Is a Refresh Token really necessary when using JWT token authentication?

Question

I am referencing another SO post that discusses using refresh tokens with JWT.

JWT (JSON Web Token) automatic prolongation of expiration

I have an application with a very common architecture where my clients (web and mobile) talk to a REST API which then talks to a service layer and data layer.

I understand JWT token authentication, but I am a little confused at how I should use refresh tokens.

I want my JWT authentication to have the following properties:

1. JWT Token has an expiration of 2 hours.

2. The token is refreshed every hour by the client.

3. If the user token is not refreshed (user is inactive and the app is not open) and expires, they will need to log in whenever they want to resume.

I see a lot of people claiming to make this a better experience using the concept of a refresh token, however, I don't see the benefit of this. It seems like an added complexity having to manage it.

My questions are the following:

1. If I WERE to use a refresh token, wouldn't it still be beneficial to have a long term expiration for good practice on that token as well?
2. If I WERE to use a refresh token, would that token be persisted with the userId and/or JWT token?
3. When I update my token every 1 hour, how does this work? Will I want to create an endpoint that takes in my JWT token or my refresh token? Will this update the expiration date of my original JWT token, or create a new token?
4. Is there the need for a refresh token given these details? It seems that If the user is just using a JWT token to grab a new token (per the link above) then the refresh token is obsolete.

Answer 1

Let me come to your questions a little later down the line and start by actually discussing the whole purpose of a refresh token.

So the situation is:

The user opens the app and provides his login credentials. Now, most probably the app is interacting with a REST backend service. REST is stateless, there isn't a way to authorize access to the APIs. Hence, so far in the discussion, there is no way to check if an authorized user is accessing the APIs or is just some random requests coming through.

Now to be able to solve this problem, we need a way to know that the requests are coming from an authorized user. So, what we did was to introduce something called an access token. So now once the user is authenticated successfully, he is issued an access token. This token is supposed to be a long and highly random token (to ensure that it can not be guessed). This is where the JWT comes into the picture. Now you may/may not want to store any user-specific details in a JWT token. Ideally, you would want to just store very simple, extremely non-sensitive details in the JWT. The manipulation of the JWT hash to retrieve other user's details (IDOR etc.) is taken care of by JWT (the library being used) itself.

So, for now, our problem with authorized access is solved.

Now we talk of an attack scenario. Let's say using all of the above user Alice, using the app, has the authorized access token and now her app can make requests to all the APIs and retrieve the data as per her authorization.

Assume that SOMEHOW Alice loses the Access Token or put another way, an adversary, Bob, gets access to Alice's access token. Now Bob, despite being unauthorized, can make requests to all the APIs that Alice was authorized to.

SOMETHING WE IDEALLY DON'T WANT.

Now the solution to this problem is :

1. Either detect that there is something of this sort happening.
2. Reduce the attack window itself.

Using just the access token alone, it is hard to achieve condition 1 above, because be it Alice or Bob, it's the same authorized token being used and hence requests form the two users are not distinguishable.

So we try achieving 2 above and hence we add an expiration to the validity of the access token, say the access token is valid for 't' (short-lived) time.

How does it help? Well, even if Bob has the access token, he can use it only while it is valid. As soon as it expires, he will have to retrieve it again. Now, of course, you could say that he can get it the same way he got it the first time. But then again there's nothing like 100% security!

The above approach still has a problem and in some cases an unacceptable one. When the access token expires, it would require the user to enter his login credentials and obtain an authorized access token again, which at least in case of mobile apps, is a bad (not acceptable) user experience.

Solution: This is where the refresh token comes in. It is again a random unpredictable token that is also issued to the app along with the access token in the first place. This refresh token is a very long-lived special token, which makes sure that as soon as the access token expires, it requests the server for a new access token, thus removing the need for the user to re-enter his login credentials to retrieve a new authorized access token, once an existing one has expired.

Now you may ask, Bob can have access to the refresh token as well, similar to the way he compromised the access token. YES. He can. However, now it becomes easy to identify such an incidence, which was not possible in the case of an access token alone, and take the necessary action to reduce the damage done.

How?

For every authenticated user (in case of a mobile app, generally), a one to one mapped refresh token and access token pair is issued to the app. So at any given point in time, for a single authenticated user, there will be only one access token corresponding to a refresh token. Now assume that if Bob has compromised the refresh token, he would be using it to generate an access token (because access token is the only thing which is authorized to access resources through the APIs). As soon as Bob (attacker) requests with the newly generated access token because Alice's (genuine user) access token is still valid, the server would see this as an anomaly, because for a single refresh token there can be only one authorized access token at a time. Identifying the anomaly, the server would destroy the refresh token in question and along with it all, it's associated access tokens will also get invalidated. Thus preventing any further access, genuine or malicious, to any authorization requiring resources. The user, Alice, would be required to once again authenticate with her credentials and fetch a valid pair of a refresh and access tokens.

Of course, you could still argue that Bob could once again get access to both refresh and access tokens and repeat the entire story above, potentially leading to a DoS on Alice, the actual genuine customer, but then again there is nothing like 100% security.

Also as a good practice, the refresh token should have an expiry, although a pretty long one.