On versions before 2.1.4, a user could log in and perform a template injection attack resulting in Remote Code Execution on the server, The attacker must successfully log into the system to launch an attack, so this is a moderate-impact vulnerability. Mitigation: all users should upgrade to 2.1.4
https://lists.apache.org/thread/n6dhnl68knpxy80t35qxkkw2691l8sfn