How is using a JSON Web Token more secure than an opaque session token, In both the scenarios the tokens are first sent to the client and then verified on the server when a client requests a protected resource.

There are several reasons people say JWTs are more secure. I’ll list them and also give you reasons why that might not really be the case as it swings both ways.

JWTs can be signed using a secret with secure algorithms like HS256 and RS256. A comprehensive list can be found here. On top of that, you can also encrypt the payload of the JSON Web token. However, session tokens can also be generated securely with a top-notch algorithm and stored in a signed cookie.
JWT can either be stored in a cookie or Web Storage( local/session Storage ). If you are not storing your JWTs in a cookie, then you are not vulnerable to CSRF. And you can decide to send them through the Authorization header for every HTTP request. However, there is still a caveat. Javascript is used to access the JWT from the Web storage, which still leaves you open to other forms of attack such as XSS (Cross-Site Scripting). Worthy of mention is that it is easier to deal with XSS than CSRF.
In a situation where your JWT has been tampered with, you can revoke or blacklist the token. This involves making a call to the DB every time you want to perform this kind of operation which I won’t advise you to do. The preferred option is to use short-lived tokens.

In the case of OAuth, opaque tokens otherwise known as bearer tokens are random strings that will be stored in some kind of hashed storage on the server together with an expiration, the scope requested (e.g. access to friend list) and the user who gave consent. Later, when the API is called, this token is sent and the server lookup on the hash-table, rehydrating the context to make the authorization decision (did it expire? does this token have the right scope associated with the API that wants to be accessed?). The main difference between opaque tokens and signed tokens(e.g JWT) is that JWTs are stateless. They don’t need to be stored on a hash-table.

Conclusion

The libraries used to sign and encrypt your JWTs should be secure to ensure your authentication process is also secure. You should also use cookies as storage mechanism rather than using them for login. You can argue that there are more benefits to using JWTs like they are easier to scale and they can be used in OAuth cases which you can find in this article. At the end of the day, In my opinion it simply falls on the developers’ reasoning/logic to ensure the right steps are followed to make an app secured regardless of what form of token is used for authentication or authorization. Use case is also key in this context!