AlmaLinux OS 9 SSH daemon must not allow compression or must only allow compression after successful authentication.
Overview
| Finding ID | Version | Rule ID | IA Controls | Severity |
| V-269260 | ALMA-09-020370 | SV-269260r1050142_rule | CCI-000366 | medium |
| Description | ||||
| 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. | ||||
| STIG | Date | |||
| CloudLinux AlmaLinux OS 9 Security Technical Implementation Guide | 2025-05-22 | |||
Details
Check Text (C-269260r1050142_chk)
Verify the SSH daemon performs compression after a user successfully authenticates with the following command:
$ sshd -T | grep compression
Compression no
If the "Compression" keyword is set to "yes", this is a finding.
Fix Text (F-73192r1048157_fix)
Configure the SSH daemon to not allow compression.
Add the following line to "/etc/ssh/sshd_config", or uncomment the line and set the value to "delayed" or preferably "no":
Compression no
Alternatively, add the setting to an include file if the line "Include /etc/ssh/sshd_config.d/*.conf" is found at the top of the "/etc/ssh/sshd_config" file:
$ echo 'Compression no' > /etc/ssh/sshd_config.d/40-compression.conf
Restart the SSH daemon for the settings to take effect:
$ systemctl restart sshd.service