OAuth 2.0 Implicit Flow Considered Harmful

8. October 2020 • edited 23. October 2020

Certainly I’m not the only one writing into the web that OAuth 2.0 Implicit Flow is bad for security reasons and deprecated by OAuth 2.0 Best Current Practices and OAuth 2.1. But this can’t be said enough times. So I’ll try my best! This post is a guide for people facing situations where random dudes asking “Why should I bother? See, Microsoft is recommending it!” After reading this post you can tell them why it is a bad idea to use implicit flow.

How Does Implicit Flow Work?

The OAuth 2.0 Implicit Flow is from ancient times when we only had limited browsers. Maybe you’re young enough and never faced the massive pain to support something like Internet Explorer 6. This was a dark time you can’t do simply cross-origin HTTP requests without jumping backwards through burning hoops and sacrifice a kitten. So OAuth 2.0 Implicit Flow was designed to work with sole browser redirects.

Let’s examine a brief example of OAuth 2.0 Implicit Flow:

Sequence diagram for OAuth 2.0 Implicit Flow

In the above sequence diagram you see the flow for a frontend application hosted at https://www.my-app.com which want to access an API at https://www.some-api.com and therefore need an access token from the security token service (STS) responsible for this API. For the sake of brevity we assume that both the API and STS are hosted at the same domain.

As you can see there are going lot of redirects back and forth. First the fronted application will redirect you to some-api.com to authenticate there (step one and two). If it is a mobile app it will open a system browser for redirecting. Why is this done there and not inside the frontend application? The reason is you do not trust the frontend application and do not want to give it the credentials for some-api.com. This is also the reason why mobile apps must open a browser for authentication and can’t provide an own login mask. Although the customer or designer dislikes this and judge it an ugly user experience. After redirected to the STS it presents you the login for authentication (Step three and four). If authenticated successfully the STS presents a so called consent dialog (step five and six). This consent dialog is the typical dialog with a message like: “do you want to allow my-app.com to access your data at some-api.com?”

Then the STS issues the access token and redirects back to my-app.com with the token appended to the URL as a fragment. This reloads the frontend application and populates it with the access token. And then the application can use it to request the API.

Why Is Implicit Flow Bad?

Disclaimer: I will not go into the problems not using TLS. Nowadays there is absolutely no good reason for not using TLS everywhere! Unless you use TLS, stop reading immediately and fix that first!

Ups, Posted the Entire URL

You know these dudes sending the whole URL with all query parameters and fragments via email or chat to tell you what they found in the internet. Despite the fact that they may leak a lot of data about them you may suspect what the problem with the access token in URLs is: If this URL leaves the browser tab in any way, anyone can use the token from the URL to access the API. Let me illustrate this by an example:

You’re sitting in the internet cafe at a shared computer and visit your loved my-app.com and do your business there. After you closed the browser and left the cafe I come in and search through the browser history. Gotcha! I have your token and can do things now in your behalf. To circumvent this one could rewrite the browsers location and history after step eight in the sequence above. But this is a quite cumbersome mitigation which may fail for various reasons.

Oh, Look This Funny Furt App

A more sophisticated attack may happen at mobile devices. With a mobile application as frontend the whole redirect stuff from the sequence above involves the system’s browser and app redirects. This means that your frontend app need to register for the call back URL used in step seven in the sequence above. In our example that would be my-app.com. So when the systems browser received a redirect to an URL with my-app.com it leaves the browser and opens the app and hand over the URL with the token.

Now imagine that someone place a malicious app on your phone which also registers for this URL and intercepts the token: Gotcha! I don’t have very deep knowledge in the field of mobile app development and maybe there are also some mitigations to this attack vector. But this does not really matter because you must not use implicit flow!

What Is the Alternative?

The alternative is the OAuth 2.0 Authorization Code Flow with PKCE. (Hint: The OAuth 2.0 dudes pronounce this “PKCE” thingy “pigsi”.) So first: For what stands PKCE? This is the abbreviation for Proof Key for Code Exchange. Ok, didn’t give as any hint how it works. First I give a short introduction how OAuth 2.0 Authorization Code Flow works and then describe why we need this “pigsi” thingy and how it works.

OAuth 2.0 Authorization Code Flow

We use the same example as above with our app hosted at https://www.my-app.com and an API we integrate hosted at https://www.some-api.com. Here is the full sequence of the code flow:

Sequence diagram for OAuth 2.0 Code Flow

The first seven steps are merely identical to the implicit flow. The main difference here is that we do not hve a frontend but a backend in this diagram. Of course does my-app.com also have a frontend, but this is rendered by the backend and does not play any role in the authorization code flow. Also a difference here is that step one is “Connect some-api account”. This is like the “connect GitHub/Twitter/Facebook/Whatever”-buttons you may have seen at other sites.

The big difference in this diagram to the previous one is that we distinguish the back and forth communication in two kinds:

  1. The front channel communication: This is all the authorization communication done via the insecure user side channel (eg. at computers browser). Why insecure? Because you have no control over this communication channel. You don’t know if there is some malware or someone eavesdropping this communication.
  2. The back channel communication: This is all the authorization communication between backend and STS. This is considered secure because you have control over the backend and you must trust the STS anyway. Also you may use mTLS to authenticate both parties (backend and STS) to each other.

If you look closely you may recognized that step nine does not redirect with an access token, but with an authorization code. That’s hence where the name authorization code flow come from. With this authorization code you can’t access the API. It is only used by the backend to authorize against the STS in combination with the client credentials. The client credentials are the backend’s “username and password” for the STS. The STS will return the access token and a refresh token, if authorization code and client credentials were valid. And then the backend may access the API with the access token.

Why is this more secure than implicit flow? The important difference is that the worthy access token is never sent over the insecure front channel. Only the authorization code is sent over it and this is only useful in combination with the secret client credentials of the backend. So sniffing this is worthless. Ok, if the client credentials leak for any reason then code flow is also doomed. But that’s always the case if someone leaks credentials. So keep your OAuth 2.0 client credentials secret or renew them!

But I Can’t Store Client Credentials!

So the code flow from above works really well, if you have a traditional server side web application with a backend where you can store your client credentials. But what if you have no such backend because you are a single page or mobile application? PKCE for the rescue!

Sequence diagram for OAuth 2.0 Code Flow

Some say PKCE is like a one time password. So let’s go into the details how it works. For the first sight it looks quite similar to the traditional code flow without PKCE. That’s no coincidence because PKCE is just a mechanism on top of code flow. The first important thing to realise: There are no client credentials in the play here. Instead the frontend generates a code verifier, which must be a nonce and derives a code challenge with a hash function from it. This code challenge is passed to the STS via the initial front channel communication when obtaining the authorization code and the STS stores this along with the issued authorization code. The remaining back and forth is the same as described above in the code flow.

The frontend sends the received authorization code together with the plain code verifier via the secure back channel to the STS. This verifies it against the code challenge stored together with the used authorization code by hashing and comparing it. If this verification was successful the STS respond the access token and the frontend can use the API.

You may ask why there is no refresh token? Same reason why there are no client credentials! With a refresh token you may extend the lifetime of an access token forever without any user interaction. So it is too risky storing it in an unsafe environment like a single page or mobile application. Also refresh tokens are only meant to be used in backends where no user interaction is possible.

Conclusion

  • PKCE is not hard to use and most frameworks or libraries should have it included. So use it! It makes no harm. You can also use it to secure traditional code flow with client credentials additionally.
  • Some say that implicit flow is not less safe than it was ever and if suitable you can use it. I say no! And some others like Dr. Fett also say so, and he’s one of the dudes writing the RFCs. So he should know.
  • Maybe you are confused because I used the terms frontend and backend alternating in the diagrams. This is to illustrate “where” the OAuth 2.0 client resides. The backend is something secured in a datacenter. Frontend is something at an insecure place like a browser or mobile device, but both are OAuth 2.0 clients.

Cover image by MasterTux from Pixabay.

SecurityOAuth 2.0PKCE
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