  In a practical CVQKD system, the optical attenuator can adjust the
Gaussian-modulated coherent states and the local oscillator signal to an
optimal value for guaranteeing the security of the system and optimizing the
performance of the system. However, the performance of the optical attenuator
may deteriorate due to the intentional and unintentional damage of the device.
In this paper, we investigate the practical security of a CVQKD system with
reduced optical attenuation. We find that the secret key rate of the system may
be overestimated based on the investigation of parameter estimation under the
effects of reduced optical attenuation. This opens a security loophole for Eve
to successfully perform an intercept-resend attack in a practical CVQKD system.
To close this loophole, we add an optical fuse at Alice's output port and
design a scheme to monitor the level of optical attenuation in real time, which
can make the secret key rate of the system evaluated precisely. The analysis
shows that these countermeasures can effectively resist this potential attack.
