The Struts team has announced the End of Life for Struts 2.3. For the next six months, they will remediate security issues as practical. After six months, approximately May 2019, Struts 2.3 will no longer be updated.
See the announcement here Update your applications to Struts 2.5 as soon as possible. There is an upgrade guide in the Struts documentation (note that this article is part of the deprecated Struts documentation Wiki. Please post if you find a more up-to-date source).