Cis Ubuntu1604 V200 5 3 17

CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 5.3.17

Description

The two options ClientAliveInterval and ClientAliveCountMax control the timeout of ssh sessions.

• ClientAliveInterval sets a timeout interval in seconds after which if no data has been received from the client, sshd will send a message through the encrypted channel to request a response from the client. The default is 0, indicating that these messages will not be sent to the client.
• ClientAliveCountMax sets the number of client alive messages which may be sent without sshd receiving any messages back from the client. If this threshold is reached while client alive messages are being sent, sshd will disconnect the client, terminating the session. The default value is 3.
  • The client alive messages are sent through the encrypted channel
  • Setting ClientAliveCountMax to 0 disables connection termination

Example: The default value is 3. If ClientAliveInterval is set to 15, and ClientAliveCountMax is left at the default, unresponsive SSH clients will be disconnected after approximately 45 seconds.

Rationale

Having no timeout value associated with a connection could allow an unauthorized user access to another user's ssh session (e.g. user walks away from their computer and doesn't lock the screen). Setting a timeout value reduces this risk.

• The recommended ClientAliveInterval setting is no greater than 900 seconds (15 minutes)
• The recommended ClientAliveCountMax setting is 0
• At the 15 minute interval, if the ssh session is inactive, the session will be terminated.

Impact

In some cases this setting may cause termination of long-running scripts over SSH or remote automation tools which rely on SSH. In developing the local site policy, the requirements of such scripts should be considered and appropriate ServerAliveInterval and ClientAliveInterval settings should be calculated to insure operational continuity.

Audit Procedure

Command Line

Run the following commands and verify ClientAliveInterval is between 1 and 900:

sshd -T -C user=root -C host="$(hostname)" -C addr="$(grep $(hostname) /etc/hosts | awk '{print $1}')" | grep clientaliveinterval

Expected Result

clientaliveinterval 900

Run the following command and verify ClientAliveCountMax is 0:

sshd -T -C user=root -C host="$(hostname)" -C addr="$(grep $(hostname) /etc/hosts | awk '{print $1}')" | grep clientalivecountmax

Expected Result

clientalivecountmax 3

Run the following commands and verify the output:

grep -Ei '^\s*ClientAliveInterval\s+(0|9[0-9][1-9]|[1-9][0-9][0-9][0-9]+|1[6-9]m|[2-9][0-9]m|[1-9][0-9][0-9]+m)\b' /etc/ssh/sshd_config

Nothing should be returned.

grep -Ei '^\s*ClientAliveCountMax\s+([1-9]|[1-9][0-9]+)\b' /etc/ssh/sshd_config

Nothing should be returned.

Remediation

Command Line

Edit the /etc/ssh/sshd_config file to set the parameters according to site policy. This should include ClientAliveInterval between 1 and 900 and ClientAliveCountMax of 0:

ClientAliveInterval 900
ClientAliveCountMax 0

Default Value

ClientAliveInterval 0

ClientAliveCountMax 3

References

1. https://man.openbsd.org/sshd_config

CIS Controls

Version 7

16.11 Lock Workstation Sessions After Inactivity - Automatically lock workstation sessions after a standard period of inactivity.

Profile Applicability

• Level 1 - Server
• Level 1 - Workstation

Assessment Status

Automated