This repository will serve as a knowledge base for everything I've learned about the T-Mobile 5G Gateway so others can start off not completely in the dark like I had to.

I've seen alot of posts on the FastMile variant manufactured by Nokia (a.k.a the Trashcan) but not much on the KVD21 from Arcadyan. I'm entirely still a novice at hardware hacking and will most likely end up bricking this thing but I work in the back of a T-Mobile doing repairs and get abandoned or devices for recycle all the time and want to try my hand at hardware hacking the exotic and uncommon devices I come across in my field of work.

Basic information and a device usage guide can be found here:

https://www.t-mobile.com/support/devices/get-to-know-your-arcadyan-kvd21-gateway

I have decided to put this project on the shelf until a Mediatek bootloader dump script supports the T75 SoC.

I tried fuzzing the routers config login page for vulnerabilities and actually got the web server to permanently crash but lost the hard drive to mechanical failure and therefore lost the wordlist i used (which was a few popular command injection wordlists for unix/linux/websites on github combined) and to make matters worse I incorrectly set the options for the command I was using (wfuzz i think) so I did not get to see what string caused the crash. I know the login page is vulnerable but do not know the string to get it to crash and the web server only displays 404s now so I cant continue that avenue. I tried getting a prompt that wasn't locked down and found some scripts for unlocking Mediatek bootloaders.

The best luck ive had with these scripts is getting the device to switch to EDL but none of the Mediatek software recognizes it and the scripts do not support rom dumps for this SoC yet.

I'm not going to be able to get into this thing until one of the Mediatek bootloader dump scripts adds support for this SoC so in the hopes someone else can do what I could not I have updated the repo with pictures of the full tear down and the logs to the bootloader (normal start, and started with mediatek preloader dump script) obtained through the UART port.

The tear down of the device was quite easy after I figured out the right ammount of pressure to apply to the area holding the plastic clips together without breaking the clips.

With the cover off we can see an array of antennas and a screen with some buttons surrounding two boards that are sandwiched together.

We can also see there is what looks like a UART port on the edge of one of the central PCBs.

Using a multimeter to monitor voltages while the device is booting I see there is 0.4v on the pin by the arrow symbol, followed by 0v, 1.8v, and 0v. The pin farthest from the arrow symbol was verrified to be a ground pin by checking for continuity between the pin and a ground plane. The two pins in the middle are fluctuating 1.8v on the left and 0v stable on the right which indicates TX/RX respectively.

I soldered headers on to the 4 pin wells so I can get a reliable connection with an FTDI I did this while drunk so it looks horrible. 😅

This is the pinout for the UART connection I used with my Adafruit FT232H:

When we open up a tty serial on the FT232H and power the device on we get what appears to be a bootloader log that stops outputting shortly after the linux kernel starts. We cannot transmit over this tty but some valuable information was obtained from the bootloader log.

We also have some interesting bits of data at the start of the log that don't seem to be garbage from an improper baud rate or something similar.

When the data was separated from the text and opened in a hex editor like xxd we see that there are bits of 0x80 and 0x00 following a brief preamble of 0x10000000 and 0xFF008800.

Moving on to the actual bootloader log we can see a return string entry from the function dump_cmdline() that provides us with several key pieces of info:

void dump_cmdline(void *):75: cmdline len=395, str="
  init=/etc/preinit
  root=/dev/mmcblk0p28
  rootwait
  loglevel=8
  androidboot.selinux=permissive
  androidboot.hardware=mt6890
  initcall_debug=1
  page_owner=on
  ramoops.mem_address=0x42880000
  ramoops.mem_size=0xe0000
  ramoops.pmsg_size=0x10000
  ramoops.console_size=0x40000
  mtk_printk_ctrl.disable_uart=0
  bootslot=a
  androidboot.hardware=mt6890
  bootprof.pl_t=1384
  bootprof.logo_t=0
  bootprof.lk_t=7882
"

• The gateway is running Android and we can use ADB once we know how to get the gateway into fastboot.
• The modem processor is a Mediatek MT6890/MT6880 T75 based modem SoC package branded as the Fibrocom FG360-NA.
• SELinux is set to permissive. 🤣 This means once we have shell access on our gateway, priveledge escalation should be trivial despite Android being the target OS.

Full pictures of the teardown and the logs for the bootloader obtained through UART are available in the repo.