Detection
On a UNIX-platform, the command "ntpdc" will query existing NTP servers for monitoring data. If the system is vulnerable to exploitation, it will respond to the "monlist" command in interactive mode. By default, most modern UNIX and Linux distributions allow this command to be used from localhost, but not from a remote host. To test for monlist support, execute the following command at the command line:
/usr/sbin/ntpdc <remote server>
monlist
Additionally, the "ntp-monlist" script is available for NMap, which will automatically display the results of the monlist command. If the system does not support the monitor query, and is therefore not vulnerable to this attack type, NMap will return an error type 4 (No Data Available) or no reply at all.
Recommended Course of Action
As all versions of ntpd prior to 4.2.7 are vulnerable by default, the simplest recommended course of action is to upgrade all versions of ntpd that are publically accessible to at least 4.2.7. However, in cases where it is not possible to upgrade the version of the service, it is possible to disable the monitor functionality in earlier versions of the software.
To disable "monlist" functionality on a public-facing NTP server that cannot be updated to 4.2.7, add the "noquery" directive to the "restrict default" line in the system's ntp.conf, as shown below:
restrict default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery
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