"Features" for Finding Trackers


One key component of the Tracking the Trackers project is building a machine learning (ML) tool to aide humans to find tracking in Android apps. One of the most important pieces of developing a machine learning tool is figuring out which “features” should be fed to the machine learning algorithms. In this context, features are constrained data sets derived from the whole data set. In our case, the whole data set is terabytes of APKs. This post is an outline of the features that we are focusing on in this current project.

Confirmed Features

These are features that we will definitely used, and already have good tooling to do the feature extraction.

Permissions

Android apps must request permissions from the Android OS to access sensitive user data as well as certain system features. This can naturally give big hints towards tracking attempts. Basically an app which does not request any permissions will have a much harder time of tracking its users, while an app aimed towards tracking will require a myriad of permissions depending on the properties it desires to track (e.g. location, contacts, phone IDs, Bluetooth IDs, WiFi IDs, camera/microphone-access, call-logs and many more)

Tracking Libraries and SDKs

Code re-use plays a big role in any software project, why write your own tracking functionality when someone else has already implemented a whole library geared towards tracking users. This functionality is provided by different SDKs, which are pre-configured bundles of functionality which in turn (for this use case) are provided by tracking companies. The app developer often has to choose the desired functionality. While importing a tracking library is no guarantee for tracking activities it is certainly a red flag. We compare the libraries imported by the app with a list of known tracking libraries.

Domain Names

Developers leave URLs in form of strings in the code to allow exchange of information with the world outside of the app. This can be used to transfer information about the user which in turn can be used for tracking. The domains often contain a hint about the purpose behind the data transfer (e.g. https://www.google-analytics.com). Domain names are data sinks for collecting data, which gives us a clear point to focus on analyzing since collecting tracking data does not matter if it never leaves the local device, while domain names are the point in the code where data leaves the device and is sent to be collected and analyzed on a remote server. Combined with other features like permissions and seeing tracking libraries being imported a human reviewer could get a pretty good idea of what type of information could be sent. This human “gut feeling” of recognizing fishy combinations of features is something a neural network can often learn to approximate by being trained on a sufficient amount of training data. Domain names known to be relevant to tracking are collected and maintaned by Exodus Privacy.

New Experimental Features

These are features that show a lot of promise, but there is not existing tooling to easily work with them. We are working to make it easier, and will cover that work in future posts.

API Key ID

The API Key ID is a string that identifies bit of authentication data for enabling access to an online service. Many online services require an API Key even if a library or SDK is not required to access it. Even when the SDK is detected, the presence of an API Key shows that the tracking function is actually enabled. For example, the Google Firebase SDK includes lots of functionality, not only tracking, each of which must be enabled with an API Key. So the presence of Firebase is not enough to confirm tracking. A current example of exactly this is the Austrian Red Cross’ Stopp Corona app to track the spread of covid-19 in Austria.

Natural Language Processing (NLP)

As mentioned under the section on domain names, domain names can already give a clear hint at intent. However since this property is well known, URLs might be obfuscated to hide this information. This domain name obfuscation is a well known technique in the world of malware, there is some evidence of use by tracking companies. In this case, a language model might learn that any URL that looks like random letters and numbers might be a sign to consider increasing the probability of classifying this app as tracking slightly, depending on other features like requested permissions, and imported tracking libraries.

Broadcast Receivers

Android provides a system for data to be broadcast to all apps on a device, this is known as a Broadcast Receiver. A wide range of data is available via this mechanism, both from the Android system as well as apps. The Android OS broadcasts detailed information about the battery level, health, and charging status, including details of how its charging. Many music apps will broadcast detailed information about the song being played, while also collecting those events from the system and other apps. The full extent of this activity is not well described, both what data is broadcasted, what apps are doing with it, and which apps are collecting. As a feature, Broadcast Receivers have a lot of promise since they fit the patterns of useful features for machine learning: small, globally unique, and easy to extract.

Feature Extraction Process

We are using tabular data to feed to the machine learning processes, so the process of extracting and pre-processing different features for classification includes similar steps for every feature. The number of features that can reasonably be processed this way is limited to probably tens of thousands of features, or perhaps even hundreds of thousands. Therefore we have collections of the top-n features where n is in the range of thousands: For example, the top thousand tracking libraries, or all built-in permissions. This way we extract the features out of the binary APK file, and source code when available, and loop over our collections of known features. If the feature was found in the APK/source the tabular data will be a 1, else a 0.