Audit changes to the system clock and time zone

Description

auditd rules in /etc/audit/rules.d/60-cirsec-time.rules, key time-change: the calls that set the clock (adjtimex, settimeofday, clock_settime, and stime for 32-bit programs) and writes to /etc/localtime.

Rationale

Every log and audit record relies on the clock. A change of time outside the time synchronisation service can hide when an attack happened or make logs disagree between systems.

Impact

Time synchronisation daemons that adjust the clock often (chrony, systemd-timesyncd) produce regular time-change records.

Setting

Audit

1. Run auditctl -l as root. It must list the rules shown below (keys time-change).
2. Run grep -h . /etc/audit/rules.d/*.rules. The same rules must be present, so they load at boot.

Expected rules, as auditctl -l prints them:

-a always,exit -F arch=b64 -S adjtimex,settimeofday,clock_settime -F key=time-change
-a always,exit -F arch=b32 -S stime,settimeofday,adjtimex,clock_settime -F key=time-change
-w /etc/localtime -p wa -k time-change

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

rules

Rules loaded in the kernel and present in /etc/audit/rules.d.

script: |
  command -v auditctl >/dev/null 2>&1 || { echo "auditctl not found: auditd is not installed"; exit 1; }
  want() {
  cat <<'RULES'
  -a always,exit -F arch=b64 -S adjtimex,settimeofday,clock_settime -F key=time-change
  -a always,exit -F arch=b32 -S stime,settimeofday,adjtimex,clock_settime -F key=time-change
  -w /etc/localtime -p wa -k time-change
  RULES
  }
  loaded=$(auditctl -l 2>&1) || { echo "$loaded"; exit 3; }
  disk=$(cat /etc/audit/rules.d/*.rules 2>/dev/null)
  n=0 rc=0
  while IFS= read -r r; do
    [ -n "$r" ] || continue
    n=$((n + 1))
    printf '%s\n' "$loaded" | grep -Fxq -- "$r" || { echo "not loaded: $r"; rc=1; }
    printf '%s\n' "$disk" | grep -Fxq -- "$r" || { echo "not in /etc/audit/rules.d: $r"; rc=1; }
  done <<EOF
  $(want)
  EOF
  [ "$rc" = 1 ] || echo "$n rules loaded and present in /etc/audit/rules.d"
  exit "$rc"

Remediation

1. Write the expected rules to /etc/audit/rules.d/60-cirsec-time.rules (mode 0640).
2. Run augenrules --load. If auditctl -s shows enabled 2, the configuration is locked: reboot to load the new rules.

Rollback

Delete /etc/audit/rules.d/60-cirsec-time.rules and run augenrules --load (or reboot if the configuration is locked).

rules-file

command -v augenrules >/dev/null 2>&1 || { echo "auditd is not installed" >&2; exit 1; }
want() {
cat <<'RULES'
-a always,exit -F arch=b64 -S adjtimex,settimeofday,clock_settime -F key=time-change
-a always,exit -F arch=b32 -S stime,settimeofday,adjtimex,clock_settime -F key=time-change
-w /etc/localtime -p wa -k time-change
RULES
}
f=/etc/audit/rules.d/60-cirsec-time.rules
t=$(mktemp)
{ echo '# Written by CIRSEC Baselines CB-DEB-AUDIT-017'; want; } >"$t"
if ! cmp -s "$t" "$f"; then
  cb_backup "$f"
  cat "$t" >"$f"
fi
rm -f "$t"
chmod 640 "$f"
if auditctl -s | grep -q '^enabled 2'; then
  echo "audit rules are locked (-e 2): reboot to load $f"
else
  augenrules --load >/dev/null
fi

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

• audit.rules(7), Debian manual pages Debian, vendor-doc, accessed 2026-10-18
• auditctl(8), Debian manual pages Debian, vendor-doc, accessed 2026-10-18
• ANSSI-BP-028 v2.0 - Recommandations de configuration d'un système GNU/Linux (R73) ANSSI, guide, accessed 2026-10-18

Mappings

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

Threats mitigated

Related controls

• requires CB-DEB-AUDIT-011 Install and run the audit daemon
• related CB-DEB-AUDIT-029 Lock the audit configuration and load every valid rule

People and lifecycle