Stability of passive LTI system
A counter-example would be an ideal LC-circuit, which is passive but not BIBO stable: if excited at its resonance frequency, the output grows without bounds. Such systems are sometimes called marginally stable, because they have poles on the stability boundary (the imaginary axis).
Note that if a system is not lossless, i.e., if there is a positive resistance such as in an RLC-circuit, the system is strictly passive, and as a consequence it is also BIBO-stable. I.e., strict passivity implies BIBO-stability (all transfer function poles are in the left half-plane).