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---
title: "Choosing Your Android-Based Operating System"
tags: ['operating systems', 'security', 'android']
author: Tommy
---
Android is a secure operating system that has strong [app sandboxing](https://source.android.com/security/app-sandbox), [Verified Boot](https://source.android.com/security/verifiedboot) (AVB), and a robust [permission](permission) control system.
When you buy an Android phone, the device's default operating system often comes with invasive integration with apps and services that are not part of the [Android Open-Source Project](https://source.android.com/). An example of such is Google Play Services, which has irrevocable privileges to access your files, contacts storage, call logs, SMS messages, location, camera, microphone, hardware identifiers, and so on. These apps and services increase the attack surface of your device and are the source of various privacy concerns with Android.
This problem could be solved by using a custom Android-based operating system that does not come with such invasive integration. Unfortunately, many custom Android-based operating systems often violate the Android security model by not supporting critical security features such as AVB, rollback protection, firmware updates, and so on. Some of them also ship [`userdebug`](https://source.android.com/setup/build/building#choose-a-target) builds which expose root over [ADB](https://developer.android.com/studio/command-line/adb) and require [more permissive](https://github.com/LineageOS/android_system_sepolicy/search?q=userdebug&type=code) SELinux policies to accommodate debugging features, resulting in a further increased attack surface and weakened security model.
When choosing a custom Android-based operating system, you should make sure that it upholds the Android security model. Ideally, the custom operating system should have subtantial privacy and security improvements to justify adding yet another party to trust.
## Baseline Security
### Verified Boot
![Verified Boot](/verified-boot.png)
[Verified Boot](https://source.android.com/security/verifiedboot) is an important part of the Android security model. It provides protection against [evil maid](https://en.wikipedia.org/wiki/Evil_maid_attack) attacks, malware persistence, and ensures security updates cannot be downgraded with [rollback protection](https://source.android.com/security/verifiedboot/verified-boot#rollback-protection).
Android 10 and above has moved away from full-disk encryption to more flexible [file-based encryption](https://source.android.com/security/encryption/file-based). Your data is encrypted using unique encryption keys, and the operating system files are left unencrypted.
Verified Boot ensures the integrity of the operating system files, thereby preventing an adversary with physical access from tampering or installing malware on the device. In the unlikely case that malware is able to exploit other parts of the system and gain higher privileged access, Verified Boot will prevent and revert changes to the system partition upon rebooting the device.
Unfortunately, OEMs are only obliged to support Verified Boot on their stock Android distribution. Only a few OEMs such as Google support custom AVB key enrollment on their devices. Additionally, some AOSP derivatives such as LineageOS or /e/ OS do not support Verified Boot even on hardware with Verified Boot support for third-party operating systems. These AOSP derivatives should be avoided at all cost.
### Firmware Updates
Firmware updates are critical for maintaining security and without them your device cannot be secure. OEMs have support agreements with their partners to provide the closed-source components for a limited support period. These are detailed in the monthly [Android Security Bulletins](https://source.android.com/security/bulletin).
On a custom Android distribution, it is the responsibility of the operating system vendor to extract the firmware from the stock operating system, test it against their Android builds, then ship them to the user.
Unfortunately, many custom Android distributions, including extremely popular ones like LineageOS and /e/ OS do not ship firmware updates for most of their supported device. Instead, they expect the user to keep track of stock OS updates, extract and flash the firmware themselves. Beyond the lack of testing, this is extremely burdensome and not feasible for most end users and is yet another reason to not use these distributions.
### Patch Levels
As the [Android Security Bulletins](https://source.android.com/security/bulletin) is updated every month, Android-Based operating systems are expected to apply all security fixes before the next bulletin update comes out. Beside extracting the firmware, testing it and shipping it to the end user as described [above](#firmware-updates), the AOSP based system also need to be updated.
This is a particularly challenging thing to do, especially around the time of a new major Android release since there are a lot of changes. Sometimes, newer firmware versions require newer major versions of AOSP, and if the developer takes too long to update their base operating system to the next major AOSP version, they cannot ship firmware updates either, leaving users vulnerable.
This has happened to CalyxOS during the Android 11 to Android 12 transition. It took them [4 months](https://github.com/privacyguides/privacyguides.org/pull/578#issue-1112002737) to update to Android 12; and during those 4 months, they could not ship any firmware updates at all, leaving the user vulnerable during that time period.
It would be much better if you just stick to the stock operating system (which got updated to Android 12 shortly after the AOSP 12 release) instead of using a custom operating system which could not keep up with updates as described.
### Chromium Webview Updates
Android comes with a system [webview](https://developer.android.com/reference/android/webkit/WebView), a component that many apps rely on to use as part of their activity layout. It effectively behaves like a minimal browser, opening random websites with arbitary code the internet. Thus, it is very important that this component is consistently kept up to dater.
Some Android-based operating systems, including ones like CalyxOS, often fall behind on security updates for this component. Particularly, this has gotten so bad that they actually fell behind for [3 months](https://github.com/privacyguides/privacyguides.org/pull/548#issuecomment-1018245074) back in January 2022 and [2 months](https://github.com/privacyguides/privacyguides.org/pull/1378) in June 2022. It is a good indication that these operating systems cannot keep up with security updates and should not be used.
### User Builds
As mentioned [above](/os/choosing-your-android-based-operating-system/), `userdebug` builds expose root over ADB and require more permissive SELinux policies to accommodate debugging features. `userdebug` builds violate the Android security model and are really only meant for developers to test out their android builds during development.
End users should be using the production `user` builds, and any distributions that fail to deliver them like LineageOS or /e/ OS should be avoided.
### SELinux in Enforcing Mode
[SELinux](https://source.android.com/security/selinux) is a critical part of the Android security model, having the Linux kernel enforcing confinement for all proccesses, including system processes running as root.
Unfortunately, many custom Android-based operating system builds (especially unofficial LineageOS builds) disables SELinux or set it into permissive mode. You can check whether SELinux is in enforcing mode or not by executing `getenforce` in the ADB shell (the expected output is `Enforcing`). You should avoid any Android-based operating system builds that do not have SELinux in enforcing mode at all cost.
![ADB SELinux](/adb-selinux.png)
## Good Android-Based Operating Systems
Currently, I am only aware of two Android-based operating systems that should be used over the stock operating systems:
### GrapheneOS
![GrapheneOS Phone & Cat](/grapheneos-phone-cat.jpg)
GrapheneOS is the **only** custom Android-based operating system you should buy a new phone for. It provides additional [security hardening](https://en.wikipedia.org/wiki/Hardening_(computing)) and privacy improvements over the stock operating system from Google. It has a [hardened memory allocator](https://github.com/GrapheneOS/hardened_malloc), network and sensor permissions, and various other [security feature](https://grapheneos.org/features). GrapheneOS also comes with full firmware updates and signed builds, so verified boot is fully supported.
For usability purposes, GrapheneOS supports [Sandboxed Google Play](https://grapheneos.org/usage#sandboxed-google-play), which runs Google Play Services fully sandboxed like any other regular app. This means you can take advantage of most Google Play Services, such as [push notifications](https://firebase.google.com/docs/cloud-messaging/), while giving you full control over their permissions and access, and while containing them to a specific work profile or user profile of your choice. Most interestingly, the [In-app Billing API](https://android-doc.github.io/google/play/billing/api.html), [Google Play Games](https://play.google.com/googleplaygames), [Play Asset Delivery](https://developer.android.com/guide/playcore/asset-delivery), [FIDO2](/knowledge/multi-factor-authentication/#fido2-fast-identity-online) all work exceptionally well. Most [Advanced Protection Program](https://landing.google.com/advancedprotection/) features, except for [Play Protect](https://support.google.com/googleplay/answer/2812853?hl=en), also work.
Recently, GrapheneOS has also added the [Storage Scopes](https://grapheneos.org/usage#storage-access) feature, allowing you to limit what legacy apps which still use the old storage API can access. With this new feature, you no longer have to grant certain apps access to all of your media or files to use them anymore.
![VLC Storage Scopes](/vlc-storage-scopes.png)
Currently, Google Pixel phones are the only devices that meet GrapheneOS's [hardware security requirements](https://grapheneos.org/faq#device-support).
### DivestOS
DivestOS is a great aftermarket operating system for devices that have gone end of life or near end of life. Note that this is a harm reduction project, run by one developer on a best effort basis, and you should not buy a new device just to run DivestOS.
Being a soft-fork of [LineageOS](https://lineageos.org/), DivestOS inherits many [supported devices](https://divestos.org/index.php?page=devices&base=LineageOS) from LineageOS. It has signed builds, making it possible to have [verified boot](https://source.android.com/security/verifiedboot) on some non-Pixel devices.
It comes with substantial hardening over AOSP. DivestOS has automated kernel vulnerability ([CVE](https://en.wikipedia.org/wiki/Common_Vulnerabilities_and_Exposures)) [patching](https://gitlab.com/divested-mobile/cve_checker), fewer proprietary blobs, a custom [hosts](https://divested.dev/index.php?page=dnsbl) file, and various security features ported from GrapheneOS. A non-exhaustive list of this includes:
- A hardened webview. [Mulch](https://gitlab.com/divested-mobile/mulch) comes with *some* patches from GrapheneOS's Vanadium browser and the [Bromite](https://github.com/bromite/bromite) project. It gets updated fairly quickly and do not fall behind nearly as much as Bromite.
- Kernel patches from GrapheneOS and enables all available kernel security features via [defconfig hardening](https://github.com/Divested-Mobile/DivestOS-Build/blob/master/Scripts/Common/Functions.sh#L758). All kernels newer than version 3.4 include full page [sanitization](https://lwn.net/Articles/334747/) and all ~22 Clang-compiled kernels have [`-ftrivial-auto-var-init=zero`](https://reviews.llvm.org/D54604?id=174471) enabled.
- GrapheneOS's [`INTERNET`](https://developer.android.com/training/basics/network-ops/connecting) and SENSORS permission toggle.
- [Hardened memory allocator](https://github.com/GrapheneOS/hardened_malloc)
- [Secure Exec-Spawning](android/grapheneos-vs-calyxos.md#additional-hardening)
- Partial [bionic](https://en.wikipedia.org/wiki/Bionic_(software)) hardening patchsets from GrapheneOS
- GrapheneOS's per-network full [MAC randomization](https://en.wikipedia.org/wiki/MAC_address#Randomization) option on version 17.1 and higher
- Automatic reboot/Wi-Fi/Bluetooth [timeout options](https://grapheneos.org/features)
With that being said, DivestOS is not without its faults. The developer does not have all of the devices he is building for, and for a lot of them he simply publishes the builds blind without actually testing them. Firmware update support [varies](https://gitlab.com/divested-mobile/firmware-empty/-/blob/master/STATUS) across devices. DivestOS also takes a very long time to update to a new major Android, and actually took longer than CalyxOS did as mentioned [above](#firmware-updates).
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