Published: 09 June 2017
In the cron package through 3.0pl1-128 on Debian, and through 3.0pl1-128ubuntu2 on Ubuntu, the postinst maintainer script allows for group-crontab-to-root privilege escalation via symlink attacks against unsafe usage of the chown and chmod programs.
CVSS 3 base score: 6.7
Launchpad, Ubuntu, Debian
|Ubuntu 21.04 (Hirsute Hippo)||
|Ubuntu 20.04 LTS (Focal Fossa)||
|Ubuntu 18.04 LTS (Bionic Beaver)||
|Ubuntu 16.04 ESM (Xenial Xerus)||
|Ubuntu 14.04 ESM (Trusty Tahr)||
This appears to be mitigated by kernel symlink restrictions. The crontabs dir has the sticky bit set drwx-wx--T root crontab crontabs which means symlinks within the dir must have the same uid as the target. It is still possible that a cron package update could trigger this race.
I believe that actually _exploiting_ the bug requires updating the cron package. So long as there's no updates for cron, the vulnerable code doesn't run. So if we find a second bug in cron then we really should fix the race condition at the same time, but so long as we don't push a cron update, the vulnerable code just plain doesn't run. the patch just narrows the time window for the race condition.