An issue was discovered in shadow 4.5. newgidmap (in shadow-utils)...

Vulnerability Description

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

• https://bugs.launchpad.net/ubuntu/+source/shadow/+bug/1729357
• https://security.gentoo.org/glsa/201805-09