Because it fails to properly sanitise user-supplied input, Apache Tomcat versions previous to 9.0.12, 8.5.34, and 7.0.91 are vulnerable to an open-redirection vulnerability.
Upgrade to Apache Tomcat 9.0.12 or later.
Upgrade to Apache Tomcat 8.5.34 or later.
Upgrade to Apache Tomcat 7.0.91 or later.
Use mapperDirectoryRedirectEnabled=”true” and mapperContextRootRedirectEnabled=”true” on the Context to ensure that redirects are issued by the Mapper rather than the default Servlet.