Fitbit Aria Wi-Fi Smart Scale

Fitbit Aria Wi-Fi Smart Scale Status: beta
Beschreibung	Fitbit WLAN Scale Reverse Engineering
Autor:	architekt, krisha
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description

the scale

The scale offered by fitbit.com measures weight and body fat. The data is then submitted to their website and can be further analysed. It is possible to use one scale for multiple users including different accounts on fitbit.com website. The scale identifies the users by their weight and maybe by their body fat.

the master plan

We want to create an own service to analyse the data in our own way. With that we're able to bypass limits and to extend the functionality. We are maybe able to allow more users and are able to identify the users in a better way. Additionally the data is not transmitted any more unencrypted to a public website (fitbit.com is password protected). One main interest is also the reversing of the protocol and maybe more...

analysis

request

• The data is transmitted with HTTP POST to http://www.fitbit.com/scale/upload
• Usually it consists of 62 bytes of binary data in little endian format
  • In case there is no connection, the data is stored inside the scale and transmitted in a bulk when it is used next time

0               4                   8              0xC
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+ 
|    Ident 1    |    ??? (1)    |   |          MAC          |  ???  >
| 02  00  00  00| 5C  00  00  00|   | 00  1D  C9  XX  XX  XX| 1C  00>
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
>  (2)  |    ??? (3)    |    Timestamp 2    |   ??? (4)     | Ident >
> 00  00| 21  00  00  00| 30  4D      90  50| 01  00  00  00| 02  00>
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
>  2    |    ??? (5)    |    Weight in g    |  Timestamp 1  | User  >
> 00  00| 00  00  00  00| 06  FE      00  00| 1C  4D  90  50| 00  00>
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
>  ID   |  Body fat 1   |    Body fat 2     |  Body fat 3   |        
> 00  00| 00  00  00  00| 00  00      00  00| 00  00  00  00|        
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+

explanation

• Ident 1: Seems to be always 2
• (1): Values: 0x5C, 0x5B, 0x4F, might be indication for errornous syncs
• (2): Values: 0x1C, 0x1D
• (3): Unknown, might saw 0x11 and 0x21
• Timestamp 2: Measurement end in unix time
• (4): Unknown (fixed?)
• Ident 2: Unknown (fixed?)
• (5): Values 0x00, 0x66
• Weight in g: self-explanatory
• Timestamp 1: Measurement start in unix time
• User ID: 0x00 for guest, values for registered users: 0x0011yyyy
• Body fat 1: Displayed body fat in per mil
• Body fat 2: Maybe physical measurement value (resistance, electrical impedance). For registered users this has always a value. For guests this is 0x0.
• Body fat 3: Value similiar to Body fat 1

response

header

0               4                   8
+---+---+---+---+---+---+---+---+   +---+---+
|  Timestamp    |  ???  |  Number of users  |
|               |       |                   |
+---+---+---+---+---+---+---+---+   +---+---+

user data

0               4                   8               12
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
|    User Id    |                    Always zero                    >
| XX  XX  YY  YY|                                                   >
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
>               | Name of user (only first three bytes used, rest   >
>               |                                                   >
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
> is filled with spaces)        |   |      ???      |      ???      |
>                               |   |    58950      |               |
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
| Age in years  |Sex|      Height in -  |  Weight in g  |  Body Fat >
|               |   |    millimeters    |               |           >
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
>   |      ???      | ??|  Some weight  |  Timestamp    |
>   |1040128544 or 0|   |               |               |
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+

padding

0               4               8
+---+---+---+---+---+---+---+---+
|            Padding            |
|                               |
+---+---+---+---+---+---+---+---+

own tools

/* to be crafted */

attacks

gather information

/* MAC - what is it, what's the idea to use it */ First results showed that... /* continue with dumping DB */. This is a base attack used later again.

fatify local target

Das geht relativ einfach: Mit tcpdump oder Wireshark die Daten eines Ziels nach der Messung mitschneiden. Da meistens WPA im WLAN zum Einsatz kommt, empfiehlt es sich hier am Router oder danach die Daten mitzusniffen. Anschließend einfach den Request klonen und ein eigenes Gewicht oder Körperfett einsetzen. Es kann sein, dass fitbit die Anzahl der Messungen beschränkt und das Gewicht der Person in einem gewissen Limit (Durschschnitt) liegen muss.

fatify the world

In der Theorie ist es möglich alle fitbit User etwas schwerer zu machen. Die Waage authentifiziert sich mit der MAC Adresse, zusätzlich gibt es eine xx Bit Userkennung. Die MAC Adresse (Herstellerkennung + Wert) kann man per Brute-Force testen, die Benutzerkennung macht den Aufwand aufgrund der vielen Bits aber sehr langwierig. Da der fitbit Server allerdings auch auf Gäste antwortet und zwar mit der kompletten Userliste inkl. der Benutzerkennung, kann man einfach einen Request als Gast (Wert 0) absetzen. Als Antwort schickt der Server die Kennungen zurück und man kann sich seine eigenen Requests zusammensetzen - für alle fitbit User weltweit ;-)