  Based on the fundamental rules of quantum mechanics, two communicating
parties can generate and share a secret random key that can be used to encrypt
and decrypt messages sent over an insecure channel. This process is known as
quantum key distribution (QKD). Contrary to classical encryption schemes, the
security of a QKD system does not depend on the computational complexity of
specific mathematical problems. However, QKD systems can be subject to
different kinds of attacks, exploiting engineering and technical imperfections
of the components forming the systems. Here, we review the security
vulnerabilities of QKD. We mainly focus on a particular effect known as
backflash light, which can be a source of eavesdropping attacks. We equally
highlight the methods for quantifying backflash emission and the different ways
to mitigate this effect.
