challenge-82.md

Challenge

class JWT {
    public function verifyToken($data, $signature) {
        $pub = openssl_pkey_get_public("file://pub_key.pem");
        $signature = base64_decode($signature);
        if (openssl_verify($data, $signature, $pub)) {
            $object = json_decode(base64_decode($data));
            $this->loginAsUser($object);
        }
    }
}

(new JWT())->verifyToken($_GET['d'], $_GET['s']);

Solution

This challenge contains a bug in the usage of the openssl_verify() function in line 5 that leads to an authentication bypass in line 7. The function has three return values: 1 if the signature is correct, 0 if the signature verification failed, and -1 if there was an error while performing the verification. So if an attacker generates a valid signature for the data using another algorithm than the one pub_key.pem is using, the openssl_verify() function returns -1 which is casted to true automatically. To avoid this problem use the type-safe comparison === to validate the return value of openssl_verify(), or consider using a different library for cryptography.

Refference

• php-security-calendar-2017 Day 18 - Sign