The PHP Ajax handler should be secured, not serving calls from anywhere other than the page that the visitor is viewing in the browser (assuming that Apache is not configured to restrict access to the handler). I have designed a security structure with this aim in mind, and would like to know if my approach is good or could be improved. Are there any weaknesses that could be exploited?
The design is quite simple. When the page is constructed for serving, a session cookie is created, and its value is an encrypted token, derived from the client's IP, and a random salt. The cookie's value, the encrypted token, is also used as the name of a temporary db record for 60 minutes, and the record's value is the salt that was used to generate the token.
$ajaxSalt = bin2hex(openssl_random_pseudo_bytes(30)); // Create random salt $cookieAndTrans = md5(crypt($_SERVER['REMOTE_ADDR'], $ajaxSalt)); // We use cookie value as token setCookie('qnrwp_ajax_cookie', $cookieAndTrans); // Set session cookie, for JS Ajax caller to echo back to us set_transient('qnrwp_ajax_temp_salt_'.$cookieAndTrans, $ajaxSalt, 60 * MINUTE_IN_SECONDS); // Save salt for 60 mins
Receiving the call, the handler will check that the transmitted cookie value matches the cookie value that was set initially. It then uses this value as the name of the temporary db record to call up (if 60 minutes haven't passed), and uses the value of the record, the salt, to confirm that the IP matches.
// ----------------------- Security check if ($_POST['qnrwp_ajax_cookie'] !== $_COOKIE['qnrwp_ajax_cookie']) wp_die(); $ajaxTrans = get_transient('qnrwp_ajax_temp_salt_'.$_POST['qnrwp_ajax_cookie']); if (!$ajaxTrans) wp_die(); if ($_POST['qnrwp_ajax_cookie'] !== md5(crypt($_SERVER['REMOTE_ADDR'], $ajaxTrans))) wp_die();
A couple of things to clarify to avoid confusion:
md5() is primarily used as replacement for
bin2hex(), its weak crypto security just an added bonus, but not being relied upon -
crypt() is used for good encryption.
crypt() is able to generate a salt, but I prefer creating my own as I'll store it in the temporary record (
set/get_transient() in the code). The salt is quite long, helping avoid clashes between different clients accessing the site at the same time. A clash is still possible, but I think unlikely, and even if it happened, it would not be catastrophic - if the clashing clients share their IP, one of them may find up to 120 minutes available after page load rather than 60, no big deal.
I believe my question is at quite an advanced level and would prefer if those with good knowledge of the subject write answers rather than try to engage in discussion in the comments. That said, if anything is unclear, feel free to ask in the comments. If you see weaknesses in the code, please try to provide concrete suggestions for improvement, rather than merely pointing out the loopholes. The most constructive answer will be accepted. I will wait a few days before accepting.
Please note I'm not open to suggestions of third-party tools or WordPress plugins - this is a case of "rolling my own".