# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-DEB-AUDIT-020
title: Audit changes to file permissions, owners and extended attributes
finding:
  title: Changes to file permissions and ownership are not recorded
  description: A user can open up access to sensitive files or hide a setuid program without a record.
status: active
profiles: [server-hardened, desktop-hardened]
risk:
  severity: low
assessment: automated
tags: [auditd, permissions]

description: |
  auditd rules in `/etc/audit/rules.d/60-cirsec-perm.rules`, for actions of users who logged in:

  - mode changes (`chmod`, `fchmod`, `fchmodat`, and `fchmodat2` where the kernel and auditd know it);
  - owner changes (`chown`, `fchown`, `lchown`, `fchownat`);
  - extended attribute changes (`setxattr`, `removexattr` and their variants), which also hold ACLs;
  - runs of `chcon`, `setfacl` and `chacl` (key `perm_chng`).

  Every system call rule exists for 64-bit and 32-bit programs (key `perm_mod`).
rationale: |
  Permission changes are how a user makes a private file readable, a script writable by others or
  a binary setuid. Recording them makes such changes visible and attributable.
impact: |
  Users who change many permissions (builds, package managers run by users, rsync) produce many records. Actions of system services (no login session) are not recorded.
notes: |
  `chcon` belongs to coreutils; `setfacl` and `chacl` come with the `acl` package, often absent. A rule on a program that is not installed is accepted and applies once it is installed.

setting:
  location: /etc/audit/rules.d/60-cirsec-perm.rules
  default_value: No audit rules
  recommended_value: "Rules loaded (auditctl -l) and present in /etc/audit/rules.d/60-cirsec-perm.rules"

audit:
  manual: |
    1. Run `auditctl -l` as root. It must list the rules shown below (keys `perm_chng`, `perm_mod`).
    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 chmod,fchmod,fchmodat -F auid>=1000 -F auid!=-1 -F key=perm_mod
    -a always,exit -F arch=b32 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=-1 -F key=perm_mod
    -a always,exit -F arch=b64 -S chown,fchown,lchown,fchownat -F auid>=1000 -F auid!=-1 -F key=perm_mod
    -a always,exit -F arch=b32 -S lchown,fchown,chown,fchownat -F auid>=1000 -F auid!=-1 -F key=perm_mod
    -a always,exit -F arch=b64 -S setxattr,lsetxattr,fsetxattr,removexattr,lremovexattr,fremovexattr -F auid>=1000 -F auid!=-1 -F key=perm_mod
    -a always,exit -F arch=b32 -S setxattr,lsetxattr,fsetxattr,removexattr,lremovexattr,fremovexattr -F auid>=1000 -F auid!=-1 -F key=perm_mod
    -a always,exit -S all -F path=/usr/bin/chcon -F perm=x -F auid>=1000 -F auid!=-1 -F key=perm_chng
    -a always,exit -S all -F path=/usr/bin/setfacl -F perm=x -F auid>=1000 -F auid!=-1 -F key=perm_chng
    -a always,exit -S all -F path=/usr/bin/chacl -F perm=x -F auid>=1000 -F auid!=-1 -F key=perm_chng
    ```
  checks:
    - id: rules
      engine: shell
      description: Rules loaded in the kernel and present in /etc/audit/rules.d.
      spec:
        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 chmod,fchmod,fchmodat -F auid>=1000 -F auid!=-1 -F key=perm_mod
          -a always,exit -F arch=b32 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=-1 -F key=perm_mod
          -a always,exit -F arch=b64 -S chown,fchown,lchown,fchownat -F auid>=1000 -F auid!=-1 -F key=perm_mod
          -a always,exit -F arch=b32 -S lchown,fchown,chown,fchownat -F auid>=1000 -F auid!=-1 -F key=perm_mod
          -a always,exit -F arch=b64 -S setxattr,lsetxattr,fsetxattr,removexattr,lremovexattr,fremovexattr -F auid>=1000 -F auid!=-1 -F key=perm_mod
          -a always,exit -F arch=b32 -S setxattr,lsetxattr,fsetxattr,removexattr,lremovexattr,fremovexattr -F auid>=1000 -F auid!=-1 -F key=perm_mod
          -a always,exit -S all -F path=/usr/bin/chcon -F perm=x -F auid>=1000 -F auid!=-1 -F key=perm_chng
          -a always,exit -S all -F path=/usr/bin/setfacl -F perm=x -F auid>=1000 -F auid!=-1 -F key=perm_chng
          -a always,exit -S all -F path=/usr/bin/chacl -F perm=x -F auid>=1000 -F auid!=-1 -F key=perm_chng
          RULES
          # fchmodat2 exists from Linux 6.6 (Debian 13); auditd 3.0 (Debian 12) does not know it
          if ausyscall x86_64 fchmodat2 >/dev/null 2>&1; then
            echo '-a always,exit -F arch=b64 -S fchmodat2 -F auid>=1000 -F auid!=-1 -F key=perm_mod'
            echo '-a always,exit -F arch=b32 -S fchmodat2 -F auid>=1000 -F auid!=-1 -F key=perm_mod'
          fi
          }
          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:
  manual: |
    1. Write the expected rules to `/etc/audit/rules.d/60-cirsec-perm.rules` (mode 0640).
    2. Run `augenrules --load`. If `auditctl -s` shows `enabled 2`, the configuration is locked:
       reboot to load the new rules.
  effects:
    restart: none
    disruption: none
    lockout_risk: false
  rollback: |
    Delete `/etc/audit/rules.d/60-cirsec-perm.rules` and run `augenrules --load` (or reboot if the configuration is locked).
  scripts:
    - id: rules-file
      engine: shell
      content: |
        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 chmod,fchmod,fchmodat -F auid>=1000 -F auid!=-1 -F key=perm_mod
        -a always,exit -F arch=b32 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=-1 -F key=perm_mod
        -a always,exit -F arch=b64 -S chown,fchown,lchown,fchownat -F auid>=1000 -F auid!=-1 -F key=perm_mod
        -a always,exit -F arch=b32 -S lchown,fchown,chown,fchownat -F auid>=1000 -F auid!=-1 -F key=perm_mod
        -a always,exit -F arch=b64 -S setxattr,lsetxattr,fsetxattr,removexattr,lremovexattr,fremovexattr -F auid>=1000 -F auid!=-1 -F key=perm_mod
        -a always,exit -F arch=b32 -S setxattr,lsetxattr,fsetxattr,removexattr,lremovexattr,fremovexattr -F auid>=1000 -F auid!=-1 -F key=perm_mod
        -a always,exit -S all -F path=/usr/bin/chcon -F perm=x -F auid>=1000 -F auid!=-1 -F key=perm_chng
        -a always,exit -S all -F path=/usr/bin/setfacl -F perm=x -F auid>=1000 -F auid!=-1 -F key=perm_chng
        -a always,exit -S all -F path=/usr/bin/chacl -F perm=x -F auid>=1000 -F auid!=-1 -F key=perm_chng
        RULES
        # fchmodat2 exists from Linux 6.6 (Debian 13); auditd 3.0 (Debian 12) does not know it
        if ausyscall x86_64 fchmodat2 >/dev/null 2>&1; then
          echo '-a always,exit -F arch=b64 -S fchmodat2 -F auid>=1000 -F auid!=-1 -F key=perm_mod'
          echo '-a always,exit -F arch=b32 -S fchmodat2 -F auid>=1000 -F auid!=-1 -F key=perm_mod'
        fi
        }
        f=/etc/audit/rules.d/60-cirsec-perm.rules
        t=$(mktemp)
        { echo '# Written by CIRSEC Baselines CB-DEB-AUDIT-020'; want; } >"$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

references:
  - title: audit.rules(7), Debian manual pages
    url: https://manpages.debian.org/trixie/auditd/audit.rules.7.en.html
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
  - title: auditctl(8), Debian manual pages
    url: https://manpages.debian.org/trixie/auditd/auditctl.8.en.html
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
  - title: ANSSI-BP-028 v2.0 - Recommandations de configuration d'un système GNU/Linux (R73)
    url: https://messervices.cyber.gouv.fr/documents-guides/fr_np_linux_configuration-v2.0.pdf
    publisher: ANSSI
    kind: guide
    accessed: 2026-10-18
mappings:
  - framework: anssi-linux
    framework_version: "2.0"
    id: "R73"
    relation: partial
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: "AU-12"
    relation: related
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "6.2.3.18"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "6.2.3.24"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "6.2.3.25"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "6.2.3.26"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "6.2.3.18"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "6.2.3.19"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "6.2.3.20"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "6.2.3.27"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "6.2.3.28"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "6.2.3.29"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1222.002
related_controls:
  - id: CB-DEB-AUDIT-011
    type: requires
  - id: CB-DEB-AUDIT-029
    type: related

authors:
  - name: Alice
    github: Sn0wAlice
verified:
  - date: 2026-10-18
    by: Sn0wAlice
    versions: ["12"]
    environment: Proxmox VM, Debian 12 genericcloud image, kernel 6.1.0-54-cloud-amd64, auditd 1:3.0.9-1, BIOS boot
    notes: "Rules fail on fresh install; fix writes /etc/audit/rules.d/60-cirsec-perm.rules and loads it; rules match auditctl -l, kept after reboot; second run idempotent."
  - date: 2026-10-18
    by: Sn0wAlice
    versions: ["13"]
    environment: Proxmox VM, Debian 13 genericcloud image, kernel 6.12.113+deb13-cloud-amd64, auditd 1:4.0.2-2+deb13u1, BIOS boot
    notes: "Rules fail on fresh install; fix writes /etc/audit/rules.d/60-cirsec-perm.rules and loads it; rules match auditctl -l, kept after reboot; second run idempotent."
introduced_in: 0.1.0
created: 2026-10-18
updated: 2026-10-18