Set SSH MaxSessions limit

Classification:

compliance

Framework:

Control:

Description

The MaxSessions parameter specifies the maximum number of open sessions permitted from a given connection. To set MaxSessions edit /etc/ssh/sshd_config as follows:

MaxSessions 10

Rationale

To protect a system from denial of service due to a large number of concurrent sessions, use the rate limiting function of MaxSessions to protect availability of sshd logins and prevent overwhelming the daemon.

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

var\_sshd\_max\_sessions='10'


if [ -e "/etc/ssh/sshd\_config" ] ; then
 
 LC\_ALL=C sed -i "/^\s\*MaxSessions\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' "MaxSessions $var\_sshd\_max\_sessions" >> "/etc/ssh/sshd\_config"
else
 head -n "$(( line\_number - 1 ))" "/etc/ssh/sshd\_config.bak" > "/etc/ssh/sshd\_config"
 printf '%s\n' "MaxSessions $var\_sshd\_max\_sessions" >> "/etc/ssh/sshd\_config"
 tail -n "+$(( 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: XCCDF Value var\_sshd\_max\_sessions # promote to variable
 set\_fact:
 var\_sshd\_max\_sessions: !!str 10
 tags:
 - always

- name: Set SSH MaxSessions limit
 block:

 - name: Check for duplicate values
 lineinfile:
 path: /etc/ssh/sshd\_config
 create: false
 regexp: (?i)^\s\*MaxSessions\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\*MaxSessions\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\*MaxSessions\s+
 line: MaxSessions {{ var\_sshd\_max\_sessions }}
 state: present
 insertbefore: ^[#\s]\*Match
 validate: /usr/sbin/sshd -t -f %s
 when: ansible\_virtualization\_type not in ["docker", "lxc", "openvz", "podman", "container"]
 tags:
 - PCI-DSSv4-2.2.6
 - configure\_strategy
 - low\_complexity
 - low\_disruption
 - medium\_severity
 - no\_reboot\_needed
 - sshd\_set\_max\_sessions