Use org.keycloak.common.util.Base64Url to encode/decode clientID - #15736

Istn't the .replace("=", "");missing here?

no, as the the method already take the part before the first "="
(see comment)

public static String encodeBase64ToBase64Url(String base64) {
    String s = base64.split("=")[0]; // Remove any trailing '='s
    s = s.replace('+', '-'); // 62nd char of encoding
    s = s.replace('/', '_'); // 63rd char of encoding
    return s;
}

You are right, I haven't checked that. I just compared it to https://github.com/keycloak/keycloak/pull/15735/files but apparently standard Java does not remove trailing =?

true, standard java doesn't, but since we know the boundaries from the base64, we don't need to keep the = for padding

Java 17 always use padding by default :

but provide a way to disable it