This tldr from Guilherme Rambo is enough, but read the full post for all the details.
TL;DR: Any app with access to Bluetooth could record your conversations with Siri and audio from the iOS keyboard dictation feature when using AirPods or Beats headsets. This would happen without the app requesting microphone access permission and without the app leaving any trace that it was listening to the microphone.
This bug has since been handled by Apple. Also, after reaching back out to Apple (on Oct 25), Guilherme was told he’d be receiving a $7,000 (USD) bug bounty payment for reporting the issue.