One may “accidentally” set up security realm / authorization in such a way that you may no longer able to reconfigure Jenkins.
When this happens, you can fix this by the following steps:

1. Stop Jenkins (the easiest way to do this is to kill the servlet container.)
2. Go to $JENKINS_HOME in the file system and find config.xml file.
3. Open this file in the editor.
4. Look for the <useSecurity>true</useSecurity> element in this file.
5. Replace true with false
6. Start Jenkins

When Jenkins comes back, it’s in the unsecured mode where everyone gets full access to the system.

So useful.

[Seen: Jenkins wiki]