Jenkins 2.274 and earlier, LTS 2.263.1 and earlier does not implement any restrictions for the URL rendering a formatted preview of markup passed as a query parameter, resulting in a reflected cross-site scripting (XSS) vulnerability if the configured markup formatter does not prohibit unsafe elements (JavaScript) in markup.
https://www.jenkins.io/security/advisory/2021-01-13/#SECURITY-2153