fix(weak-id): replace predictable mt_rand with cryptographically secure random_bytes

mt_rand() is seeded with a predictable value and not suitable for
security-sensitive contexts. random_bytes() uses the OS CSPRNG instead, which
is cryptographically unpredictable.

Author: IdkWhatAmIDoin

vulnerabilities/weak_id/source/impossible.php

@@ -3,7 +3,7 @@
 $html = "";
 
 if ($_SERVER['REQUEST_METHOD'] == "POST") {
-	$cookie_value = sha1(mt_rand() . time() . "Impossible");
+	$cookie_value = bin2hex(random_bytes(20));
 	setcookie("dvwaSession", $cookie_value, time()+3600, "/vulnerabilities/weak_id/", $_SERVER['HTTP_HOST'], true, true);
 }
 ?>