Jenkins versions 2.56 and earlier as well as 2.46.1 LTS and earlier are vulnerable to an issue in the Jenkins user database authentication realm: create an account if signup is enabled; or create an account if the victim is an administrator, possibly deleting the existing default admin user in the process and allowing a wide variety of impacts.
https://github.com/advisories/GHSA-85wq-pqhp-hmq6
https://jenkins.io/security/advisory/2017-04-26/