Limit SSH connections and close dead sessions

Description

These OpenSSH server options limit resources used per client and detect dead connections:

• ClientAliveInterval (1 to {{ ssh_client_alive_interval }} seconds) and ClientAliveCountMax (1 to {{ ssh_client_alive_count_max }}): the server checks that the client still answers, and closes the session after ClientAliveCountMax unanswered checks;
• MaxStartups at most 10:30:60: beyond 10 unauthenticated connections, new ones are refused with a probability of 30 %, rising to 100 % at 60;
• MaxSessions at most {{ ssh_max_sessions }}: shells, commands and subsystems multiplexed on one connection.

OpenSSH's own default for ClientAliveInterval is 0 (no check), and Ubuntu keeps it, cloud images included. Idle shell sessions are closed by TMOUT (CB-UBU-AUTH-020), not by these options, since a live client always answers keepalive checks. The fix only changes the options that are out of bounds, in /etc/ssh/sshd_config.d/00-cirsec.conf.

Rationale

Sessions whose client has gone away (laptop closed, network cut) keep a shell open on the server; closing them limits what a later session hijack can reach. Connection limits keep a brute force tool or a flood of half-open connections from making the server unreachable for administrators.

Impact

Sessions of clients that stop answering for more than {{ ssh_client_alive_interval }} × {{ ssh_client_alive_count_max }} seconds are closed. Automation that opens many parallel connections or multiplexed sessions (Ansible pipelining, ControlMaster) may hit the limits; raise them through tailoring if needed.

Applies when

Detected: when: OpenSSH server is installed A scanner reports the control not applicable when one is false.

Setting

Parameters

The value used is your override (tailoring file), else the profile's, else the one of the profile it extends, else the default. Bounds limit what an override may set.

Audit

Run sshd -T | grep -E '^(clientaliveinterval|clientalivecountmax|maxstartups|maxsessions) '. clientaliveinterval must be between 1 and {{ ssh_client_alive_interval }}, clientalivecountmax between 1 and {{ ssh_client_alive_count_max }}, each field of maxstartups at most 10:30:60, and maxsessions at most {{ ssh_max_sessions }}.

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

client-alive-interval-set

ClientAliveInterval is not 0 (keepalive off).

key: clientaliveinterval
operator: '>='
value: 1

client-alive-interval

ClientAliveInterval upper bound.

key: clientaliveinterval
operator: <=
value: '{{ ssh_client_alive_interval }}'

client-alive-count-set

ClientAliveCountMax is not 0.

key: clientalivecountmax
operator: '>='
value: 1

client-alive-count

ClientAliveCountMax upper bound.

key: clientalivecountmax
operator: <=
value: '{{ ssh_client_alive_count_max }}'

max-startups

MaxStartups at most 10:30:60.

key: maxstartups
operator: match
value: ^([1-9]|10):([1-9]|[12][0-9]|30):([1-9]|[1-5][0-9]|60)$

max-sessions

MaxSessions upper bound.

key: maxsessions
operator: <=
value: '{{ ssh_max_sessions }}'

Remediation

Add to /etc/ssh/sshd_config.d/00-cirsec.conf:

ClientAliveInterval {{ ssh_client_alive_interval }}
ClientAliveCountMax {{ ssh_client_alive_count_max }}
MaxStartups 10:30:60
MaxSessions {{ ssh_max_sessions }}

Check with sshd -t, then run systemctl reload ssh (on 24.04 and later, when ssh.service is not running yet, the next connection starts it with the new settings).

Rollback

Remove the lines from /etc/ssh/sshd_config.d/00-cirsec.conf and run systemctl reload ssh.

limits

# CB-UBU-NET-017: bring SSH keepalive and connection limits within bounds, changing only what is out of bounds.
PATH="$PATH:/usr/sbin"
# sshd -T and -t need /run/sshd, which only exists once ssh.service has run (ssh.socket on 24.04 and later).
[ -d /run/sshd ] || mkdir -p -m 0755 /run/sshd
f=/etc/ssh/sshd_config.d/00-cirsec.conf
grep -Eqi '^[[:space:]]*Include[[:space:]]+/etc/ssh/sshd_config.d/\*\.conf' /etc/ssh/sshd_config || {
  echo '/etc/ssh/sshd_config does not include sshd_config.d/*.conf' >&2
  exit 1
}
cfg=$(sshd -T 2>/dev/null) || { echo 'sshd -T failed' >&2; exit 1; }
get() { printf '%s\n' "$cfg" | awk -v k="$1" '$1 == k { print $2; exit }'; }
# in_range VALUE MAX: VALUE is an integer between 1 and MAX.
in_range() { case $1 in '' | *[!0-9]*) return 1 ;; esac; [ "$1" -ge 1 ] && [ "$1" -le "$2" ]; }
changes=
in_range "$(get clientaliveinterval)" "$CB_SSH_CLIENT_ALIVE_INTERVAL" || changes="$changes clientaliveinterval=$CB_SSH_CLIENT_ALIVE_INTERVAL"
in_range "$(get clientalivecountmax)" "$CB_SSH_CLIENT_ALIVE_COUNT_MAX" || changes="$changes clientalivecountmax=$CB_SSH_CLIENT_ALIVE_COUNT_MAX"
in_range "$(get maxsessions)" "$CB_SSH_MAX_SESSIONS" || changes="$changes maxsessions=$CB_SSH_MAX_SESSIONS"
get maxstartups | grep -Eq '^([1-9]|10):([1-9]|[12][0-9]|30):([1-9]|[1-5][0-9]|60)$' || changes="$changes maxstartups=10:30:60"
[ -n "$changes" ] || exit 0
mkdir -p /etc/ssh/sshd_config.d
cb_backup "$f"
touch "$f"
chmod 600 "$f"
for c in $changes; do
  k=${c%%=*}
  sed -i "/^[[:space:]]*${k}[[:space:]]/Id" "$f"
  printf '%s %s\n' "$k" "${c#*=}" >>"$f"
done
if ! sshd -t; then
  cb_restore "$f"
  echo 'sshd -t rejected the change, reverted' >&2
  exit 1
fi

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

• sshd_config(5), Ubuntu manual pages Canonical, vendor-doc, accessed 2026-10-19
• NIST IR 7966 - Security of Interactive and Automated Access Management Using Secure Shell (SSH) NIST, guide, 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-AUTH-020 Close idle shell sessions automatically

People and lifecycle