A CLI application that automatically prepares Android APK files for HTTPS inspection
npm install apk-mitm> A CLI application that automatically prepares Android APK files for HTTPS inspection
Inspecting a mobile app's HTTPS traffic using a proxy is probably the easiest way to figure out how it works. However, with the [Network Security Configuration][network-security-config] introduced in Android 7 and app developers trying to prevent MITM attacks using [certificate pinning][certificate-pinning], getting an app to work with an HTTPS proxy has become quite tedious.
apk-mitm automates the entire process. All you have to do is give it an APK file and apk-mitm will:
- decode the APK file using [Apktool][apktool]
- replace the app's [Network Security Configuration][network-security-config] to allow user-added certificates
- modify the source code to disable various [certificate pinning][certificate-pinning] implementations
- encode the patched APK file using [Apktool][apktool]
- sign the patched APK file using [uber-apk-signer][uber-apk-signer]
You can also use apk-mitm to patch apps using Android App Bundle and rooting your phone is not required.
Modifying the regular behavior of an app to work around security features like certificate pinning requires either modifying the app's source code _before_ running it or ["hooking"][hooking] the app _while_ it is running to intercept function calls.
apk-mitm is currently taking the former approach of modifying the app's code by first "disassembling" the app using [Apktool][apktool], making changes to several files, and then assembling it back together (again using Apktool).
This approach has the benefit that it doesn't require a rooted device but it also has several drawbacks. The biggest problem is that apps aren't really _meant_ to be disassembled. Apktool tries to achieve that anyway but, especially with big and complex apps, it often runs into problems.
Another issue with this approach is that some certificate pinning methods, like checks performed within native binaries (as is the case for frameworks like Flutter), are either very hard or impossible to circumvent. For this reason, it can be beneficial to try out other approaches for more tricky apps.
Specifically, I'd like to highlight [mitmproxy's android-unpinner project][android-unpinner] and [the underlying frida-interception-and-unpinning scripts by HTTP Toolkit][frida-interception-and-unpinning]. Their approach applies patches at runtime using [Frida][frida], which is a lot more powerful but also means that it either can't be used or is more difficult to use without a rooted device.
If you have an up-to-date version of [Node.js][node] (14+) and [Java][java] (8+), you can install apk-mitm by running:
``shell`
npm install -g apk-mitm
Once installed, you can run this command to patch an app:
`shell`
apk-mitm
Patching an APK file called example.apk might look like this:
`shell
$ apk-mitm example.apk
✔ Decoding APK file
✔ Modifying app manifest
✔ Replacing network security config
✔ Disabling certificate pinning
✔ Encoding patched APK file
✔ Signing patched APK file
Done! Patched APK: ./example-patched.apk
`
You can now install the example-patched.apk file on your Android device and use a proxy like [Charles][charles] or [mitmproxy][mitmproxy] to look at the app's traffic.
You can also patch apps using Android App Bundle with apk-mitm by providing it with a .xapk file (for example from [APKPure][apkpure]) or a .apks file (which you can export yourself using [SAI][sai]). If you're doing this on Linux, make sure that both zip and unzip are installed.
Sometimes you'll need to make manual changes to an app in order to get it to work. In these cases the --wait option is what you need. Enabling it will make apk-mitm wait before re-enconding the app, allowing you to make changes to the files in the temporary directory.
If you want to experiment with different changes to an APK, then using --wait is probably not the most convenient option as it forces you to start from scratch every time you use it. In this case you might want to take a look at [APKLab][apklab]. It's an Android reverse engineering workbench built on top of VS Code that comes with [apk-mitm support][apklab-mitm] and should allow you to iterate much more quickly.
On some devices (like Android TVs) you might not be able to add a new certificate to the system's root certificates. In those cases you can still add your proxy's certificate [directly to the app's Network Security Config][network-security-config-custom-ca] since that will work on any device. You can accomplish this by running apk-mitm with the --certificate flag set to the path of the certificate (.pem or .der file) used by your proxy.
- If the app uses Google Maps and the map is broken after patching, then the app's API key is probably [restricted to the developer's certificate][google-api-key-restrictions]. You'll have to [create your own API key][google-maps-android] without restrictions and run apk-mitm with the --wait option to be able to replace the com.google.android.geo.API_KEY value in the app's AndroidManifest.xml file.
- If apk-mitm crashes while decoding or encoding the issue is probably related to [Apktool][apktool]. Check [their issues on GitHub][apktool-issues] to find possible workarounds. If you happen to find an Apktool version that's not affected by the issue, you can instruct apk-mitm to use it by specifying the path of its JAR file through the --apktool` option.
- Connor Tumbleson for making [an awesome APK decompiler][apktool]
- Patrick Favre-Bulle for making [a very simple tool for signing APKs][uber-apk-signer]
- Ryan Welton for inspiring most of the certificate pinning removal code
MIT © Niklas Higi
[network-security-config]: https://developer.android.com/training/articles/security-config
[network-security-config-custom-ca]: https://developer.android.com/training/articles/security-config#ConfigCustom
[certificate-pinning]: https://owasp.org/www-community/controls/Certificate_and_Public_Key_Pinning#what-is-pinning
[hooking]: https://en.wikipedia.org/wiki/Hooking
[android-unpinner]: https://github.com/mitmproxy/android-unpinner
[frida-interception-and-unpinning]: https://github.com/httptoolkit/frida-interception-and-unpinning
[frida]: https://frida.re/
[node]: https://nodejs.org/en/download/
[java]: https://www.oracle.com/technetwork/java/javase/downloads/index.html
[apklab]: https://github.com/Surendrajat/APKLab
[apklab-mitm]: https://github.com/Surendrajat/APKLab#apply-mitm-patch
[google-maps-android]: https://console.cloud.google.com/google/maps-apis/apis/maps-android-backend.googleapis.com
[google-api-key-restrictions]: https://cloud.google.com/docs/authentication/api-keys#api_key_restrictions
[android-app-bundle]: https://developer.android.com/platform/technology/app-bundle/
[apkpure]: https://apkpure.com/
[sai]: https://github.com/Aefyr/SAI
[charles]: https://www.charlesproxy.com/
[mitmproxy]: https://mitmproxy.org/
[apktool]: https://ibotpeaches.github.io/Apktool/
[apktool-issues]: https://github.com/iBotPeaches/Apktool/issues
[uber-apk-signer]: https://github.com/patrickfav/uber-apk-signer