Brief reverse engineering work on FIMI A3

( Original text by Konrad Iturbe )

This is the start of a new series on reverse engineering consumer products, mainly to enhance their use but also to expose data leaks and vulnerabilities.

Something caught my eye last week. Xiaomi-backed FIMI, a Shenzhen company, released a drone. I tend to avoid most cheap drones since they tend to suck (bad quality camera, bad UX…) but this one is different.

This drone has a “DIY” port. This is an UART/PWM/GPIO port with what I assume are two ports for power. FIMI showcases how the user can attach a fireworks igniter or LEDs, but my mind went instantly to this DEF CON presentation from this year. The project is about a drone which is difficult to intercept but the author of the presentation also shows some offensive uses of drones. This drone can theoretically have a WiFi jammer or a promiscuous WiFi packet sniffer which can be activated from the ground. Possibilities are endless when you have this sort of port. The A3 also does not need a smartphone for operation, it includes a remote controller with an LCD panel. Having a smartphone controlling the drone opens a new vector for attackers (wifi network between the remote controller and cellphone can be brute forced, phone can have malware…). DJI does not yet have a all-in-one remote controller but Xiaomi has outsmarted them. Chinese innovation at its finest.

DIY port. Time to attach a RPI Zero with a pinneapple.
Some payloads that can be attached to drones. Source: David Melendez’s DEF CON talk PDF

On the camera side for those interested this drone is rocking the AMBA A12 chipset with a 1440p Sony CMOS sensor. Takes 8MP stills and can record 1080p video at 30FPS with a bitrate of 60 MB/s. The gimbal is 2 axis, just like the DJI Spark but at ~$250 this drone is a worthy competitor of the DJI Spark. If it only shot 4K video and had 3 axis gimbal, but we can’t have everything in life.


Part 1: The firmware(s):

FIMI makes the firmware available for downloading to anyone here.

The firmware is split into 3: The AMBA A12 firmware, the Drone Cortex A7 firmware and the remote controller firmware.

wget https://www.fimi.com/media/Productattachments//f/2/f21a-a-v010sp12rtm181027r16987-cn-rtm_u-release-741c119eb4d25878e21045e3f3c485d4.zip -P drone_fw/

wget https://www.fimi.com/media/Productattachments//f/i/firmware.zip -P cam_fw/

wget https://www.fimi.com/media/Productattachments//r/2/r21a-a-v010sp13rc181024r16900-cn-b_250k-release-ota-97b6c6c59241976086fabdc41472150c.zip -P remotecontrol_fw/

The firmwares are highly compressed. The filesize of each one is:

3.8M firmware.zip

488K f21a-a-v010sp12rtm181027r16987-cn-rtm_u-release-741c119eb4d25878e21045e3f3c485d4.zip

728K r21a-a-v010sp13rc181024r16900-cn-b_250k-release-ota-97b6c6c59241976086fabdc41472150c.zip

First step is to decompress each firmware zip file. After the decompression is done the remote firmware yields a 1.2M BFU file, the drone firmware is now a 492K BIN file and the camera firmware, which contains code relevant to the AMBA ISP yields 3 files: a 3.8M firmware.bin and two 0-byte files: rollback.txt and update.txt. Looking deeper at the firmware.bin file using binwalk 3 files are compressed: amba_ssp_svc.bin, dsp.bin.gz, rom.bin.gz

amba_ssp_svc.bin is a gz file, so the name should be amba_ssp_svc.bin.gz

Using extract_fw.sh we can get the gz files and their contents:

firmware.bin SHA512: 1cba74305d0491b957f1805c84e9b1cf5674002fc4f0de26905a16fb40303376187f1c35085b7455bff5c4de23cf8faa9479e4f78fd50dbf69947deb27f5d687

From here I used dd to extract the files. The “skip” flag is the location and the count is the next location — location.

dd if=[firmware.bin] of=out/amba_ssp_svc.bin.gz bs=1 skip=508 count=1812019
dd if=[firmware.bin] of=out/dsp.bin.gz bs=1 skip=1812527 count=1988127
dd if=[firmware.bin] of=out/rom.bin.gz bs=1 skip=3800654 count=143664

NOTE: All the files needed are on my github repository.

Now there is something to work with. Extrac each file with gunzip and we end up with:

4.3M amba_ssp_svc.bin
4.9M dsp.bin
2.3M rom.bin

Part 2: Ambarella chipset:

This is as far as we get. The 4.3M file is the AMBA chipset firmware. From here we can use some of the methods I used in reverse engineering GoPro camera firmwares:

strings amba_ssp_svc.bin | grep “c:\\\\”

The aim of this reverse engineering work is to:

  • See if we can flash our own images onto the drone
  • See what kind of resolutions and frame rates are available
  • See if we can run custom commands or get a telnet/RTOS session
  • Disable NFZ (No Fly Zones) and enable FCC (US 5GHz mode) in Europe
  • Can we root the drone?

It appears we can flash images from the SD card to the drone:

C:\version.txt
C:\update.txt
C:\rollback.txt
C:\firmware.bin

As per the resolutions, see cam_fw/out/README.md in the GitHub repository.

The AMBA A12 chipset accepts some commands, including what appears to be RTOS USB Shell.

The drone firmware and controller firmware don’t have such ways of getting into. The drone firmware is a bin file with no sections and the remote controller firmware is a bfu file.

It’d be interesting if the drone can fly on offline waypoints. Attaching a WiFi sniffer just got a whole lot easier.

I ordered this drone but it won’t arrive soon since it’s a pre-order.

Stay tuned for part 2!

GitHub repository: https://github.com/KonradIT/fimi_a3

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