  We consider the security of practical continuous-variable quantum key
distribution implementation with the local oscillator (LO) fluctuating in time,
which opens a loophole for Eve to intercept the secret key. We show that Eve
can simulate this fluctuation to hide her Gaussian collective attack by
reducing the intensity of the LO. Numerical simulations demonstrate that, if
Bob does not monitor the LO intensity and does not scale his measurements with
the instantaneous intensity values of LO, the secret key rate will be
compromised severely.
