I'm interested what others think of their existing design and whether there are any fundamental security issues that will be resolved by their proposed change.

They are proposing requiring a secret signed certificate to carry out any actions beyond monitoring for both the cloud and local (on printer) MQTT servers. These certificates would be issued at the discretion of Bambu by their CSR, currently only for "Bambu Studio" their slicer, Bambu Handy (their mobile app) and "Bambu Connect" which will enable upload G-Code generated by third party slicer (a workaround for existing functionality being removed). This "secret" certificate has already been extracted from the Bambu Connect application as per the article as their new security model requires embedded this certificate into desktop applications.

The current design:

https://github.com/Doridian/OpenBambuAPI/blob/main/mqtt.md

Connecting to their cloud MQTT requires a username and token already. These details are obtained via a HTTPS request to their login server using your bambu account (which requires a valid email & possibly captcha) to obtain a token. The cloud MQTT is TLS secured, although this is just to encrypt the traffic (aka HTTPS), it is not mutual authentication.

Connecting to the MQTT server hosted on the printer (aka LAN mode) requires a fixed username and a local access token (a random 8 digit number). This can be found via the physical display of the printer in a menu (or apparently cloud MQTT!?). This access token can be refreshed via a menu option again physically at the printer. To be clear, this token only allows to you connect directly to the local MQTT server running on the IP address of the printer, so in most environments this should only be the local network. This is also the password for the FTP server that can be used to upload/download sliced 3mf/gcode files.

Personally - this design seems ok to me? With an MQTT service properly configured to isolate user accounts from each other, this is a pattern widely deployed for embedded devices (Azure IoT, AWS IoT etc).

I don't see how the "DDOS" related issues they are claiming would be related to this specific design. If the issue is in the login server - well, that's prior to authentication anyway so nothing they are doing here will fix that.

If it's problems with your cloud MQTT service not being properly isolated - maybe fix that? If the DDOS is at L2, auth isn't going to help.
You require logins tied to an email, you can block clients that misbehave once they are logged in.

Nobody is brute forcing the local MQTT server via XSS or something, because JS doesn't allow for raw TCP connections. Are they concerned about malicious software already on the network? Then rate limiting on the printer side or switch to a random length alphanum LAN token to increase keyspace.

I'm curious what more qualified people think, I cannot see any justications for their proposed design improving security. So either;

a) They've decided they are incapable of properly securing their MQTT cloud stuff and instead of fixing that just want to assume every client connected to their cloud MQTT servers is fully trusted. I'm sure that'll work great. Doesn't justify adding this to the local MQTT servers on the printers - if anything that reduces security, as to roll certificates you now have a long tail of printer firmware updates.

b) It's not about security