Title

The SSH daemon must not permit Kerberos authentication unless needed. (Cat III impact)

Discussion

Kerberos authentication for SSH is often implemented using GSSAPI. If Kerberos is enabled through SSH, the SSH daemon provides a means of access to the system's Kerberos implementation. Vulnerabilities in the system's Kerberos implementation may then be subject to exploitation. To reduce the attack surface of the system, the Kerberos authentication mechanism within SSH must be disabled for systems not using this capability.

Check Content

Check the SSH daemon configuration. Note that keywords are case-insensitive and arguments (args) are case-sensitive. keyword=KerberosAuthentication arg(s)=no Default values include: "no" 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. If the system does not use Kerberos authentication, this is not applicable. Examine the file. # cat /opt/ssh/etc/sshd_config | tr '\011' ' ' | tr -s ' ' | sed -e 's/^[ \t]*//' | grep -v '^#' | grep -i "KerberosAuthentication" If the return value is yes, this is a finding.

Fix Text

Edit the SSH daemon configuration and set (add if necessary) a KerberosAuthentication directive set to no.

Additional Identifiers

Rule ID: SV-35075r1_rule

Vulnerability ID: V-22475

Group Title: GEN005526

Expert Comments

CCIs tied to check.

Controls tied to check. These are derived from the CCIs shown above.