  A well-protected and characterised source in a quantum key distribution
system is needed for its security. Unfortunately, the source is vulnerable to
light-injection attacks, such as Trojan-horse, laser-seeding, and laser-damage
attacks, in which an eavesdropper actively injects bright light to hack the
source unit. The hacking laser could be a high-power one that can modify
properties of components via the laser-damage attack and also further help the
Trojan-horse and other light-injection attacks. Here we propose a
countermeasure against the light-injection attacks, consisting of an additional
sacrificial component placed at the exit of the source. This component should
either withstand high-power incoming light while attenuating it to a safe level
that cannot modify the rest of the source, or get destroyed into a permanent
high-attenuation state that breaks up the line. We demonstrate experimentally
that off-the-shelf fiber-optic isolators and circulators have these desired
properties, at least under attack by a continuous-wave high-power laser.
