The HudsonPrivateSecurityRealm class in Jenkins before 1.600 and LTS before 1.596.1 does not restrict access to reserved names when using the "Jenkins' own user database" setting, which allows remote attackers to gain privileges by creating a reserved name.
https://wiki.jenkins-ci.org/display/SECURITY/Jenkins+Security+Advisory+2015-02-27
https://bugzilla.redhat.com/show_bug.cgi?id=1205627