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V-271717

CAT II (Medium)

OL 9 SSH daemon must not allow compression or must only allow compression after successful authentication.

Rule ID

SV-271717r1091863_rule

STIG

Oracle Linux 9 Security Technical Implementation Guide

Version

V1R5

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.

Check Content

Verify that OL 9 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.

Add the following line to "/etc/ssh/sshd_config" or to a file in "/etc/ssh/sshd_config.d" or uncomment the line 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