Restrict write access to APT keys and repository lists

Description

APT trusts the keyrings in /etc/apt/trusted.gpg.d/, /etc/apt/keyrings/, /usr/share/keyrings/ (and the legacy /etc/apt/trusted.gpg), and downloads from the repositories listed in /etc/apt/sources.list and /etc/apt/sources.list.d/. These directories should be root:root mode 0755 and their files root:root mode 0644 or stricter.

/usr/share/keyrings/ also holds the Ubuntu Pro keys (ubuntu-pro-*.gpg) used by the ESM repositories. /etc/apt/keyrings/ does not exist on 20.04 (not applicable until created). The check follows symbolic links and checks their target.

Rationale

A user able to add a key or a source entry can make APT install a package of their choice, with maintainer scripts running as root, at the next update. Only root may change them. Reading them is harmless: keys are public and APT's unprivileged _apt user needs to read them.

Impact

None known. Ubuntu ships these files with these permissions.

Setting

Audit

1. stat -c '%U:%G %a %n' /etc/apt/trusted.gpg.d /etc/apt/keyrings /usr/share/keyrings /etc/apt/sources.list.d shows root:root and 755 or stricter.
2. stat -L -c '%U:%G %a %n' /etc/apt/trusted.gpg /etc/apt/trusted.gpg.d/* /etc/apt/keyrings/* /usr/share/keyrings/* /etc/apt/sources.list /etc/apt/sources.list.d/* shows root:root and 644 or stricter for every existing file.

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

trusted-gpg-d

The /etc/apt/trusted.gpg.d directory.

path: /etc/apt/trusted.gpg.d
owner: root
group: root
mode: '0755'

etc-keyrings

The /etc/apt/keyrings directory.

path: /etc/apt/keyrings
owner: root
group: root
mode: '0755'

usr-share-keyrings

The /usr/share/keyrings directory.

path: /usr/share/keyrings
owner: root
group: root
mode: '0755'

key-files

Every keyring file APT can use, following symbolic links.

script: |
  d="/etc/apt/trusted.gpg /etc/apt/trusted.gpg.d /etc/apt/keyrings /usr/share/keyrings"
  # shellcheck disable=SC2086
  bad=$(find -L $d -maxdepth 1 -type f \( ! -user root -o ! -group root -o -perm /7133 \) \
        -exec stat -L -c '%n %U:%G %a' {} + 2>/dev/null)
  if [ -n "$bad" ]; then
    printf '%s\n' "$bad"
    exit 1
  fi
  # shellcheck disable=SC2086
  echo "$(find -L $d -maxdepth 1 -type f 2>/dev/null | wc -l) key files root:root 644 or stricter"

sources-list

The /etc/apt/sources.list file.

path: /etc/apt/sources.list
owner: root
group: root
mode: '0644'

sources-list-d

The /etc/apt/sources.list.d directory.

path: /etc/apt/sources.list.d
owner: root
group: root
mode: '0755'

sources-list-d-files

Files in /etc/apt/sources.list.d.

path: /etc/apt/sources.list.d/*
owner: root
group: root
mode: '0644'

Remediation

1. chown root:root /etc/apt/trusted.gpg.d /etc/apt/keyrings /usr/share/keyrings /etc/apt/sources.list.d and chmod go-w on them.
2. For each key file and source file: chown root:root <file> and chmod 644 <file> (or stricter).

Rollback

Restore the previous owner and mode with chown and chmod. Nothing else depends on them.

trusted-gpg-d

Generated from check trusted-gpg-d: the engine writes the fix for every output, it sets the expected value and the check runs again after.

etc-keyrings

Generated from check etc-keyrings: the engine writes the fix for every output, it sets the expected value and the check runs again after.

usr-share-keyrings

Generated from check usr-share-keyrings: the engine writes the fix for every output, it sets the expected value and the check runs again after.

key-files

find -L /etc/apt/trusted.gpg /etc/apt/trusted.gpg.d /etc/apt/keyrings /usr/share/keyrings \
  -maxdepth 1 -type f \( ! -user root -o ! -group root -o -perm /7133 \) 2>/dev/null |
  while IFS= read -r f; do
    chown root:root "$f" && chmod u-xs,go-wxs,o-t "$f" || exit 1
  done

sources-list

Generated from check sources-list: the engine writes the fix for every output, it sets the expected value and the check runs again after.

sources-list-d

Generated from check sources-list-d: the engine writes the fix for every output, it sets the expected value and the check runs again after.

sources-list-d-files

Generated from check sources-list-d-files: the engine writes the fix for every output, it sets the expected value and the check runs again after.

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

• apt-secure(8), Ubuntu manual pages Canonical, vendor-doc, accessed 2026-10-19
• sources.list(5), Ubuntu manual pages Canonical, vendor-doc, accessed 2026-10-19
• Ubuntu Server documentation - Third-party repository usage Canonical, vendor-doc, accessed 2026-10-19

Mappings

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

Threats mitigated

Related controls

• related CB-UBU-PKG-005 Restrict access to APT repository credentials

People and lifecycle