  We propose a novel attack that compromises the physical layer security of
downlink (DL) communications in wireless systems. This technique is based on
the transmission of a slowly-varying random symbol by the eavesdropper during
its uplink transmission, so that the equivalent fading channel observed at the
base station (BS) has a larger variance. Then, the BS designs the secure DL
transmission under the assumption that the eavesdropper's channel experiences a
larger fading severity than in reality. We show that this approach can lead the
BS to transmit to Bob at a rate larger than the secrecy capacity, thus
compromising the system secure operation. Our analytical results, corroborated
by simulations, show that the use of multiple antennas at the BS may partially
alleviate but not immunize against these type of attacks.
