fwiw, you probably don't need to worry on input devices where you can feasibly flash the firmware. homebrew keyboards and keyboards that support this, you're probably in the clear in terms of security concerns (... I think)
@cwebberChristine Lemmer-Webber https://zmk.dev/docs/features/bluetooth
ZMK documents security concerns around Bluetooth connections.
TL;DR: there are no known vulnerabilities after a bond has been made - a bond should be made in a controlled environment to avoid MITM
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