  Imperfect devices in commercial quantum key distribution systems open
security loopholes that an eavesdropper may exploit. An example of one such
imperfection is the wavelength dependent coupling ratio of the fiber beam
splitter. Utilizing this loophole, the eavesdropper can vary the transmittances
of the fiber beam splitter at the receiver's side by inserting lights with
wavelengths different from what is normally used. Here, we propose a wavelength
attack on a practical continuous-variable quantum key distribution system using
homodyne detection. By inserting light pulses at different wavelengths, this
attack allows the eavesdropper to bias the shot noise estimation even if it is
done in real time. Based on experimental data, we discuss the feasibility of
this attack and suggest a prevention scheme by improving the previously
proposed countermeasures.
