  We introduce an attack scheme for eavesdropping the ping-pong quantum
communication protocol proposed by Bostr$\ddot{o}$m and Felbinger [Phys. Rev.
Lett. \textbf{89}, 187902 (2002)] freely in a noise channel. The vicious
eavesdropper, Eve, intercepts and measures the travel photon transmitted
between the sender and the receiver. Then she replaces the quantum signal with
a multi-photon signal in a same state, and measures the photons return with the
measuring basis with which Eve prepares the fake signal except for one photon.
This attack increase neither the quantum channel losses nor the error rate in
the sampling instances for eavesdropping check. It works for eavesdropping the
secret message transmitted with the ping-pong protocol. Finally, we propose a
way for improving the security of the ping-pong protocol.
