A session fixation vulnerability exists in Jenkins 2.145 and earlier, LTS 2.138.1 and earlier in core/src/main/java/hudson/security/HudsonPrivateSecurityRealm.java that prevented Jenkins from invalidating the existing session and creating a new one when a user signed up for a new user account.
https://github.com/advisories/GHSA-rr6r-p7rw-369c
https://jenkins.io/security/advisory/2018-10-10/#SECURITY-1158