An issue was discovered in shadow 4.5. newgidmap is setuid and allows an unprivileged user to be placed in a user namespace where setgroups 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" to restrict access to paths. This flaw effectively reverts a security feature in the kernel to prevent this sort of privilege escalation.