CVE-2018-11567
Prior to 2018-04-27, the reprompt feature in Amazon Echo devices could be misused by a custom Alexa skill. The reprompt feature is designed so that if Alexa does not receive an input within 8 seconds, the device can speak a reprompt, then wait an additional 8 seconds for input; if the user still does not respond, the microphone is then turned off. The vulnerability involves empty output-speech reprompts, custom wildcard ("gibberish") input slots, and logging of detected speech. If a maliciously designed skill is installed, an attacker could obtain transcripts of speech not intended for Alexa to process, but simply spoken within the device’s hearing range.
Source: CVE-2018-11567