Title

RHEL 9 SSH daemon must not allow compression or must only allow compression after successful authentication. (Cat II impact)

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.

Check Content

Verify the SSH daemon performs compression after a user successfully authenticates with the following command: $ sudo /usr/sbin/sshd -dd 2>&1 | awk '/filename/ {print $4}' | tr -d '\r' | tr '\n' ' ' | xargs sudo grep -iH '^\s*compression' Compression delayed If the "Compression" keyword is set to "yes", is missing, or the returned line is commented out, this is a finding.

Fix Text

Configure the SSH daemon to not allow compression. Uncomment the "Compression" keyword in "/etc/ssh/sshd_config" 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

Additional Identifiers

Rule ID: SV-258002r952200_rule

Vulnerability ID: V-258002

Group Title: SRG-OS-000480-GPOS-00227

Expert Comments

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