Bad documentation of error handling in ParseWithClaims can lead to potentially dangerous situations

Summary

Unclear documentation of the error behavior in ParseWithClaims can lead to situation where users are potentially not checking errors in the way they should be. Especially, if a token is both expired and invalid, the errors returned by ParseWithClaims return both error codes. If users only check for the jwt.ErrTokenExpired using error.Is, they will ignore the embedded jwt.ErrTokenSignatureInvalid and thus potentially accept invalid tokens.

Fix

We have back-ported the error handling logic from the v5 branch to the v4 branch. In this logic, the ParseWithClaims function will immediately return in "dangerous" situations (e.g., an invalid signature), limiting the combined errors only to situations where the signature is valid, but further validation failed (e.g., if the signature is valid, but is expired AND has the wrong audience). This fix is part of the 4.5.1 release.

Workaround

We are aware that this changes the behaviour of an established function and is not 100 % backwards compatible, so updating to 4.5.1 might break your code. In case you cannot update to 4.5.0, please make sure that you are properly checking for all errors ("dangerous" ones first), so that you are not running in the case detailed above.

token, err := /* jwt.Parse or similar */
if token.Valid {
	fmt.Println("You look nice today")
} else if errors.Is(err, jwt.ErrTokenMalformed) {
	fmt.Println("That's not even a token")
} else if errors.Is(err, jwt.ErrTokenUnverifiable) {
	fmt.Println("We could not verify this token")
} else if errors.Is(err, jwt.ErrTokenSignatureInvalid) {
	fmt.Println("This token has an invalid signature")
} else if errors.Is(err, jwt.ErrTokenExpired) || errors.Is(err, jwt.ErrTokenNotValidYet) {
	// Token is either expired or not active yet
	fmt.Println("Timing is everything")
} else {
	fmt.Println("Couldn't handle this token:", err)
}