This vulnerability arises because the 'su_lightbox' shortcode in the Shortcodes Ultimate plugin does not properly sanitize user input or escape output within the 'src' attribute. As a result, a user with contributor level access or above can introduce a script that permanently resides in the post or page content (a form of Stored XSS). When unsuspecting users visit the page containing the script, the script executes in their browser context, potentially leading to session hijacking, redirections, or other malicious activities. Since WordPress contributors typically have the permission to author content without publishing rights, this vulnerability can be exploited by contributors aiming to run malicious scripts under the assumption that their contributions will eventually be published. XSS vulnerabilities of this nature can be manipulated to steal users' credentials, manipulate page content, or perform actions on behalf of other users.

To mitigate this vulnerability, restrict contributor level access to only trusted users. Implement additional security policies that require manual review of content changes by higher-level administrators before publication. Utilize WordPress security plugins to monitor and block suspicious activities and regularly scan for vulnerabilities. Educate site users about the dangers of executing JavaScript embedded in potentially compromised sites.