We recently assisted a client who had lost their administrator password for Jenkins. Luckily, the solution is rather straightforward - as long as you have root access to the server.
While Jenkins security could be disabled altogether with a simple configuration file switch, the easiest workaround is just resetting the password hash to a known value. This will ensure that all other security settings stay intact.
Please note that these instructions require you to have SSH access into the Jenkins server.
Here’s a 1-liner to reset the admin password to ‘releaseworks’ on most systems:
sudo 'sed -i "" /var/lib/jenkins/users/admin*/config.xml && service jenkins restart'
The longer story
$ sudo su jenkins # cd /var/lib/jenkins/users # ls admin_94129024 #