Covert Channels in Network Time Security

Network Time Security (NTS) specified in RFC8915 is a mechanism to provide cryptographic security for clock synchronization using the Network Time Protocol (NTP) as foundation. By using Transport Layer Security (TLS) and Authenticated Encryption with Associated Data (AEAD) NTS is able to ensure integrity and authenticity between server and clients synchronizing time. However, in the past it was shown that time synchronisation protocols such as the Network Time Protocol (NTP) and the Precision Time Protocol (PTP) might be leveraged as carrier for covert channels, potentially infiltrating or exfiltrating information or to be used as Command-and-Control channels in case of malware infections. By systematically analyzing the NTS specification, we identified 12 potential covert channels, which we describe and discuss in this paper. From the 12 channels, we exemplary selected an client-side approach for a proof-of-concept implementation using NTS random UIDs. Further, we analyze and investigate potential countermeasures and propose a design for an active warden capable of mitigating the covert channels described in this paper.