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cis-ubuntu1804-v220-4-2-6

DevOps & Security
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Ensure sshd ClientAliveInterval and ClientAliveCountMax are configured

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CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0 - Control 4.2.6

Profile Applicability

  • Level 1 - Server
  • Level 1 - Workstation

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.

Rationale

Having no timeout value associated with a connection could allow an unauthorized user access to another user's SSH session. Setting a timeout value reduces this risk.

The ClientAliveCountMax parameter sets a limit on the number of times the server will attempt to send alive messages before disconnecting.

Audit Procedure

Command Line

Run the following command and verify the output:

sshd -T | grep -Pi '(clientaliveinterval|clientalivecountmax)'

Expected Result

clientaliveinterval 15
clientalivecountmax 3

OR site-appropriate values. clientaliveinterval should be between 1 and 300. clientalivecountmax should be between 1 and 3.

Remediation

Command Line

Edit the /etc/ssh/sshd_config file to set the parameters according to site policy:

ClientAliveInterval 15
ClientAliveCountMax 3

Default Value

ClientAliveInterval 0 ClientAliveCountMax 3

References

  1. NIST SP 800-53 Rev. 5: CM-6

CIS Controls

None

Profile Applicability

  • Level 1 - Server
  • Level 1 - Workstation

Assessment Status

Automated