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# Install System CA Certificate on Android Emulator

Since Android 7, apps ignore user provided certificates, unless they are configured to use them. As most applications do not explicitly opt in to use user certificates, we need to place our mitmproxy CA certificate in the system certificate store, in order to avoid having to patch each application, which we want to monitor.

Please note, that apps can decide to ignore the system certificate store and maintain their own CA certificates. In this case you have to patch the application.

# 1. Prerequisites

Android Studio/Android Sdk is installed (tested with Version 4.1.3 for Linux 64-bit)
An Android Virtual Device (AVD) was created. Setup documentation available here
- The AVD must not run a production build (these will prevent you from using adb root)
- The proxy settings of the AVD are configured to use mitmproxy. Documentation here
Emulator and adb executables from Android Sdk have been added to $PATH variable
- emulator usually located at /home/<your_user_name>/Android/Sdk/emulator/emulator on Linux systems
- adb usually located at /home/<your_user_name>/Android/Sdk/platform-tools/adb on Linux systems
- I added these lines to my .bashrc
```
export PATH=$PATH:$HOME/Android/Sdk/platform-tools
export PATH=$PATH:$HOME/Android/Sdk/emulator
```
Mitmproxy CA certificate has been created
- Usually located in ~/.mitmproxy/mitmproxy-ca-cert.cer on Linux systems
- If the folder is empty or does not exist, run mitmproxy in order to generate the certificates

# 2. Rename certificate

CA Certificates in Android are stored by the name of their hash, with a ‘0’ as extension (Example: c8450d0d.0). It is necessary to figure out the hash of your CA certificate and copy it to a file with this hash as filename. Otherwise Android will ignore the certificate. By default, the mitmproxy CA certificate is located in this file: ~/.mitmproxy/mitmproxy-ca-cert.cer

# Instructions

Enter your certificate folder: cd ~/.mitmproxy/
Generate hash and copy certificate : hashed_name=`openssl x509 -inform PEM -subject_hash_old -in mitmproxy-ca-cert.cer | head -1` && cp mitmproxy-ca-cert.cer $hashed_name.0

# 3. Insert certificate into system certificate store

Now we have to place our CA certificate inside the system certificate store located at /system/etc/security/cacerts/ in the Android filesystem. By default, the /system partition is mounted as read-only. The following steps describe how to gain write permissions on the /system partition and how to copy the certificate created in the previous step.

# Instructions for API LEVEL > 28

Starting from API LEVEL 29 (Android 10), it seems to be impossible to mount the “/” partition as read-write. Google provided a workaround for this issue using OverlayFS. Unfortunately, at the time of writing this (11. April 2021), the instructions in this workaround will result in your emulator getting stuck in a boot loop. Some smart guy on Stackoverflow found a way to get the /system directory writable anyway.

Keep in mind: You always have to start the emulator using the -writable-system option if you want to use your certificate. Otherwise Android will load a “clean” system image.

Tested on emulators running API LEVEL 29 and 30

# Instructions

List your AVDs: emulator -list-avds (If this yields an empty list, create a new AVD in the Android Studio AVD Manager)
Start the desired AVD: emulator -avd <avd_name_here> -writable-system (add -show-kernel flag for kernel logs)
restart adb as root: adb root
disable secure boot verification: adb shell avbctl disable-verification
reboot device: adb reboot
restart adb as root: adb root
perform remount of partitions as read-write: adb remount. (If adb tells you that you need to reboot, reboot again adb reboot and run adb remount again.)
push your renamed certificate from step 2: adb push <path_to_certificate> /system/etc/security/cacerts
set certificate permissions: adb shell chmod 664 /system/etc/security/cacerts/<name_of_pushed_certificate>
reboot device: adb reboot

# Instructions for API LEVEL <= 28

Tested on emulators running API LEVEL 26, 27 and 28

Keep in mind: You always have to start the emulator using the -writable-system option if you want to use your certificate. Otherwise Android will load a “clean” system image.

List your AVDs: emulator -list-avds (If this yields an empty list, create a new AVD in the Android Studio AVD Manager)
Start the desired AVD: emulator -avd <avd_name_here> -writable-system (add -show-kernel flag for kernel logs)
restart adb as root: adb root
perform remount of partitions as read-write: adb remount. (If adb tells you that you need to reboot, reboot again adb reboot and run adb remount again.)
push your renamed certificate from step 2: adb push <path_to_certificate> /system/etc/security/cacerts
set certificate permissions: adb shell chmod 664 /system/etc/security/cacerts/<name_of_pushed_certificate>
reboot device: adb reboot