Do Not Allow SSH Environment Options

Classification:

compliance

Framework:

Control:

Description

Ensure that users are not able to override environment variables of the SSH daemon.

The default SSH configuration disables environment processing. The appropriate configuration is used if no value is set for PermitUserEnvironment.

To explicitly disable Environment options, add or correct the following

/etc/ssh/sshd_config:

PermitUserEnvironment no

Rationale

SSH environment options potentially allow users to bypass access restriction in some configurations.

Remediation

Shell script

The following script can be run on the host to remediate the issue.

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

if [ -e "/etc/ssh/sshd\_config" ] ; then
 
 LC\_ALL=C sed -i "/^\s\*PermitUserEnvironment\s\+/Id" "/etc/ssh/sshd\_config"
else
 touch "/etc/ssh/sshd\_config"
fi
# make sure file has newline at the end
sed -i -e '$a\' "/etc/ssh/sshd\_config"

cp "/etc/ssh/sshd\_config" "/etc/ssh/sshd\_config.bak"
# Insert before the line matching the regex '^Match'.
line\_number="$(LC\_ALL=C grep -n "^Match" "/etc/ssh/sshd\_config.bak" | LC\_ALL=C sed 's/:.\*//g')"
if [ -z "$line\_number" ]; then
 # There was no match of '^Match', insert at
 # the end of the file.
 printf '%s\n' "PermitUserEnvironment no" >> "/etc/ssh/sshd\_config"
else
 head -n "$(( line\_number - 1 ))" "/etc/ssh/sshd\_config.bak" > "/etc/ssh/sshd\_config"
 printf '%s\n' "PermitUserEnvironment no" >> "/etc/ssh/sshd\_config"
 tail -n "+$(( line\_number ))" "/etc/ssh/sshd\_config.bak" >> "/etc/ssh/sshd\_config"
fi
# Clean up after ourselves.
rm "/etc/ssh/sshd\_config.bak"

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

Ansible playbook

The following playbook can be run with Ansible to remediate the issue.

- name: Do Not Allow SSH Environment Options
 block:

 - name: Check for duplicate values
 lineinfile:
 path: /etc/ssh/sshd\_config
 create: false
 regexp: (?i)^\s\*PermitUserEnvironment\s+
 state: absent
 check\_mode: true
 changed\_when: false
 register: dupes

 - name: Deduplicate values from /etc/ssh/sshd\_config
 lineinfile:
 path: /etc/ssh/sshd\_config
 create: false
 regexp: (?i)^\s\*PermitUserEnvironment\s+
 state: absent
 when: dupes.found is defined and dupes.found > 1

 - name: Insert correct line to /etc/ssh/sshd\_config
 lineinfile:
 path: /etc/ssh/sshd\_config
 create: true
 regexp: (?i)^\s\*PermitUserEnvironment\s+
 line: PermitUserEnvironment no
 state: present
 insertbefore: ^[#\s]\*Match
 validate: /usr/sbin/sshd -t -f %s
 when: ansible\_virtualization\_type not in ["docker", "lxc", "openvz", "podman", "container"]
 tags:
 - CJIS-5.5.6
 - DISA-STIG-UBTU-20-010047
 - NIST-800-171-3.1.12
 - NIST-800-53-AC-17(a)
 - NIST-800-53-CM-6(a)
 - NIST-800-53-CM-7(a)
 - NIST-800-53-CM-7(b)
 - PCI-DSS-Req-2.2.4
 - PCI-DSSv4-2.2.6
 - low\_complexity
 - low\_disruption
 - medium\_severity
 - no\_reboot\_needed
 - restrict\_strategy
 - sshd\_do\_not\_permit\_user\_env