The logrotation script (/etc/cron.daily/upstart) in the Ubuntu Upstart package before 1.13.2-0ubuntu9, as used in Ubuntu Vivid 15.04, allows local users to execute arbitrary commands and gain privileges via a crafted file in /run/user/*/upstart/sessions/.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2015-2383
https://bugs.launchpad.net/ubuntu/+source/upstart/+bug/1425685
http://www.halfdog.net/Security/2015/UpstartLogrotationPrivilegeEscalation/
http://seclists.org/fulldisclosure/2015/Mar/7
http://packetstormsecurity.com/files/130587/Ubuntu-Vivid-Upstart-Privilege-Escalation.html