Title

The 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

Check the SSH daemon configuration. Note that keywords are case-insensitive and arguments (args) are case-sensitive. keyword=Compression arg(s)="no" OR "delayed", IE: mutually exclusive arguments, should never occur together Default values include: "delayed" Note: When the default "arg" value exactly matches the required "arg" value (see above), the <keyword=arg> entry is not required to exist (commented or uncommented) in the ssh (client) or sshd (server) configuration file. While not required, it is recommended that the configuration file(s) be populated with all keywords and assigned arg values as a means to explicitly document the ssh(d) binary's expected behavior. Examine the file. # cat /opt/ssh/etc/sshd_config | tr '\011' ' ' | tr -s ' ' | sed -e 's/^[ \t]*//' | grep -v '^#' | grep -i "Compression" If configuration information is not returned or the return value is yes, this is a finding.

Fix Text

Edit the SSH daemon configuration and add or edit the Compression setting value to no or delayed.

Additional Identifiers

Rule ID: SV-35146r1_rule

Vulnerability ID: V-22488

Group Title: GEN005539

Expert Comments

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