The loadUserByUsername function in hudson/security/HudsonPrivateSecurityRealm.java in Jenkins before 1.551 and LTS before 1.532.2 allows remote attackers to determine whether a user exists via vectors related to failed login attempts.
https://wiki.jenkins-ci.org/display/SECURITY/Jenkins+Security+Advisory+2014-02-14
https://github.com/jenkinsci/jenkins/commit/fbf96734470caba9364f04e0b77b0bae7293a1ec