If it's unverified, then you can't trust the header to begin with. You can't trust that it even forms any kind of valid format. It could be a payload of "naughtystrings" for all you know.

And if you really must decode it, then you can call the base64 decode yourself - and know what you're doing is unsafe.

In the case of an unverified token, it is best to pretend the token doesn't even exist.

Literally incorrect. You can't verify the jwt prior to reading the header, because you don't know which of the advertised keys was used to sign the jwt.

Check out e.g. auth0's doc. https://auth0.com/docs/tokens/json-web-tokens/validate-json-...

I assume you're refering to this part of the spec:

> If the JOSE Header contains a "cty" (content type) value of "JWT", then the Message is a JWT that was the subject of nested signing or encryption operations. In this case, return to Step 1, using the Message as the JWT. [0]

This is also step 8 of validating a JWT. There is a number of validation steps you need to take before you can decide how to process the payload.

You'll also note that none of the previous 7 steps actually decode the payload ("the Message") - they only find it. And 4 of those steps are about verifying that the header isn't dangerous.

[0] https://tools.ietf.org/html/rfc7519#section-7.2

Again, the library can provide that functionality in a better way. In this case, the library could provide a means for the consuming code to be queried about what keys are available. E.g., in JS-like pseudo-code:

  validator = new JwtValidator();
  // By default, the validator should reject everything, until you call…
  validator.add_signing_key(new RsaSigned((a strongly-typed RSA key));
  // I wouldn't even add this to the library, but let's
  // say you want to comply with the RFC fully:
  validator.add_signing_key(new CompletelyUnsafeNoneAlgorithm());
  // (And I'd name it that explicitly, to let the consumer know that
  // what they're doing is nuts.)

This validator knows exactly which algorithms — and keys — are valid.

If the full keyset is somehow not known ahead of time, a callback to enumerate keys of a certain type could be configured.

Better API design here prevents this vuln. from ever happening.

(Personally, I wish the RFC had never added "alg": "none"; it's a mistake waiting to happen. I wish JWS/JWT could be updated to remove it.)