How to secure microphones on smart home devices?

Question

I have a number of smart home devices in my house. Some of which are a Samsung refrigerator, a Whirlpool washer, a Bose Bluetooth speaker with wifi and a Roku TV all of which I assume have microphones.

What would be the best way at securing these devices from being hacked and listened in on short of completely dismantling said devices and disconnecting the microphone?

In Android, there are settings to change the permissions of the microphone. However, I am assuming an attacker who has compromised a device would be able to change these permissions? Do smart home devices normally have more then one microphone?

IN RESPONSE TO ANSWERS: Well if these devices were comprimised, then it would be safe to say that the router is most likely compramised as well wouldnt it, and any firewall that is used would not be effective? If smart home devices have connection to the internet, would you be able to SSH into the device and change default passwords, assuming they were not compramised. Would you also be able to disable the microphone in a way that it would be very difficult to re-enable? For this entire question we will just assume nothing is comprimised and other then setting a firewall to stop the transfer of data are there other methods that can be taken to devices individually?

Answer 1

As schroeder mentionned in a comment, you cannot realistically achieve what you want.

However, if what you want is to prevent those device to spy (listen and report) on you, then you can often prevent them to exfiltrate data if they become compromised or misused. So the devices will still be able listen, but not to report anything they hear.

To do so, you can configure a firewall on the WiFi router of your LAN. Your existing router might not provide this functionality, so you might need to buy one. For this, your router will most likely need to assign static IP adresses to your devices (based on their MAC adresses), then prevent them to communicate with internet while still allowing them to be accessible from inside the LAN. If needed, you can add an exception to allow them to communicate with specific servers, for example to allow them to fetch firmware updates. How to do so exaclty depends on your router, and would be a question better suited for superuser.com.

---

When the devices communicate with bluetooth, they cannot use this protocol to directly access internet. They can however connect to another nearby blutooth device (like your mobile phone), which in turn could exfiltrate data to internet. To prevent an app on your phone to do so, you can either block its access to internet in its settings, or you can use a configurable ad-blocker to prevent it to connect to the domain names of your choosing.

Bluetooth being a security nightmare, in theory your Bluetooth devices could be hacked from a nearby attacker and used to exfiltrate data directly via Bluetooth to a nearby attacker. If this is a threat for you (most likely it is not), I can only recommend to disable Bluetooth on your devices.