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.
https://bugs.launchpad.net/ubuntu/+source/shadow/+bug/1729357
Source: MITRE
Published: 2018-02-15
Updated: 2019-10-03
Type: CWE-732
Base Score: 5
Vector: AV:N/AC:L/Au:N/C:P/I:N/A:N
Impact Score: 2.9
Exploitability Score: 10
Severity: MEDIUM
Base Score: 5.3
Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Impact Score: 1.4
Exploitability Score: 3.9
Severity: MEDIUM