The Nextend Social Login and Register plugin for WordPress is vulnerable to a self-based Reflected Cross-Site Scripting via the ‘error_description’ parameter in all versions up to, and including, 3.1.12 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers, with access to a subscriber-level account, to inject arbitrary web scripts in pages that execute if they can successfully trick a user into performing an action such as clicking on a link. NOTE: This vulnerability can be successfully exploited on a vulnerable WordPress instance against an OAuth pre-authenticated higher-level user (e.g., administrator) by leveraging a cross-site request forgery in conjunction with a certain social engineering technique to achieve a critical impact scenario (cross-site scripting to administrator-level account creation). However, successful exploitation requires "Debug mode" to be enabled in the plugin's "Global Settings".