This vulnerability arises from how the Content Blocks plugin processes user input into content blocks via shortcodes, without adequate sanitization or escaping. Because the input is not properly sanitized, an attacker with sufficient access privileges can inject scripts that are stored within the WordPress database. When another user accesses a page with the injected malicious content, the scripts execute in their browser's context, potentially leading to session hijacking, defacement, or other malicious activities. Stored XSS vulnerabilities pose a significant threat as they affect all users who load the affected page, bypassing standard security protocols like content security policy (CSP) that rely on static content analysis. The fact that this vulnerability requires contributor-level access limits its exploitability but can be severely damaging within multi-author environments.

To mitigate this vulnerability, restrict the use of the vulnerable plugin to trusted users only and consider employing a security plugin that provides input validation and output sanitization features. Regularly review user roles and permissions to minimize the number of accounts with contributor access. Additionally, educate users on recognizing potentially malicious activity and adhering to best practices when creating content.