NTP - Network Time Protocol - can be abused for attacks on HTTPS, DNSSEC, and Bitcoin.
Researchers at University of Boston describe how unencrypted NTP traffic can be intercepted and then used to change the time of clients. For example, the clock can be turned back to a point where the host would accept a fraudulent digital certificate that has been revoked.
Or by advancing the time on a DNS resolver the DNSSEC validation can be made to fail.
The researches also give advice on how to protect yourself against these various attacks.
Attacking the Network Time Protocol
Abstract—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 a 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 on 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.
▻http://www.cs.bu.edu/~goldbe/papers/NTPattack.pdf
backup: ▻http://docdro.id/Cf0QqBD