hacktricks/mobile-pentesting/android-app-pentesting/install-burp-certificate.md

# Install Burp Certificate

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## On a Virtual Machine

First of all you need to download the Der certificate from Burp. You can do this in _**Proxy**_ --> _**Options**_ --> _**Import / Export CA certificate**_

![](<../../.gitbook/assets/image (367).png>)

**Export the certificate in Der format** and lets **transform** it to a form that **Android** is going to be able to **understand.** Note that **in order to configure the burp certificate on the Android machine in AVD** you need to **run** this machine **with** the **`-writable-system`** option.\
For example you can run it like:

{% code overflow="wrap" %}
```bash
C:\Users\<UserName>\AppData\Local\Android\Sdk\tools\emulator.exe -avd "AVD9" -http-proxy 192.168.1.12:8080 -writable-system
```
{% endcode %}

Then, to **configure burps certificate do**:

{% code overflow="wrap" %}
```bash
openssl x509 -inform DER -in burp_cacert.der -out burp_cacert.pem
CERTHASHNAME="`openssl x509 -inform PEM -subject_hash_old -in burp_cacert.pem | head -1`.0"
mv burp_cacert.pem $CERTHASHNAME #Correct name
adb root && sleep 2 && adb remount #Allow to write on /syste
adb push $CERTHASHNAME /sdcard/ #Upload certificate
adb shell mv /sdcard/$CERTHASHNAME /system/etc/security/cacerts/ #Move to correct location
adb shell chmod 644 /system/etc/security/cacerts/$CERTHASHNAME #Assign privileges
adb reboot #Now, reboot the machine
```
{% endcode %}

Once the **machine finish rebooting** the burp certificate will be in use by it!

## Using Magisc

If you **rooted your device with Magisc** (maybe an emulator), and you **can't follow** the previous **steps** to install the Burp cert because the **filesystem is read-only** and you cannot remount it writable, there is another way.

Explained in [**this video**](https://www.youtube.com/watch?v=qQicUW0svB8) you need to:

1. **Install a CA certificate**: Just **drag\&drop** the DER Burp certificate **changing the extension** to `.crt` in the mobile so it's stored in the Downloads folder and go to `Install a certificate` -> `CA certificate`

<figure><img src="../../.gitbook/assets/image (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1).png" alt="" width="164"><figcaption></figcaption></figure>

* Check that the certificate was correctly stored going to `Trusted credentials` -> `USER`

<figure><img src="../../.gitbook/assets/image (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1).png" alt="" width="334"><figcaption></figcaption></figure>

2. **Make it System trusted**: Download the Magisc module [MagiskTrustUserCerts](https://github.com/NVISOsecurity/MagiskTrustUserCerts) (a .zip file), **drag\&drop it** in the phone, go to the **Magics app** in the phone to the **`Modules`** section, click on **`Install from storage`**, select the `.zip` module and once installed **reboot** the phone:

<figure><img src="../../.gitbook/assets/image (2) (1) (1) (1) (1) (1) (1) (1) (1).png" alt="" width="345"><figcaption></figcaption></figure>

* After rebooting, go to `Trusted credentials` -> `SYSTEM` and check the Postswigger cert is there

<figure><img src="../../.gitbook/assets/image (3) (1) (1) (1) (1).png" alt="" width="314"><figcaption></figcaption></figure>

## Post Android 14

Changes:

* Until now, system-trusted CA certificates lived in **`/system/etc/security/cacerts/`**. On a standard AOSP emulator, those could be **modified directly with root access** with minimal setup, immediately taking **effect everywhere**.
* In Android 14, system-trusted CA certificates will generally live in **`/apex/com.android.conscrypt/cacerts`**, and all of **`/apex` is immutable**.
* That **APEX cacerts path cannot be remounted as rewritable** - remounts simply fail. In fact, even if you unmount the entire path from a root shell, apps can still read your certificates just fine.
* The alternative technique of **mounting a tmpfs directory over the top also doesn't work** - even though this means that `ls /apex/com.android.conscrypt/cacerts` might return nothing (or anything else you like), apps will still see the same original data.
  * Because the `/apex` mount is [explicitly mounted](https://cs.android.com/android/platform/superproject/main/+/main:system/core/init/mount\_namespace.cpp;l=97;drc=566c65239f1cf3fcb0d8745715e5ef1083d4bd3a) **with PRIVATE propagation**, so that all changes to mounts inside the `/apex` path are never shared between processes.

That's done by the `init` process which starts the OS, which then launches the [Zygote process](https://en.wikipedia.org/wiki/Booting\_process\_of\_Android\_devices#Zygote) (with a new mount namespace copied from the parent, so including its **own private `/apex` mount**), which then in turn **starts each app process** whenever an app is launched on the device (who each in turn then **copy that same private `/apex` mount**).

### Recursively remounting mountpoints

* You can remount `/apex` manually, removing the PRIVATE propagation and making it writable (ironically, it seems that entirely removing private propagation _does_ propagate everywhere)
* You copy out the entire contents of `/apex/com.android.conscrypt` elsewhere
* Then you unmount `/apex/com.android.conscrypt` entirely - removing the read-only mount that immutably provides this module
* Then you copy the contents back, so it lives into the `/apex` mount directly, where it can be modified (you need to do this quickly, as [apparently](https://infosec.exchange/@g1a55er/111069489513139531) you can see crashes otherwise)
* This should take effect immediately, but they recommend killing `system_server` (restarting all apps) to get everything back into a consistent state

```bash
# Create a separate temp directory, to hold the current certificates
# Otherwise, when we add the mount we can't read the current certs anymore.
mkdir -p -m 700 /data/local/tmp/tmp-ca-copy

# Copy out the existing certificates
cp /apex/com.android.conscrypt/cacerts/* /data/local/tmp/tmp-ca-copy/

# Create the in-memory mount on top of the system certs folder
mount -t tmpfs tmpfs /system/etc/security/cacerts

# Copy the existing certs back into the tmpfs, so we keep trusting them
mv /data/local/tmp/tmp-ca-copy/* /system/etc/security/cacerts/

# Copy our new cert in, so we trust that too
mv $CERTIFICATE_PATH /system/etc/security/cacerts/

# Update the perms & selinux context labels
chown root:root /system/etc/security/cacerts/*
chmod 644 /system/etc/security/cacerts/*
chcon u:object_r:system_file:s0 /system/etc/security/cacerts/*

# Deal with the APEX overrides, which need injecting into each namespace:

# First we get the Zygote process(es), which launch each app
ZYGOTE_PID=$(pidof zygote || true)
ZYGOTE64_PID=$(pidof zygote64 || true)
# N.b. some devices appear to have both!

# Apps inherit the Zygote's mounts at startup, so we inject here to ensure
# all newly started apps will see these certs straight away:
for Z_PID in "$ZYGOTE_PID" "$ZYGOTE64_PID"; do
    if [ -n "$Z_PID" ]; then
        nsenter --mount=/proc/$Z_PID/ns/mnt -- \
            /bin/mount --bind /system/etc/security/cacerts /apex/com.android.conscrypt/cacerts
    fi
done

# Then we inject the mount into all already running apps, so they
# too see these CA certs immediately:

# Get the PID of every process whose parent is one of the Zygotes:
APP_PIDS=$(
    echo "$ZYGOTE_PID $ZYGOTE64_PID" | \
    xargs -n1 ps -o 'PID' -P | \
    grep -v PID
)

# Inject into the mount namespace of each of those apps:
for PID in $APP_PIDS; do
    nsenter --mount=/proc/$PID/ns/mnt -- \
        /bin/mount --bind /system/etc/security/cacerts /apex/com.android.conscrypt/cacerts &
done
wait # Launched in parallel - wait for completion here

echo "System certificate injected"
```

### Bind-mounting through NSEnter

*   First, we need set up a writable directory somewhere. For easy compatibility with the existing approach, I'm doing this with a `tmpfs` mount over the (still present) non-APEX system cert directory:

    ```bash
    mount -t tmpfs tmpfs /system/etc/security/cacerts
    ```
* Then you place the CA certificates you're interested in into this directory (e.g. you might want copy all the defaults out of the existing `/apex/com.android.conscrypt/cacerts/` CA certificates directory) and set permissions & SELinux labels appropriately.
*   Then, use `nsenter` to enter the Zygote's mount namespace, and bind mount this directory over the APEX directory:

    ```bash
    nsenter --mount=/proc/$ZYGOTE_PID/ns/mnt -- \
        /bin/mount --bind /system/etc/security/cacerts /apex/com.android.conscrypt/cacerts
    ```

    The Zygote process spawns each app, copying its mount namespace to do so, so this ensures all newly launched apps (everything started from now on) will use this.
*   Then, use `nsenter` to enter each already running app's namespace, and do the same:

    ```bash
    nsenter --mount=/proc/$APP_PID/ns/mnt -- \
        /bin/mount --bind /system/etc/security/cacerts /apex/com.android.conscrypt/cacerts
    ```

    Alternatively, if you don't mind the awkward UX, you should be able to do the bind mount on `init` itself (PID 1) and then run `stop && start` to soft-reboot the OS, recreating all the namespaces and propagating your changes everywhere (but personally I do mind the awkward reboot, so I'm ignoring that route entirely).

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