  Fiber optic networks are used worldwide and have been regarded as excellent
media for transmitting time-frequency (TF) signals. In the past decades,
fiber-based TF synchronization techniques have been extensively studied.
Instruments based on these techniques have been successfully applied. With the
increasing application of TF synchronization instruments, their security has
become an important issue. Unfortunately, the security risks of fiber-based
frequency synchronization (FbFS) instruments have been overlooked. This paper
proposes a frequency tampering method called "frequency lens". On a 200 km
fiber link, we demonstrate a frequency tampering scenario using a frequency
lens-enabled frequency tampering module (FTM). On the user side, the frequency
value of the recovered 100 MHz signal can be stealthily altered within a range
of 100 MHz-100 Hz to 100 MHz+100 Hz, while the frequency dissemination
stability of the system remains normal. Related to this tampering risk,
potential hazards in three different application scenarios, which rely on
precise frequency references, are analyzed. Two countermeasures are also
proposed to solve this tampering risk.
