Researchers at security firm Checkmarx say they built a proof-of-concept skill for Amazon’s Echo devices that in theory could have voice assistant Alexa listen to, transcribe, and report what users said after they thought they had finished using a legitimate service.

They took advantage of a feature that allows a skill to extend the time it listens to users after it’s been activated if it prompts them for more information by playing an inaudible prompt. That way, their skill, which offered a simple calculator, could keep getting transcripts from Alexa of what users said without them getting any audio cue that the device was still listening. A light would likely have been visible on affected devices, Threatpost reports, but users wouldn’t notice it unless they looked at the device.

Amazon also allows developers to build custom 'skills,' applications for Alexa, which is the brain behind millions of voice-activated smart devices including Amazon Echo Show, Echo Dot, and Amazon Tap.
However, security researchers at cybersecurity firm Checkmarx created a proof-of-concept voice-driven 'skill' for Alexa that forces device to indefinitely record surround voice to secretly eavesdrop on users’ conversations and then also sends the complete transcripts to a third-party website.

Disguised as a simple calculator for solving maths problems, the malicious skill, if installed, immediately gets activated in the background after a user says "Alexa, open calculator."
"The calculator skill is initialized, and the API\Lambda-function that's associated with the skill receives a launch request as an input," researchers said in its report.
In a video demonstration, researchers show that when a user opens up a session with the calculator app (in the background), it also creates a second session without verbally indicating the user that the microphone is still active.

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