JWT doesn’t have a benefit over using “sessions” per se. JWTs provide a means of maintaining session state on the client instead of doing it on the server.
What people often mean when asking this is “What are the benefits of using JWTs over using Server-side sessions“.
With server-side sessions, you will either have to store the session identifier in a database, or else keep it in memory and make sure that the client always hits the same server. Both of these have drawbacks. In the case of the database (or other centralised storage), this becomes a bottleneck and a thing to maintain – essentially an extra query to be done with every request.
With an in-memory solution, you limit your horizontal scaling, and sessions will be affected by network issues (clients roaming between Wifi and mobile data, servers rebooting, etc).
Moving the session to the client means that you remove the dependency on a server-side session, but it imposes its own set of challenges.
- Storing the token securely.
- Transporting it securely.
- JWT sessions can sometimes be hard to invalidate.
- Trusting the client’s claim.
These issues are shared by JWTs and other client-side session mechanisms alike.
JWT, in particular, addresses the last of these. It may help to understand what a JWT is:
It is a bit of information. For user sessions, you could include the username and the time when the token expires. But it could conceivably be anything, even the session ID or the user’s entire profile (please don’t do that though).
It has got a secure signature that prevents malicious parties from generating fake tokens (you need access to the server’s private key to sign them and you can verify that they were not modified after they were signed).
You send them with every request, just like a cookie or Authorization Header would be sent. In fact, they are commonly sent in the HTTP Authorization header but using a cookie is fine too.
The token is signed and so the server can verify its origin. We will assume that the server trusts its own ability to sign securely (you should use a standard library: don’t try to do it yourself, and secure the server properly).
On the issue with securely transporting the token, the answer is commonly to send it via an encrypted channel, usually httpS.
Regarding securely storing the token in the client, you need to ensure that the bad guys can’t get to it. This (mostly) means preventing JS from bad web sites from reading the token to send it back to them. This is mitigated using the same strategies used to mitigate other kinds of XSS attacks.
If you have a need to invalidate JWTs, there are definitely ways this can be achieved. Storing a per-user epoch for only users who have requested to have their “other sessions terminated” is a very efficient method that will probably be good enough. If an application needs per-session invalidation, then a session ID can be maintained in the same way and the “killed tokens” table can still be maintained to be much smaller than the full user table (you only need to retain records newer than the longest allowed token lifetime). So the ability to invalidate the token partially negates the benefit of client-side sessions in that you would have to maintain this session killed state. This will more than likely be a much smaller table than the original session state table, so the lookups are still more efficient though.
One other benefit of using JWT tokens is that it is reasonably easy to implement using libraries available in probably every language you can expect to have it. It is also completely divorced from your initial user authentication scheme – if you move to a fingerprint-based system, you do not need to make any changes to the session management scheme.
A more subtle benefit: Because the JWT can carry “information” and this can be accessed by the client, you can now start doing some smart things. For example, remind the user that their session will be expiring a few days before they are logged out, giving them the option to re-authenticate, based on the expiry date in the token. Whatever you can imagine.
So in short: JWTs answers some of the questions and shortcomings of other session techniques.
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“Cheaper” authentication because you can eliminate a DB round trip (or at least have a much smaller table to query!), which in turns enable horizontal scalability.
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Tamper-proof client-side claims.
While JWTs does not answer the other issues like secure storage or transport, it does not introduce any new security issues.
A lot of negativity exists around JWTs, but if you implement the same security that you would for other types of authentication, you will be fine.
One final note: It is also not Cookies vs Tokens. Cookies is a mechanism for storing and transporting bits of information and can be used to store and transport JWT tokens too.