https://www.newscientist.com/article/2146658-siri-and-alexa-can-be-turned-against-you-by-ultrasound-whispers/

Assistants falling for the ploy included Amazon Alexa, Apple’s Siri, Google Now, Samsung S Voice, Microsoft Cortana and Huawei HiVoice, as well as some voice control systems used in cars.

The hack was created by Guoming Zhang, Chen Yan and their team at Zhejiang University in China. Using ultrasound, an inaudible command can be used to wake the assistant, giving the attacker control of the speaker, smartphone or other device, as well as access to any connected systems.