  Quantum key distribution (QKD) based on the fundamental laws of quantum
physics can allow the distribution of secure keys between distant users.
However, the imperfections in realistic devices may lead to potential security
risks, which must be accurately characterized and considered in practical
security analysis. High-speed optical modulators, being as one of the core
components of practical QKD systems, can be used to prepare the required
quantum states. Here, we find that optical modulators based on LiNbO3,
including phase modulators and intensity modulators, are vulnerable to
photorefractive effect caused by external light injection. By changing the
power of external light, eavesdroppers can control the intensities of the
prepared states, posing a potential threat to the security of QKD. We have
experimentally demonstrated the influence of light injection on LiNbO3-based
optical modulators and analyzed the security risks caused by the potential
green light injection attack, along with the corresponding countermeasures.
