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V-252943

CAT II (Medium)

The TOSS SSH daemon must not allow compression or must only allow compression after successful authentication.

Rule ID

SV-252943r1155807_rule

STIG

Tri-Lab Operating System Stack (TOSS) 4 Security Technical Implementation Guide

Version

V2R5

CCIs

CCI-000366

Discussion

If compression is allowed in an SSH connection prior to authentication, vulnerabilities in the compression software could result in compromise of the system from an unauthenticated connection, potentially with root privileges. Compression options are: no - disables compression delayed - allow compression only after authentication yes - enables compression before authentication, which can leak sensitive metadata and is not recommended

Check Content

Verify the SSH daemon disables compression or only performs compression after a user successfully authenticates with the following command:

$ sudo grep -ir compression /etc/ssh/sshd_config /etc/ssh/sshd_config

/etc/ssh/sshd_config:Compression no

If the "Compression" keyword is set to "yes", is missing, or the returned line is commented out, this is a finding.

Fix Text

Uncomment the "Compression" keyword in "/etc/ssh/sshd_config" (this file may be named differently or be in a different location if using a version of SSH that is provided by a third-party vendor) on the system and set the value to "delayed" or "no":

Compression no

The SSH service must be restarted for changes to take effect:

$ sudo systemctl restart sshd.service