Using a technique called the DolphinAttack, a team from Zhejiang University translated typical vocal commands into ultrasonic frequencies that are too high for the human ear to hear, but perfectly decipherable by the microphones and software powering our always-on voice assistants. This relatively simple translation process lets them take control of gadgets with just a few words uttered in frequencies none of us can hear.

The researchers didn’t just activate basic commands like “Hey Siri” or “Okay Google,” though. They could also tell an iPhone to “call 1234567890” or tell an iPad to FaceTime the number. They could force a Macbook or a Nexus 7 to open a malicious website. They could order an Amazon Echo to “open the backdoor” (a pin would also be required, an August spokesperson clarifies). Even an Audi Q3 could have its navigation system redirected to a new location. “Inaudible voice commands question the common design assumption that adversaries may at most try to manipulate a [voice assistant] vocally and can be detected by an alert user,” the research team writes in a paper just accepted to the ACM Conference on Computer and Communications Security.

In other words, Silicon Valley has designed human-friendly UI with a huge security oversight. While we might not hear the bad guys talking, our computers clearly can. “From a UX point of view, it feels like a betrayal,” says Ame Elliott, design director at the nonprofit SimplySecure. “The premise of how you interact with the device is ‘tell it what to do,’ so the silent, surreptitious command is shocking.”

To hack each voice assistant, the researchers used a smartphone with about $3 of additional hardware, including a tiny speaker and amp. In theory, their methods, which are now public, are duplicatable by anyone with a bit of technical know-how and just a few bucks in their pocket.

In some cases, these attacks could only be made from inches away, though gadgets like the Apple Watch were vulnerable from within several feet. In that sense, it’s hard to imagine an Amazon Echo being hacked with DolphinAttack. An intruder who wanted to “open the backdoor” would already need to be inside your home, close to your Echo. But hacking an iPhone seems like no problem at all. A hacker would nearly need to walk by you in a crowd. They’d have their phone out, playing a command in frequencies you wouldn’t hear, and you’d have your own phone dangling in your hand. So maybe you wouldn’t see as Safari or Chrome loaded a site, the site ran code to install malware, and the contents and communications of your phone were open season for them to explore.

The exploit is enabled by a combination of hardware and software problems, the researchers explain in their paper. The microphones and software that power voice assistants like Siri, Alexa, and Google Home can pick up inaudible frequencies–specifically, frequencies above the 20KhZ limits of human ears. (How high is 20kHz? It’s just above the mosquito ringtone that went viral a few years ago, which allowed young students who hadn’t damaged their hearing yet to text message friends without their teachers hearing.)