Joomla Extension - j2commerce.com - Unauthenticated PayPal callback forgery leading to order confirmation fraud in J2Store 1.0.0-3.3.21, 4.0

Joomla Extension - j2commerce.com - Unauthenticated PayPal callback forgery leading to order confirmation fraud in J2Store 1.0.0-3.3.21, 4.0.0-4.0.21, 4.1.0-4.1.6 - The PayPal IPN listener's signature check (_validateIPN()) accepted UNVERIFIED and any non-INVALID response as valid, made its verification request with CURLOPT_SSL_VERIFYPEER disabled, and stored its verdict in a field nothing downstream ever checked — so processing continued regardless of the outcome. Separately, the paid-amount comparison only ran when mc_gross was a positive number; omitting the field from the POST body (floatval(null) == 0) skipped the check entirely. Combined with a merchant-configured receiver_email and a sequential, enumerable order id read from the custom field, an anonymous POST was enough to move a pending order straight to CONFIRMED with no payment, or force another customer's pending order to FAILED. paypalv2.php performed no amount check under any circumstances.