  In ultralong fiber laser key distribution, two sides use standard optical
equipment to create kilometer long fiber lasers in a communication link to
establish a secret key. Its security rests on the assumption that any attacker
would need much more sophisticated equipment and techniques than those of the
legitimate user in order to discover the generated key. We present a challenge
to that assumption with a hidden probe attack in which the eavesdropper hides a
weak signal in the unavoidable noise floor that appears in the laser during
amplification and probes with it the configuration of one or both communication
parties. We comment how this attack can compromise different proposals for
ultralong laser key distribution and propose possible countermeasures.
