Description
The INFO parameter specifices that record login and logout activity will be logged.
The default SSH configuration sets the log level to INFO. The appropriate
configuration is used if no value is set for LogLevel.
To explicitly specify the log level in SSH, add or correct the following line in
/etc/ssh/sshd_config:
Rationale
SSH provides several logging levels with varying amounts of verbosity. DEBUG is specifically
not recommended other than strictly for debugging SSH communications since it provides
so much data that it is difficult to identify important security information. INFO level is the
basic level that only records login activity of SSH users. In many situations, such as Incident
Response, it is important to determine when a particular user was active on a system. The
logout record can eliminate those users who disconnected, which helps narrow the field.
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\*LogLevel\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 "$line\_number" ]; then
# There was no match of '^Match', insert at
# the end of the file.
printf '%s\n' "LogLevel INFO" >> "/etc/ssh/sshd\_config"
else
head -n "$(( line\_number - 1 ))" "/etc/ssh/sshd\_config.bak" > "/etc/ssh/sshd\_config"
printf '%s\n' "LogLevel INFO" >> "/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: Set LogLevel to INFO
block:
- name: Check for duplicate values
lineinfile:
path: /etc/ssh/sshd\_config
create: false
regexp: (?i)^\s\*LogLevel\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\*LogLevel\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\*LogLevel\s+
line: LogLevel INFO
state: present
insertbefore: ^[#\s]\*Match
validate: /usr/sbin/sshd -t -f %s
when: ansible\_virtualization\_type not in ["docker", "lxc", "openvz", "podman", "container"]
tags:
- NIST-800-53-AC-17(a)
- NIST-800-53-CM-6(a)
- low\_complexity
- low\_disruption
- low\_severity
- no\_reboot\_needed
- restrict\_strategy
- sshd\_set\_loglevel\_info
