The following flaw was found in Jenkins:
When administrators accessed a URL like /user/example via HTTP GET, a user with the ID example was created if it did not exist. While this user record was only retained until restart in most cases, administrators' web browsers could be manipulated to create a large number of user records.
Accessing these URLs now no longer results in a user record getting created, Jenkins will respond with 404 Not Found if no such user exists. When using the internal Jenkins user database, new users can be created via Manage Jenkins ยป Manage Users. To restore the previous (unsafe) behavior, set the system property hudson.model.User.allowUserCreationViaUrl to true as described on Features controlled by system properties.
External References:
https://wiki.jenkins-ci.org/display/SECURITY/Jenkins+Security+Advisory+2017-02-01
Upstream patch:
https://github.com/jenkinsci/jenkins/commit/b88b20ec473200db35d0a0d29dcf192069106601