  Global navigation satellite systems (GNSS) provide pervasive accurate
positioning and timing services for a large gamut of applications, from Time
based One-Time Passwords (TOPT), to power grid and cellular systems. However,
there can be security concerns for the applications due to the vulnerability of
GNSS. It is important to observe that GNSS receivers are components of
platforms, in principle having rich connectivity to different network
infrastructures. Of particular interest is the access to a variety of timing
sources, as those can be used to validate GNSS-provided location and time.
Therefore, we consider off-the-shelf platforms and how to detect if the GNSS
receiver is attacked or not, by cross-checking the GNSS time and time from
other available sources. First, we survey different technologies to analyze
their availability, accuracy, and trustworthiness for time synchronization.
Then, we propose a validation approach for absolute and relative time.
Moreover, we design a framework and experimental setup for the evaluation of
the results. Attacks can be detected based on WiFi supplied time when the
adversary shifts the GNSS provided time, more than 23.942us; with Network Time
Protocol (NTP) supplied time when the adversary-induced shift is more than
2.046ms. Consequently, the proposal significantly limits the capability of an
adversary to manipulate the victim GNSS receiver.
