The URLValidator class in Apache Struts 2 2.3.20 through 2.3.28.1 and 2.5.x before 2.5.13 allows remote attackers to cause a denial of service via a null value for a URL field.
jvn.jp/en/jp/JVN12352818/index.html
jvndb.jvn.jp/jvndb/JVNDB-2016-000114
www-01.ibm.com/support/docview.wss?uid=swg21987854
bugzilla.redhat.com/show_bug.cgi?id=1348253
github.com/apache/struts
github.com/apache/struts/commit/a0fdca138feec2c2e94eb75ca1f8b76678b4d152
github.com/apache/struts/commit/eccc31ebce5430f9e91b9684c63eaaf885e603f9
nvd.nist.gov/vuln/detail/CVE-2016-4465
struts.apache.org/docs/s2-041.html