The login command available in the remoting-based CLI stored the encrypted user name of the successfully authenticated user in a cache file used to authenticate further commands. Users with sufficient permission to create secrets in Jenkins, and download their encrypted values (e.g. with Job/Configure permission), were able to impersonate any other Jenkins user on the same instance.
This has been fixed by storing the cached authentication as a hash-based MAC with a key specific to the Jenkins instance and the CLI authentication cache.
Previously cached authentications are invalidated when upgrading Jenkins to a version containing a fix for this.