Comment 26 for bug 1729357

CVE-2018-7169

An issue was discovered in shadow 4.5. newgidmap (in shadow-utils) is setuid and
allows an unprivileged user to be placed in a user namespace where setgroups(2)
is permitted. This allows an attacker to remove themselves from a supplementary
group, which may allow access to certain filesystem paths if the administrator
has used "group blacklisting" (e.g., chmod g-rwx) to restrict access to paths.
This flaw effectively reverts a security feature in the kernel (in particular,
the /proc/self/setgroups knob) to prevent this sort of privilege escalation.

References:
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2018-7169
http://www.cvedetails.com/cve/CVE-2018-7169/
https://bugs.launchpad.net/ubuntu/+source/shadow/+bug/1729357