  We introduce the so far most efficient attack against the
Kirchhoff-law-Johnson-noise (KLJN) secure key exchange system. This attack
utilizes the lack of exact thermal equilibrium in practical applications and is
based on cable resistance losses and the fact that the Second Law of
Thermodynamics cannot provide full security when such losses are present. The
new attack does not challenge the unconditional security of the KLJN scheme,
but it puts more stringent demands on the security/privacy enhancing protocol
than for any earlier attack. In this paper we present a simple defense protocol
to fully eliminate this new attack by increasing the noise-temperature at the
side of the smaller resistance value over the noise-temperature at the at the
side with the greater resistance. It is shown that this simple protocol totally
removes Eve's information not only for the new attack but also for the old
Bergou-Scheuer-Yariv attack. The presently most efficient attacks against the
KLJN scheme are thereby completely nullified.
