There's been 3 recent vulnerabilities in the NTP reference implementation. Namely:
Is ntpd, or ntpdate, running as a client vulnerable to any of these exploit if the server it's connected to can't be trusted? Assume ntpd is configured with the default configuration on most Linux server and thus doesn't use any sort of "cryptographic protection" and can't itself be queried by untrusted peers.
This means connecting to volunteer ntp servers provided by ntp.org, which can't be trusted.