  Avalanche photodiode based single photon detectors, as crucial and practical
components, are widely used in quantum key distribution (QKD) systems. For
effective detection, most of these SPDs are operated in the gated mode, in
which the gate is added to obtain high avalanche gain, and is removed to quench
the avalanche. The avalanche transition region (ATR) is a certain existence in
the process of adding and removing the gate. We first experimentally
investigate the characteristic of the ATR, including in the commercial SPD and
high-speed SPD, and then propose an ATR attack to control the detector. In the
experiment of hacking the plug-and-play QKD system, Eve only introduces less
than 0.5 % quantum bit error rate, and almost leaves no traces of her presence
including the photocurrent and afterpulse probability. We finally give possible
countermeasures against this attack.
