  We present a method to control the detection events in quantum key
distribution systems that use gated single-photon detectors. We employ bright
pulses as faked states, timed to arrive at the avalanche photodiodes outside
the activation time. The attack can remain unnoticed, since the faked states do
not increase the error rate per se. This allows for an intercept-resend attack,
where an eavesdropper transfers her detection events to the legitimate receiver
without causing any errors. As a side effect, afterpulses, originating from
accumulated charge carriers in the detectors, increase the error rate. We have
experimentally tested detectors of the system id3110 (Clavis2) from ID
Quantique. We identify the parameter regime in which the attack is feasible
despite the side effect. Furthermore, we outline how simple modifications in
the implementation can make the device immune to this attack.
