Attacking the Network Time Protocol

We explore the risk that network attackers can exploit unauthenticated Network Time Protocol (NTP) traffic to alter the time on client systems. We first discuss how an onpath attacker, that hijacks traffic to an NTP server, can quickly shift time on the server's clients. Then, we present an extremely low-rate (single packet) denial-of-service attack that an off-path attacker, located anywhere on the network, can use to disable NTP clock synchronization on a client. Next, we show how an off-path attacker can exploit IPv4 packet fragmentation to dramatically shift time on a client. We discuss the implications of these attacks on other core Internet protocols, quantify their attack surface using Internet measurements, and suggest a few simple countermeasures that can improve the security of NTP. Permission to freely reproduce all or part of this paper for noncommercial purposes is granted provided that copies bear this notice and the full citation on the first page. Reproduction for commercial purposes is strictly prohibited without the prior written consent of the Internet Society, the first-named author (for reproduction of an entire paper only), and the author's employer if the paper was prepared within the scope of employment.