Five Tools for Starting Security Analysis of Android Apps

Here are five, easy to use, tools to start security analysis of a Android apps. While they are basic, they allow to do the initial checking for things like lack of SSL, sensitive files, broadcast issues and secrets in code. We also highly recommend buying a cheap Android device for testing instead of/in addition to an emulator.

As always, please obey all relevant laws and do not use these tools for illegal activity.

On-device MITM proxy – PacketCapture

An MITM proxy is used to inspect network traffic going from/to a particular mobile device, or perhaps a specific application on the device. Normally, an MITM proxy requires setting up a separate test machine with the proxy and then pointing traffic from the test device to that machine. However, PacketCapture, is a free and easy to use MITM proxy that runs on the Android device itself, can optionally inspect SSL traffic and can also be selectively applied to a specific app. It lacks the bells and whistles of other proxies, but it is very easy to use. Behind the scenes it works by creating a VPN connection to itself on the device.

One thing to keep in mind: the next version of Android (Android P) will enable TLS by default. Apps can still opt out via a network security policy (see here). Once that changes takes place, you are advised to check the network security policy first before trying this tool.

On-device Broadcasts Monitor – Android Broadcasts Monitor

One of the common pitfalls in Android development is using global broadcasts when exchanging data between different components of the application. Because global broadcasts can be seen by other apps, they can leak sensitive data. An easy way to look for these is to install the Android Broadcasts Monitor app (Google Play link here) which will show you all global broadcasts as they happen.

On-device File Manager

Another useful tool in your toolbox is an on-device file manager. This can be used to check if a particular application leaves any sensitive data on the SD card where it can be accessed by other apps. In particular, you should inspect the “/Android/” directory. We are fans of the Amaze File Manager (source at GitHub) but you can use any other as well.

If you do end up using Amaze, it has a nice feature where you can backup an installed app to the SD card, which allows you to get an APK of an app for further analysis with tools like JADX.

On-device Video Recorder – Telecine

Recording on-device videos comes really useful when making demos or doing bug bounties. One useful tool we use is Telecine by Jake Wharton which can record all screen activity (except FLAG_SECURE). One useful tip is to use “ffmpeg” or a similar tool to downscale the resolution like this example:

ffmpeg -i Telecine_video.mp4 -crf 40 -an final.mp4

Android Decompiler – JADX

JADX is a Java decompiler which can take an Android APK and decompile it back to Java source code. One useful thing this can be used for is to analyze possible secrets that are included in the Android resources (not code). Often, there may be sensitive data that is easier to find instead of searching through source code. The “/strings” and “/raw” folders are usually the best place to start.

Keep in mind that Android uses a custom JVM which is not the same one as normal Java. Therefore things relevant to security like cryptography, SSL connections, etc. do not necessarily behave the same way as in regular JVMs.

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