# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-DEB-FS-006
title: Mount separate filesystems with nodev, nosuid and noexec where they are not needed
finding:
  title: Separate filesystems are mounted without restrictive options
  description: Device files, setuid programs or executables can be used from filesystems that should only hold data.
status: active
profiles: [core]
risk:
  severity: low
assessment: automated
tags: [mount, partitions, fstab]

description: |
  When these directories are separate filesystems, they are mounted with the options listed in
  `{{ mount_options }}` (format `directory:option,option`). By default:

  | Directory | Options |
  |---|---|
  | `/home`, `/var`, `/opt`, `/srv` | `nodev,nosuid` |
  | `/var/tmp`, `/var/log`, `/var/log/audit`, `/boot` | `nodev,nosuid,noexec` |
  | `/usr` | `nodev` |

  The hardened profiles also require `noexec` on `/home`. Every other local filesystem backed by a
  block device, except `/`, must be mounted with `nodev`. A directory that is not a separate
  filesystem is skipped; when there is none besides `/`, the control does not apply.
rationale: |
  Only `/dev` needs device files, and only system directories need setuid programs or executables.
  `nodev` stops a device node created on a data filesystem (or brought on removable media) from
  giving raw access to disks or memory. `nosuid` stops setuid programs planted there from granting
  privileges, and `noexec` stops tools dropped there from being run directly.
impact: |
  `noexec` on `/home` (hardened profiles) prevents users from running their own scripts and
  programs directly, including tools installed under `~/.local/bin`. `noexec` on `/boot` and
  `/var/log` has no known effect on Debian.
notes: |
  ANSSI also recommends `noexec` on `/var`. It is left out: dpkg runs package maintainer scripts
  from `/var/lib/dpkg/info/`, so package installation and upgrades fail with it. Container engines
  that store images under `/var/lib` need `exec` as well.

setting:
  location: /etc/fstab
  default_value: Options chosen at installation (usually defaults)
  recommended_value: "{{ mount_options }}; nodev on every other local filesystem except /"

parameters:
  - name: mount_options
    type: list
    description: Required options per mount point, as directory:option,option.
    default:
      - /home:nodev,nosuid
      - /var:nodev,nosuid
      - /var/tmp:nodev,nosuid,noexec
      - /var/log:nodev,nosuid,noexec
      - /var/log/audit:nodev,nosuid,noexec
      - /boot:nodev,nosuid,noexec
      - /opt:nodev,nosuid
      - /srv:nodev,nosuid
      - /usr:nodev
    profile_defaults:
      server-hardened: &hardened
        - /home:nodev,nosuid,noexec
        - /var:nodev,nosuid
        - /var/tmp:nodev,nosuid,noexec
        - /var/log:nodev,nosuid,noexec
        - /var/log/audit:nodev,nosuid,noexec
        - /boot:nodev,nosuid,noexec
        - /opt:nodev,nosuid
        - /srv:nodev,nosuid
        - /usr:nodev
      desktop-hardened: *hardened

audit:
  manual: |
    1. Run `findmnt -kn -o TARGET,SOURCE,OPTIONS`.
    2. For each mounted directory of `{{ mount_options }}`, the options must contain the listed ones.
    3. Every other line whose source is a `/dev/` device, except `/`, must contain `nodev`.
  checks:
    - id: options
      engine: shell
      description: Mount options of the listed directories and nodev on other local block-device filesystems.
      spec:
        script: |
          rc=0 seen=
          for e in $CB_MOUNT_OPTIONS; do
            p=${e%%:*}
            o=$(findmnt -kn -o OPTIONS --mountpoint "$p" | tail -n 1)
            [ -n "$o" ] || continue
            seen="$seen $p "
            miss=
            for opt in $(printf '%s' "${e#*:}" | tr , ' '); do
              case ,$o, in *,$opt,*) ;; *) miss="$miss $opt" ;; esac
            done
            if [ -z "$miss" ]; then echo "$p: ok"; else echo "$p: missing$miss"; rc=1; fi
          done
          other=$(findmnt -knr -o TARGET,SOURCE,OPTIONS | awk '$2 ~ "^/dev/" && $1 != "/"')
          while read -r t s o; do
            [ -n "$t" ] || continue
            case $seen in *" $t "*) continue ;; esac
            seen="$seen $t "
            case ,$o, in *,nodev,*) echo "$t: ok" ;; *) echo "$t: missing nodev"; rc=1 ;; esac
          done <<EOF
          $other
          EOF
          [ -n "$seen" ] || { echo "no separate filesystem besides /"; exit 2; }
          exit $rc

remediation:
  manual: |
    For each reported directory:
    1. Add the missing options to the fourth field of its line in `/etc/fstab` (for a mount made by
       a systemd `.mount` unit, to its `Options=`).
    2. Run `systemctl daemon-reload` and `mount -o remount <directory>`.
    3. Check with `findmnt -kn <directory>`, and reboot at the next maintenance window to confirm.

    Test `noexec` and `nosuid` first on systems that run software from `/opt`, `/srv` or `/home`.

references:
  - title: mount(8), Debian manual pages (filesystem-independent mount options)
    url: https://manpages.debian.org/trixie/mount/mount.8.en.html
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
  - title: fstab(5), Debian manual pages
    url: https://manpages.debian.org/trixie/mount/fstab.5.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 (R28)
    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: R28
    relation: partial
    note: Ours leaves out noexec on /var (breaks dpkg) and noexec on /home outside the hardened profiles.
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: CM-7
    relation: related
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.1.2.3.2"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.1.2.3.3"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.1.2.4.2"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.1.2.4.3"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.1.2.5.2"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.1.2.5.3"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.1.2.5.4"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.1.2.6.2"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.1.2.6.3"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.1.2.6.4"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.1.2.7.2"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.1.2.7.3"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.1.2.7.4"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.1.2.3.2"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.1.2.3.3"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.1.2.4.2"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.1.2.4.3"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.1.2.5.2"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.1.2.5.3"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.1.2.5.4"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.1.2.6.2"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.1.2.6.3"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.1.2.6.4"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.1.2.7.2"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.1.2.7.3"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.1.2.7.4"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1548.001
  - framework: cwe
    id: CWE-732
related_controls:
  - id: CB-DEB-FS-005
    type: related
  - id: CB-DEB-FS-003
    type: related

authors:
  - name: Alice
    github: Sn0wAlice
verified:
  - date: 2026-10-18
    by: Sn0wAlice
    versions: ["12"]
    environment: Proxmox VM 922, Debian 12 genericcloud image, kernel 6.1.0-54-cloud-amd64
    notes: "Fails on fresh install (/boot/efi without nodev); not applicable with /boot/efi unmounted; loop-mounted /var/tmp fails without options and passes after remount with nodev,nosuid,noexec."
  - date: 2026-10-18
    by: Sn0wAlice
    versions: ["13"]
    environment: Proxmox VM 932, Debian 13 genericcloud image, kernel 6.12.113+deb13-cloud-amd64
    notes: "Fails on fresh install (/boot/efi without nodev); not applicable with /boot/efi unmounted; loop-mounted /var/tmp fails without options and passes after remount with nodev,nosuid,noexec."
introduced_in: 0.1.0
created: 2026-10-18
updated: 2026-10-18