The plugin provides an Admin Search page that allows users with the `edit_posts` capability to run Relevanssi searches from the WordPress dashboard. The AJAX handler accepts a URL-encoded `args` parameter, parses it into a `WP_Query`, and then passes user-controlled taxonomy query data into Relevanssi's taxonomy restriction builder. The taxonomy value is sanitized as text but is not parameterized for SQL before being interpolated into a term taxonomy lookup query. This allows an authenticated contributor-level attacker to inject SQL through the Admin Search AJAX request and execute time-based blind SQL injection against the WordPress database.
https://plugins.trac.wordpress.org/browser/relevanssi/tags/4.27.1/lib/search.php#L969
https://plugins.trac.wordpress.org/browser/relevanssi/tags/4.27.1/lib/search-tax-query.php#L411
https://plugins.trac.wordpress.org/browser/relevanssi/tags/4.27.1/lib/admin-ajax.php#L195