The User Registration & Membership plugin for WordPress is vulnerable to Arbitrary Shortcode Execution in versions up to, and including, 5.2.8 This is due to insufficient input sanitization in the 'referrerurl' branch of URSmartTags::process(), where the attacker-controlled HTTPREFERER header is only passed through sanitizetextfield() — which does not strip WordPress shortcode delimiters — before being substituted into a Content Restriction message that is subsequently executed via applyshortcodes()/doshortcode(). This makes it possible for unauthenticated attackers to execute arbitrary registered shortcodes on any restricted page whose message uses the supported {{referrerurl}} smart tag.