Yes, the base NTP protocol is unauthenticated UDP. So, that's pretty insecure.
Properly configured, with sufficient upstream time servers, etc... it's still pretty robust against DoS attacks and evil maid attacks, so you'll have to do some work to trick clients into following your fake NTP server. And it will be hard to hide what you're doing while you do it.
It took a while, but I think we've actually solved that security problem with NTS. Now we just have to get the vendors and the community to support and deploy NTS widely.
Properly configured, with sufficient upstream time servers, etc... it's still pretty robust against DoS attacks and evil maid attacks, so you'll have to do some work to trick clients into following your fake NTP server. And it will be hard to hide what you're doing while you do it.
It took a while, but I think we've actually solved that security problem with NTS. Now we just have to get the vendors and the community to support and deploy NTS widely.