A7: 2021 | JWT Tokens (10) | Cycubix Docs


In the past assignments we learned to NOT trust the libraries to do the correct thing for us. In both cases we saw that even specifying the JWT key and passing the correct algorithm. Even using the token:


  "alg" : "HS256"
  "admin" : true,
  "iat" : 1516239022,
  "sub" : "1234567890",
  "user" : "John Doe"

And the following Java code:

Jwt jwt = Jwts.parser().setSigningKey(JWT_PASSWORD).parse(accessToken);

You see we set the signing key with setSigningKey the library still skips the validation of the signature.

It is not only limited to the traditional alg: none attack, but it also works with the alg: HS256.


When you have chosen a library to help dealing with JWT tokens make sure to:

  • use the correct method in your code when validating tokens.

  • add test cases and validate the algorithm confusion is not possible.

  • as a security team write a utility methods to be used by the teams which encapsulate the library to make sure the teams use the correct parsing logic.

Alternative: Paseto

The algorithm confusion is a real problem when dealing with JWTs it can be avoided by using PASETO (Platform-Agnostic SEcurity TOkens), which is currently implemented in 10 programming languages. One of the drawbacks of using this method is that JWT is widely spread for example think about using OAuth, so it might not be the best solution to use.

For more information take a look at the following video:

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