When the default servlet in Apache Tomcat versions 9.0.0.M1 to 9.0.11, 8.5.0 to 8.5.33 and 7.0.23 to 7.0.90 returned a redirect to a directory (e.g. redirecting to "/foo/" when the user requested "/foo") a specially crafted URL could be used to cause the redirect to be generated to any URI of the attackers choice.