  Establishing an information-theoretic secret key between two parties using a
quantum key distribution (QKD) system is only possible when an accurate
characterization of the quantum channel and proper device calibration routines
are combined. Indeed, security loopholes due to inappropriate calibration
routines have been shown for discrete-variable QKD. Here, we propose and
provide experimental evidence of an attack targeting the local oscillator
calibration routine of a continuous-variable QKD system. The attack consists in
manipulating the classical local oscillator pulses during the QKD run in order
to modify the clock pulses used at the detection stage. This allows the
eavesdropper to bias the shot noise estimation usually performed using a
calibrated relationship. This loophole can be used to perform successfully an
intercept-resend attack. We characterize the loophole and suggest possible
countermeasures.
