s2Member <= 260927 - Unauthenticated Remote Code Execution via 'item_name' IPN Parameter

The s2Member plugin for WordPress is vulnerable to Remote Code Execution in all versions up to, and including, <= 260927 via the 'item_name' parameter parameter. This is due to insufficient sanitization in strip_php_tags_deep(), which only strips literal '<?' sequences, combined with a post-interpolation placeholder cleanup that reassembles a valid '<?php' opener after the attacker-controlled value has already been embedded in the email template. This makes it possible for authenticated attackers, with custom-level access and above, to execute code on the server.