  This paper presents a novel attack on voice-controlled digital assistants
using nonsensical word sequences. We present the results of experimental work
which demonstrates that it is possible for malicious actors to gain covert
access to a voice-controlled system by hiding commands in apparently
nonsensical sounds of which the meaning is opaque to humans. Several instances
of nonsensical word sequences were identified which triggered a target command
in a voice-controlled digital assistant, but which were incomprehensible to
humans, as shown in tests with human experimental subjects. Our work confirms
the potential for hiding malicious voice commands to voice-controlled digital
assistants or other speech-controlled devices in speech sounds which are
perceived by humans as nonsensical.
