Security?

[luke-jr]

From reading the instructions, it sounds like there's zero interaction with the device itself to flash? What prevents a malicious actor from putting their own firmware on? Is it even possible to secure these devices?

[pvvx]

The original firmware has no protection.
For the provided firmware, protection is provided by installing a PIN code.

https://github.com/pvvx/ATC_MiThermometer#custom-firmware-for-ble-thermometers-on-the-telink-chipset -> Key features:
Supports encrypted connections using PIN-code:
#174 (comment)

If the PIN code in the thermometer is active, setup and other functions (firmware updates) will not be available without connecting to a device that has the correct PIN code.
Warning: If you forgot the PIN code, then only hardware firmware with Flash cleaning will help.

[luke-jr]

Ok, so basically just do the firmware switch in a safe place. Is it possible without using a website?

[pvvx]

It is also possible to enter a PIN code in the "nRFConnect" program when connected to a thermometer by a special command.
You can download the catalog https://github.com/pvvx/pvv.github.io/tree/master/ATC_MiThermometer to the local disk and run "TelinkMiFlasher.html" without an internet connection.
Only these files are needed: TelinkMiFlasher.html, styles.css, core.js

Plus, it is possible to enter a PIN code using a variety of Bluetooth utilities.
Specification of the format of the 0x70 Set PinCode command - on demand or see the sources code.