More and more embedded devices store sensitive information that is protected by means of cryptography. The confidentiality of this data is threatened by information leakage via side channels like the power consumption or the electromagnetic radiation. In this paper, we show that the side-channel leakage in the power consumption is not limited to the power-supply lines and that any input/output (I/O) pin can comprise secret information. The amount of leakage depends on the design and on the state of the I/O pin. All devices that we examined leaked secret information through their I/O pins. This implies that any I/O pin that is accessible for an adversary could be a security hole. Moreover, we demonstrate that the leakage is neither prevented by transmitter/receiver circuits as they are used in serial interfaces, nor by a galvanic isolation of a chip and its output signals via optocouplers. An adversary that is able to manipulate, for example, the pins of a PC's I/O port, can attack any device that is connected to this port without being detected from outside.