The Relevanssi – A Better Search plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the highlights functionality in all versions up to, and including, 4.24.3 (Free) and <= 2.27.4 (Premium), due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page via the search results.
https://www.relevanssi.com/user-manual/installing-relevanssi-and-adjusting-the-settings/
https://plugins.trac.wordpress.org/changeset/3283795/
https://plugins.trac.wordpress.org/browser/relevanssi/tags/4.24.3/lib/excerpts-highlights.php#L683
https://plugins.trac.wordpress.org/browser/relevanssi/tags/4.24.3/lib/excerpts-highlights.php#L508