A flaw was found in Jenkins weekly up to and including 2.106 and Jenkins LTS up to and including 2.89.3. Jenkins did not properly prevent specifying relative paths that escape a base directory for URLs accessing plugin resource files. This allowed users with Overall/Read permission to download files from the Jenkins master they should not have access to.
On Windows, any file accessible to the Jenkins master process could be downloaded. On other operating systems, any file within the Jenkins home directory accessible to the Jenkins master process could be downloaded.
References:
https://jenkins.io/security/advisory/2018-02-14/