cis-ubuntu2004-v300-5-1-17

DevOps & Security

Ensure sshd MaxSessions is configured

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Ensure sshd MaxSessions is configured (Automated)

Profile Applicability

Level 1 - Server
Level 1 - Workstation

Description

The MaxSessions parameter specifies the maximum number of open sessions permitted from a given connection.

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.

Audit Procedure

Command Line

Run the following command and verify that MaxSessions is 10 or less:

# sshd -T | grep maxsessions

maxsessions 10

Run the following command and verify the output:

grep -Psi -- '^\h*MaxSessions\h+\"?([1[1-9]|[2-9][0-9]|[1-9][0-9][0-9]+)\b' /etc/ssh/sshd_config /etc/ssh/sshd_config.d/*.conf

Nothing should be returned.

IF - Match set statements are used in your environment, specify the connection parameters to use for the -T extended test mode and run the audit to verify the setting is not incorrectly configured in a match block. Example additional audit needed for a match block for the user sshuser:

# sshd -T -C user=sshuser | grep maxsessions

Note: If provided, any Match directives in the configuration file that would apply are applied before the configuration is written to standard output. The connection parameters are supplied as keyword=value pairs and may be supplied in any order, either with multiple -C options or as a comma-separated list. The keywords are addr (source address), user (user), host (resolved source host name), laddr (local address), lport (local port number), and rdomain (routing domain)

Expected Result

maxsessions 10

(or a value of 10 or less)

Remediation

Command Line

Edit the /etc/ssh/sshd_config file to set the MaxSessions parameter to 10 or less above any Include and Match entries as follows:

MaxSessions 10

Note: First occurrence of an option takes precedence, Match set statements withstanding. If Include locations are enabled, used, and order of precedence is understood in your environment, the entry may be created in a file in Include location.

Default Value

MaxSessions 10

References

SSHD_CONFIG(5)
NIST SP 800-53 Rev. 5: CM-1, CM-2, CM-6, CM-7, IA-5

CIS Controls

v8 - 0.0 Explicitly Not Mapped

v7 - 0.0 Explicitly Not Mapped

MITRE ATT&CK Mappings: T1499, T1499.002 | TA0040