Researchers have demonstrated how garbled speech commands hidden in radio or video broadcasts could be used to control a smartphone.
The clips, which sound like the Daleks from Doctor Who, can be difficult for humans to understand but still trigger a phone's voice control functionality.
The commands could make a smartphone share its location data, make calls and access compromised websites.
The researchers - from the University of California, Berkeley and Georgetown University - explored whether audio commands "unintelligible to human listeners" were still interpreted by smartphones as voice commands.
They took a series of voice commands, such as: "OK Google, call 911," which would activate an Android phone's voice control if enabled, and heavily distorted the audio so that it was difficult for human listeners to understand.
The low-pitched speech could be hidden among background noise and still trigger smartphone features.