A PHP man-in-the-middle attack on the Fitbit Aria scale that logs and spoofs guest weights.
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An imaginatively titled privacy tool for spoofing guest weight data sent to Fitbit by the Aria wifi scales.


My use case is pretty specific. Another member of my household owns a set of Fitbit Aria scales. This is a set of bathroom scales that detects the user of a Fitbit device, and uploads their weight to their account on Fitbit's servers, although it will happily weigh anyone, regardless of whether or not they are a Fitbit user.

Like all things that use my network for internet access, I've been analysing its traffic, and it was very obvious from the outset (even without doing any traffic analysis) that the device uploads 'guest' weight readings as well as readings from people with Fitbit accounts.

I keep track of my health. In fact, I did my PhD in lifelogging back in 2008 and have an awful lot of data on my activity and wherabouts for the past ten years. Unlike many, I like to do this in private, without uploading all of my personal data to some cloud-based service. This is why I do not own a Fitbit device, it insists all your health data processing happens on their server where you have no control over it. However, now the bathroom scales have been replaced with the Aria device, I, as a non-Fitbit-user, am getting the worst of both worlds; my weight is being uploaded to Fitbit's server every time I use them, and I'm not getting an automatically generated log of my weight, like the Fitbit owner of the household is.

If I were the only person in the house, I could use Helvetic, which completely replaces the Fitbit servers with a local network alternative. However, the owner of the scales would still like their data uploaded to Fitbit's cloud, so I needed something that ensures the Aria works as designed when a Fitbit user steps on them, but intercepts all other weight events, logging them locally and not uploading them to Fitbit.

This script is tested and works with the Fitbit Aria running firmware version 39 (protocol v3). It is not tested with an Aria 2.


Many thanks are due to architekt and krisha, authors of this document for an earlier version of the firmware than the one I'm using.

Thanks are also due to micolous, author of Helvetic, whose documentation tipped me off that I should be using CRC16/XModem to calculate the checksum.


The scales are pretty good with network connection, or lack of it. Our bathroom is quite a way from our wifi access point, so maybe 10% of the time the scales will weigh you and fail to upload the data because they can't connect to the wifi. This is fine, because they will store the failed attempt, and the next time someone weighs themself, the Aria will upload all previously stored data.

So it's not just a case of we can replay a 'success' response if a guest user weighs themself, because the data may include the previous user's weight, which they do want uploaded to Fitbit. Additionally, the time stamp is returned as part of the response, and the scales set themselves to this and use the timestamp in the next request. So we have to actually interpret and modify the data being uploaded.

We could cleanly remove all guest data from uploads, and craft a fake 'OK' message from the server to send to the scales if only guest data is included in a particular upload. But this is complicated, it's far easier to just replace all guest weights with a random number, similar to how PDroid for Android works. So that's what this script does.


The script only uploads the weight of Fitbit users to Fitbit's server unmodified, but keeps all users' data. Thinking about it, this is kind of a dick move. The script should only really log the guest user's data locally; just because the Fitbit user of the house has given consent for Fitbit to process her personal data, I should not assume this consent applies to me too. For this reason, the 'interpret_data' function in htdocs/functions.php has an $ignore_registered_users argument, which is set to true by default. This ensures no data from registered Fitbit users ends up in the JSON (although it's still in the raw data). If you want to store everyone's data in the JSON, and have obtained their consent, change the $ignore_registered_users value to false.

Installation and Configuration

The first bit is the hardest, and that's to get some kind of local DNS spoofing set up. Basically, the server on which this script will be running needs to respond to HTTP requests destined for www.fitbit.com. The way I've done this is to set up a Raspberry Pi on my local network with an IP address beginning with 192.168, and then configure my router to use that IP address for DNS queries. The Raspberry Pi needs to be set up running dnsmasq as shown in the following link


But you'll need to add an entry in the hosts file on the Pi so that www.fitbit.com resolves to the host on which Aria-Spoof will be running.

You can test this is working by pinging www.fitbit.com from any place on your network, and it should resolve to a local (eg 192.168.x.x) IP address. If you want, you can set up the DNS resolver so that it only responds with the local address to the Aria, but this is a bit more complicated, and you only need to do it if you really want to view Fitbit's website. If you're as worried about the security of IoT devices as I am, you probably already have a separate wifi network for the Aria anyway!

Once this is all done, you need to go to the device whose IP address is resolved as www.fitbit.com by your DNS server, and ensure it's running Apache2 and PHP. Configuration of these is out of scope of these instructions but information is commonplace on Google.

Clone this repo into a directory accessible by the www-data user, and, within that directory, also create a directory called data to which the www-data user has write access (ideally, www-data should be the owner.) Now configure a virtual host in the Apache config file that sends HTTP requests on port 80 with the host www.fitbit.com to the htdocs directory within the repo. An example apache sites config file, taken from my Raspberry Pi DNS server, 'hook', which also handles the Aria spoofing, is below

# Ensure that Apache listens on port 80
Listen 80

<VirtualHost *:80>
  DocumentRoot /home/pi/websites/hook/htdocs
  ServerName hook

<VirtualHost *:80>
  DocumentRoot /home/pi/websites/www.fitbit.com/htdocs
  ServerName www.fitbit.com
  <Directory />
    Options FollowSymLinks
    AllowOverride All
  <Directory /home/pi/websites/www.fitbit.com/htdocs>
    Options Indexes FollowSymLinks MultiViews
    AllowOverride All
    Order allow,deny
    allow from all

Once you've got it installed, you can test it by going to www.fitbit.com in the browser of a device on the same network as the Aria. It should try to redirect to the https version and then fail because we aren't doing any SSL stripping and we don't have a valid certificate.

Using the Script

Once all is in place and working, step on the scales (as a guest). You won't get any real confirmation that the weight is being spoofed, but if you get a tick on the screen of the scales and a timestamped directory gets created in the data directory you created on installation, then all went well.

In the timestamped directory, you'll find basically a complete copy of the exchange that went between the scales and the Fitbit server, unmodified. In addition, you'll get a JSON file, request_data.json, which contains the same data as the binary file, but nicely formatted into a JSON object which can be read into pretty much anything. See the 'Privacy' section above if you expected to see more data in this file.