The URLValidator class in Apache Struts 2 2.3.20 through 2.3.28.1 and 2.5.x before 2.5.1 allows remote attackers to cause a denial of service via a null value for a URL field.
https://struts.apache.org/docs/s2-041.html
https://bugzilla.redhat.com/show_bug.cgi?id=1348253
http://www.securityfocus.com/bid/91278
http://www.oracle.com/technetwork/security-advisory/cpujul2017-3236622.html
http://www-01.ibm.com/support/docview.wss?uid=swg21987854