A security issue affects these releases of Ubuntu and its derivatives:
Ubuntu 16.04 LTS
LightDM could be made to run programs as an administrator.
lightdm – Display Manager
It was discovered that LightDM incorrectly handled home directory creation for guest users. A local attacker could use this issue to gain ownership of arbitrary directory paths and possibly gain administrative privileges.
The problem can be corrected by updating your system to the following package version: