Set a restrictive default umask for login sessions

Description

The umask removes permission bits from every file and directory a process creates. Ubuntu login sessions get 022 (files 644, everyone can read them), and pam_umask with USERGROUPS_ENAB yes even gives 002 to regular users who have their own group. This control sets {{ umask }} from /etc/profile.d/, which applies to the login shells of every user, root included.

Services started by systemd do not read /etc/profile; set their umask with UMask= in their unit.

Rationale

Users and administrators regularly create files with sensitive content (exports, dumps, configuration copies) in their home or in /tmp. A restrictive umask keeps them private unless someone opens them on purpose.

Impact

Files created in shared directories are no longer readable by other users. Teams that share files through a common group need chmod g+r or a setgid group directory.

Setting

Parameters

The value used is your override (tailoring file), else the profile's, else the one of the profile it extends, else the default. Bounds limit what an override may set.

Audit

Run su -s /bin/sh -l nobody -c umask and su -l root -c umask. Both must print 0{{ umask }} (or a stricter value).

Automated: every check must pass. Scanners report each check by its id.

user

Umask of a login shell for a regular account.

script: |
  u=$(su -s /bin/sh -l nobody -c umask 2>/dev/null | tail -n 1) || exit 3
  echo "nobody login umask: $u"
  [ $(( 0$u & 0$CB_UMASK )) -eq $(( 0$CB_UMASK )) ]

root

Umask of a root login shell.

script: |
  u=$(su -l root -c umask 2>/dev/null | tail -n 1) || exit 3
  echo "root login umask: $u"
  [ $(( 0$u & 0$CB_UMASK )) -eq $(( 0$CB_UMASK )) ]

Remediation

Create /etc/profile.d/60-cirsec-umask.sh containing umask {{ umask }}. New login sessions pick it up.

Rollback

Delete /etc/profile.d/60-cirsec-umask.sh.

profile

f=/etc/profile.d/60-cirsec-umask.sh
cb_backup "$f"
printf '%s\n' '# Written by CIRSEC Baselines CB-UBU-AUTH-019' "umask $CB_UMASK" >"$f"
chmod 644 "$f"

Handwritten code reads parameters from CB_<NAME> environment variables. Each release turns these fixes into remediate.sh and an Ansible role per profile: dry run by default, a fix runs only when its check fails.

References

• pam_umask(8), Ubuntu manual pages Canonical, vendor-doc, accessed 2026-10-19
• bash(1), Ubuntu manual pages (umask, INVOCATION) Canonical, vendor-doc, accessed 2026-10-19
• ANSSI-BP-028 v2.0 - Recommandations de configuration d'un système GNU/Linux (R36) ANSSI, guide, accessed 2026-10-19

Mappings

How this control relates to the other framework's: equivalent, stricter, looser, partial or related.

Threats mitigated

People and lifecycle