  We propose an efficient strategy to attack a continuous-variable quantum key
distribution (CV-QKD) system, that we call homodyne detector blinding. This
attack strategy takes advantage of a generic vulnerability of homodyne
receivers: a bright light pulse sent on the signal port can lead to a
saturation of the detector electronics. While detector saturation has already
been proposed to attack CV-QKD, the attack we study in this paper has the
additional advantage of not requiring an eavesdropper to be phase locked with
the homodyne receiver. We show that under certain conditions, an attacker can
use a simple laser, incoherent with the homodyne receiver, to generate bright
pulses and bias the excess noise to arbitrary small values, fully comprising
CV-QKD security. These results highlight the feasibility and the impact of the
detector blinding attack. We finally discuss how to design countermeasures in
order to protect against this attack.
