PRACTICAL API SECURITY WALKTHROUGH — PART 1

Friday 12 January 2018 By Simon Rigg

Topics: Mobile App Authentication, Integration, Repackaged Apps, TLS, A Series - ShipFast

Fake Mobile Apps

Welcome! A quick question: Do you know what’s using your API? Really?

In this mini series I am going to use a fictional product, “ShipFast”, to walk you through the process of defending against various exploits in a mobile application to gain access to data on a remote server allowing real users of the system to gain an unfair business advantage at the expense of the company.

In this first post I will provide some context before we dive into the walkthrough.

We’ll be using the Android platform for the mobile app written in the Kotlin programming language and an Express Node.js backend server.

Full source code for the demo is available on github if you want to play around and try things out for yourself, but it’s not necessary to gain an understanding of the exploits and defenses I’ll be demonstrating.

Enjoy! :-)

THE SERVICE

"ShipFast" is a shipment delivery service company who subcontract the pickup and delivery of shipments to anyone wishing to earn some additional income. These guys are the "Shippers". They earn a wage for the delivery of shipments, but also have the opportunity to earn an additional bonus for some shipments which include an associated gratuity. Not all shipments have a gratuity, and the gratuity is different for each shipment, therefore not all shipments are equal.

ShipFast provides the actual delivery service to customers on a subscription basis. Shippers are paid their standard wage, and any gratuity provided by customers ahead of time is passed on, 100%, to Shippers. Good for them.

ShipFast needs to run a tight ship, so they are keen to maintain optimal efficiency in their service and therefore ensure that distances covered by Shippers to pick up shipments from one location and deliver them to another location are kept to an absolute minimum. Therefore, Shippers have no way to access the list of available shipments and thus pick and choose them: they are always given the available shipment which is closest to their current location.

THE EXPLOIT

There is no hijacking of user credentials here or tricking users to tap things in the app they shouldn't. The attack is much more subtle, because it is exploited by real trusted users of the system.

Recall in the explanation above ("The Service") that shipments are not created equally: some have gratuity associated with them, but the Shippers have no way to pick and choose as they simply get the nearest available shipment to their location...or do they?

Enter: ShipRaider. And an evil pirate laugh I will resist imitating.

Since the ShipFast server API has an endpoint to acquire the nearest available shipment given a Shipper's location data (expressed by latitude and longitude), reverse engineering the server API tells us it is possible to drive out the backend server data and enumerate the list of available shipments by sweeping over a geographical area, sending multiple fake Shipper location values to the server endpoint.

Once we can enumerate this data, we can grab the shipments in turn with the highest associated gratuity and earn as much cash as possible, as real legitimate Shippers, but at the expense of the ShipFast company who will be hurt by the increased distances Shippers are travelling and the increased time it takes shipments to be delivered. Bad for profit. Bad for customers. Bad for business. Great for us Shippers though!

THE BATTLEFIELD

For this demonstration, where no Shippers will be hurt...much, we will show various stages to defend the ShipFast mobile app and server API, and the attacks used to work around these defences.

STAGE 1

In the first stage, we will show how ShipFast secure access to their server by authenticating users and by providing an API key to identify what is talking to the service. The API key will be present in the app's metadata.

We will then show how easy it is to extract this from the app statically and use it against the server API.

STAGE 2

In the second stage, we will show how ShipFast secure access to their server API by introducing a Keyed-Hash Message Authentication Code (HMAC) to authenticate API requests by digitally signing them and prevent hijacking and tampering.

HMAC's use a secret and a message to produce a cryptographic signature, and therefore tell you two things: the integrity of the message (has it been modified?) and the authenticity of the message (is the person who signed it in possession of the secret key?).

The secret key is embedded statically in app code, and we will show how this can be extracted, along with the message components, statically from the app and used against the server API.

STAGE 3

In the third stage, we will extend what we did in Stage 2 but instead of using a static secret we will use a dynamic secret, that is, a secret which is computed at app runtime and therefore is not known until the app is actually running, so cannot easily be extracted statically by looking at the app package on disk.

We will then show how an obfuscated and digitally-signed app can be repackaged to support debugging, then debug the app by introducing a breakpoint at the creation of the HMAC in order to steal the dynamic secret. We will use our knowledge from static analysis of the app in Stage 2 to guide the dynamic analysis in Stage 3.

STAGE 4

Clearly, we need something stronger! Stepping back from the frontline of the battlefield for a moment, and gathering the troops and the strategic plans, we conclude that our server API must know reliably what is talking to it: is it ShipFast, or is it ShipRaider (or indeed something else). What we really need, is a way of authenticating the running mobile application in addition to the user and the network channel. We need something which digitally fingerprints the app reliably and without using behavioral heuristics which aren't always accurate and take time to warm up to genuine and rogue app behavior, and then communicates that digital fingerprint to our API servers so we can verify it and decide how to respond.

SETTING UP THE DEMO

THINGS YOU WILL NEED

At the risk of sounding like a flat-packed furniture instruction manual, there are some things you will need to run the demo yourself. The good news is that these are easy to get hold of, and are free!

(1) An Auth0 account, which you can get from https://auth0.com

(2) A Google Maps API key, which you can get from https://developers.google.com/maps/documentation/android-api/signup

(3) Android Studio 3, which you can get from https://developer.android.com/studio

(4) Node.js, which you can get from https://nodejs.org

CONFIGURING AUTH0

(1) Create a new Native Client in the Auth0 dashboard and name it "ShipFast"

(2) Take careful note of your Auth0 Domain and Client ID as these will be required for user authentication

(3) In the "Allowed Callback URLs" field, enter

demo://YOUR-ACCOUNT.auth0.com/android/com.criticalblue.shipfast/callback, http://127.0.0.1

replacing YOUR-ACCOUNT with your Auth0 account name

(4) Auth0 should already be pre-configured to include Google and GitHub social accounts allowing you to log in to ShipFast with those, but go ahead and add more if you wish

CONFIGURING THE DEMO

(1) Open the file server/shipfast-api/demo-configuration.js in your favorite text editor and change the server host name to the name of the machine which you intend to use to host the ShipFast and ShipRaider servers:

  config.serverHostName = "PUT-YOUR-SERVER-HOSTNAME-HERE"
(2) In the same "demo-configuration.js" file, enter your Auth0 domain:
  config.auth0Domain = "PUT-YOUR-DOMAIN-HERE"
(3) Open the file server/shipraider-rogue-web/web/js/shipraider.js in your favorite text editor and enter your Auth0 client ID and domain:
  // The Auth0 client ID
  const AUTH0_CLIENT_ID = "PUT-YOUR-CLIENT-ID-HERE"
  // The Auth0 domain
  const AUTH0_DOMAIN = "PUT-YOUR-DOMAIN-HERE"
view rawshipraider.js hosted with ❤ by GitHub
(4) Generate a self-signed certificate and private key so that you can host your server over HTTPS using TLS to protect the network channel:

 

node generate-cert.js

(5) Configure an Android Emulator to run Android 6 Marshmallow and install the .crt file generated in the previous step onto the emulator

(6) Ensure the Android Emulator has sufficient permission to use high accuracy location data (Settings->Location->Mode set to "High accuracy")

(7) Open the Android Studio ShipFast project in app/android/kotlin

(8) In Android Studio, open the app's manifest "app/manifests/AndroidManifest.xml" and enter your Google Maps API key:

<meta-data android:name="com.google.android.geo.API_KEY" android:value="PUT-YOUR-API-KEY-HERE"/>

(9) In Android Studio, open the string resource file "app/res/values/strings.xml" and enter your Auth0 client ID and domain:

<string name="com_auth0_client_id">PUT-YOUR-CLIENT-ID-HERE"</string> <string name="com_auth0_domain">PUT-YOUR-DOMAIN-HERE"</string>

RUNNING THE DEMO

The demo has three components: the mobile app, the ShipFast server and the ShipRaider rogue web server.

(1) Launch the ShipFast server as follows:

cd shipfast-api-protection/server/shipfast-api
npm install
node api-server.js

(the server may need to run as admin to host on port 443 as HTTPS)

(2) Launch the ShipFast mobile app in Android Studio using an x86 emulator running Android 6 Marshmallow

(3) Launch the ShipRaider rogue web server as follows:

cd shipfast-api-protection/server/shipraider-rogue-web
npm install
node rogue-web-server.js

The ShipFast server generates sample shipment data, held in memory, when the first request for the nearest shipment is made to the server. The sample shipment pickup and delivery locations are generated around an origin point given by the first call to fetch the nearest shipment, so if the actual client location changes dramatically, you will probably need to restart the server to regenerate sample data around a new origin point.

DIRECTORY STRUCTURE

For reference, this is the structure of the git repo directory:

NEXT TIME...

Now that we have some context, we’ll dive into the first attack of our fictional ShipFast product in my next post and how we can defend against it.

Thank you for reading and stay tuned!

 

 Learn More about Mobile API Security!