containing a hash fragment, only the browser is aware of the hash fragment. Browsers will pass the hash fragment directly to the destination webpage (the redirect URI / the client's webpage). Hash fragment have the following properties:

• They are not part of the HTTP request therefore they can't be read by servers and because of that they cannot be intercepted by intermediary servers/routers (this is important).
• They only exist on the browser - client side - so the only way to read the hash fragment is using JavaScript that runs on the page.

This makes it possible to pass an Access Token directly to the client without the risk of it being intercepted by an intermediary server. This has the caveat of only being possible client side and needs javascript running client side to use the access token.

In the authorization code flow it is not possible to pass an access token directly in a URL parameter because URL parameters are part of the HTTP Request, therefore any intermediary server/routers by which your request would pass (could be hundreds) could be able to read the access token if you are not using en encrypted connection (HTTPS) allowing what's known as Man-in-the-middle attacks.

Passing the access token directly in a URL param could in theory be possible but the auth sever would have to make sure the redirect URI is using HTTPS with TLS encryption and a 'trusted' SSL certificate (typically from a Certificate Authority that is not free) to be sure that the destination server is legitimate and that the HTTP request is fully encrypted. Having all developers purchase an SSL certificate and properly configure SSL on their domain would be a huge pain and would slow adoption down tremendously. This is why an intermediary one-time-use "authorization code" is provided that only the legitimate receiver will be able to exchange (because you need the client secret) and that the code will be useless to potential hackers intercepting the requests over unencrypted transactions (because they don't know the client secret).

You could also argue that the implicit flow is less secure, there are potential attack vectors like spoofing the domain upon redirect - for example by hijacking the IP address of the client's website. This is one of the reasons why the implicit flow only grants access tokens (which are supposed to have a limited time use) and never refresh tokens (which are unlimited in time). To remedy this issue, I advise you to host your webpages on an HTTPS-enabled server whenever possible.

Answer 3

The Implicit Flow makes the whole flow pretty easy, but also less secure.
As the client application, which is typically JavaScript running within a Browser is less trusted, no refresh tokens for long-lived access are returned.
You should use this flow for applications that need temporary access (a few hours) to the user’s data.
Returning an access token to JavaScript clients also means that your browser-based application needs to take special care – think of XSS Attacks that could leak the access token to other systems.

https://labs.hybris.com/2012/06/05/oauth2-the-implicit-flow-aka-as-the-client-side-flow