Flavio Copes
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Indie Hacker, Computer Engineer, I write tutorials for devs on http://flaviocopes.com
· 5 min read

JWT authentication: When and how to use it

Learn when JWT when is best used, when it's best to use something else, and how to prevent the most basic security issues.

JWT authentication: When and how to use it

JWT (JSON Web Token) is a very popular technology not without it share of controversy.  

Some people say you should never use it while others say it's amazing.

What's the truth? Should you use it or not? That's why we're here.

Brief introduction to JWT

A JWT technically is a mechanism to verify the owner of some JSON data. It's an encoded string, which is URL safe, that can contain an unlimited amount of data (unlike a cookie), and it's cryptographically signed.

When a server receives a JWT, it can guarantee the data it contains can be trusted because it's signed by the source. No middleman can modify a JWT once it's sent.

It's important to note that a JWT guarantees data ownership but not encryption: The JSON data you store into a JWT can be seen by anyone that intercepts the token, as it's just serialized, not encrypted.

For this reason, it's highly recommended to use HTTPS with JWTs (and HTTPS in general, by the way).

We're not going to cover how JWTs are generated in details. There are various guides about this, I can suggest this one: "what the heck is JWT anyway?"

TL;DR: What are they good for?

JWT is a great technology for API authentication and server-to-server authorization.

It's not a good choice for sessions.

Read on to understand the nitty gritty details about those affirmations.

Using JWT for API authentication

A very common use of a JWT token, and the one you should probably only use JWT for, is to use as an API authentication mechanism.

Just to give you an idea, it's so popular and widely used that Google uses it to let you authenticate to their APIs.

The idea is simple: you get a secret token from the service when you set up the API.

On the client side, you create the token (there are many libraries for this), using the secret token to sign it.

You pass it as part of the API request, and the server will know it's that specific client because the request is signed with its unique identifier:

How to expire a single token

How can you invalidate a single token? A no-effort solution is to change the server secret key, which invalidates all tokens. Not really nice for users that should not have their token expired for no reason.

One way to do it is to add a property to your user object in the server database, to reference the datetime the token was created at.

A token automatically stores this value in the iat property.

Every time you check the token, you can compare its iat value with the server-side user property.

To invalidate the token, just update the server-side value, and if iat is older than this, you can reject the token.

Another way to achieve this is by having a blacklist in your database cached in memory (or even better, a whitelist).

Store JWTs securely

A JWT needs to be stored in a safe place inside the user's browser.

If you store it inside localStorage, it's accessible by any script inside your page (which is as bad as it sounds as an XSS attack can let an external attacker get access to the token).

Don't store it in local storage (or session storage). If any of the 3rd part scripts you include in your page gets compromised, it can access all your users' tokens.

The JWT needs to be stored inside an HttpOnly cookie, a special kind of cookie that's only sent in HTTP requests to the server, and it's never accessible (both for reading or writing) from JavaScript running in the browser.

Using JWT to securely exchange information between two servers

Since JWT are signed, the receiver can be sure the client is really who it thinks it is.

Using JWT for SPA authentication

JWTs can be used as an authentication mechanism that does not require a database. The server can avoid using a database because the data store in the JWT sent to the client is safe.

Using JWT to authorize operations across servers

Say you have one server where you are logged in, SERVER1, which redirects you to another server SERVER2 to perform some kind of operation.

SERVER1 can issue you a JWT that authorizes you to SERVER2. Those 2 servers don't need to share a session or anything to authenticate you. The token is perfect for this use case.

When not to use JWTs: session tokens for regular web apps

You might think that using JWTs for session tokens might appear like a good idea at first:

  • you can store any kind of user details on the client
  • the server can trust the client, since the JWT is signed, and there is no need to call the database to retrieve the information you already stored in the JWT
  • you don't need to coordinate sessions in a centralized database when you get to the eventual problem of horizontal scaling

Ultimately, if you already have a database for your application, just use a sessions table and use regular sessions as provided by the server-side framework of choice.

Why? There is a cost involved in using JWTs: they are sent for every request to the server, and it's always a high cost compared to server-side sessions.

Also, while the security risks are minimized sending JWTs using HTTPS, there is always the possibility  that it's intercepted, and the data deciphered, exposing your user's data.

I recommend you to read these two articles on the subject if you want to get into more details about JWTs and sessions:

How to choose the perfect JWT library

Head to jwt.io and look at the Libraries list. The site contains a list of the most popular libraries that implement JWT.

Select your language of choice and pick the library that you prefer, which ideally has the highest number of green checks.

Conclusion

JWT is a very popular standard you can use to trust requests by using signatures, and exchange information between parties. Make sure you know when it's best used, when it's best to use something else, and how to prevent the most basic security issues.


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