The WPOptin plugin, used for creating top bars, pop-ups, and generating leads on WordPress websites, has a security issue called Stored Cross-Site Scripting. This means that the plugin does not properly check for harmful code in its input and output, making it possible for hackers to insert their own code into pages and potentially access user information. This vulnerability exists in all versions up to 2.0.8, allowing attackers to execute their scripts whenever a user visits the affected page.