Enable Kernel Parameter to Enforce DAC on Symlinks

An XCCDF Rule

Description

To set the runtime status of the fs.protected_symlinks kernel parameter, run the following command:

$ sudo sysctl -w fs.protected_symlinks=1

To make sure that the setting is persistent, add the following line to a file in the directory /etc/sysctl.d:

fs.protected_symlinks = 1

Rationale

By enabling this kernel parameter, symbolic links are permitted to be followed only when outside a sticky world-writable directory, or when the UID of the link and follower match, or when the directory owner matches the symlink's owner. Disallowing such symlinks helps mitigate vulnerabilities based on insecure file system accessed by privileged programs, avoiding an exploitation vector exploiting unsafe use of open() or creat().

ID: xccdf_org.ssgproject.content_rule_sysctl_fs_protected_symlinks
Severity: Medium
References: BP28(R23)

CCI-002165

CIP-003-8 R5.1.1 CIP-003-8 R5.3 CIP-004-6 R2.3 CIP-007-3 R2.1 CIP-007-3 R2.2 CIP-007-3 R2.3 CIP-007-3 R5.1 CIP-007-3 R5.1.1 CIP-007-3 R5.1.2

AC-6(1) CM-6(a)

SRG-OS-000312-GPOS-00122 SRG-OS-000312-GPOS-00123 SRG-OS-000324-GPOS-00125
Updated: about 1 year ago

# Remediation is applicable only in certain platforms
if [ ! -f /.dockerenv ] && [ ! -f /run/.containerenv ]; then

# Comment out any occurrences of fs.protected_symlinks from /etc/sysctl.d/*.conf files

for f in /etc/sysctl.d/*.conf /run/sysctl.d/*.conf /usr/local/lib/sysctl.d/*.conf; do
  matching_list=$(grep -P '^(?!#).*[\s]*fs.protected_symlinks.*$' $f | uniq )
  if ! test -z "$matching_list"; then
    while IFS= read -r entry; do
      escaped_entry=$(sed -e 's|/|\\/|g' <<< "$entry")
      # comment out "fs.protected_symlinks" matches to preserve user data
      sed -i "s/^${escaped_entry}$/# &/g" $f
    done <<< "$matching_list"
  fi
done

#
# Set sysctl config file which to save the desired value
#

SYSCONFIG_FILE="/etc/sysctl.conf"


#
# Set runtime for fs.protected_symlinks
#
/sbin/sysctl -q -n -w fs.protected_symlinks="1"

#
# If fs.protected_symlinks present in /etc/sysctl.conf, change value to "1"
#	else, add "fs.protected_symlinks = 1" to /etc/sysctl.conf
#

# Strip any search characters in the key arg so that the key can be replaced without
# adding any search characters to the config file.
stripped_key=$(sed 's/[\^=\$,;+]*//g' <<< "^fs.protected_symlinks")

# shellcheck disable=SC2059
printf -v formatted_output "%s = %s" "$stripped_key" "1"

# If the key exists, change it. Otherwise, add it to the config_file.
# We search for the key string followed by a word boundary (matched by \>),
# so if we search for 'setting', 'setting2' won't match.
if LC_ALL=C grep -q -m 1 -i -e "^fs.protected_symlinks\\>" "${SYSCONFIG_FILE}"; then
    escaped_formatted_output=$(sed -e 's|/|\\/|g' <<< "$formatted_output")
    LC_ALL=C sed -i --follow-symlinks "s/^fs.protected_symlinks\\>.*/$escaped_formatted_output/gi" "${SYSCONFIG_FILE}"
else
    if [[ -s "${SYSCONFIG_FILE}" ]] && [[ -n "$(tail -c 1 -- "${SYSCONFIG_FILE}" || true)" ]]; then
        LC_ALL=C sed -i --follow-symlinks '$a'\\ "${SYSCONFIG_FILE}"
    fi
    printf '%s\n' "$formatted_output" >> "${SYSCONFIG_FILE}"
fi

else
    >&2 echo 'Remediation is not applicable, nothing was done'
fi

- name: List /etc/sysctl.d/*.conf files
  find:
    paths:
    - /etc/sysctl.d/
    - /run/sysctl.d/
    - /usr/local/lib/sysctl.d/    contains: ^[\s]*fs.protected_symlinks.*$
    patterns: '*.conf'
    file_type: any
  register: find_sysctl_d
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_fs_protected_symlinks

- name: Comment out any occurrences of fs.protected_symlinks from config files
  replace:
    path: '{{ item.path }}'
    regexp: ^[\s]*fs.protected_symlinks
    replace: '#fs.protected_symlinks'
  loop: '{{ find_sysctl_d.files }}'
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_fs_protected_symlinks

- name: Ensure sysctl fs.protected_symlinks is set to 1
  sysctl:
    name: fs.protected_symlinks
    value: '1'
    sysctl_file: /etc/sysctl.conf
    state: present
    reload: true
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_fs_protected_symlinks

Enable Kernel Parameter to Enforce DAC on Symlinks

Description

Rationale

Remediation - Shell Script

Remediation - Ansible