auth/index.php

<?php
if (empty($_REQUEST['user']) || empty($_REQUEST['pw']))
    die('0');

function authenticate($user, $pass){
  // run shell command to output shadow file, and extract line for $user
  // then spit the shadow line by $ or : to get component parts
  // store in $shad as array
  $shad =  preg_split("/[$:]/",`cat /etc/shadow | grep "^$user\:"`);
  // use mkpasswd command to generate shadow line passing $pass and $shad[3] (salt)
  // split the result into component parts
  $mkps = preg_split("/[$:]/",trim(`mkpasswd -m sha-512 $pass $shad[3]`));
  // compare the shadow file hashed password with generated hashed password and return
  return ($shad[4] == $mkps[3]);
}

if (isset($_REQUEST["json"])) {
  $auth = authenticate($_REQUEST['user'], $_REQUEST['pw']);
  echo json_encode([
    "authenticated" => $auth,
    "sudoer" => $auth && in_array($_REQUEST["user"], posix_getgrnam("sudo")["members"])
  ]);
} else { // v1 api
  echo authenticate($_REQUEST['user'], $_REQUEST['pw']) ? '1' : '0';
}