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various enhancements

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Wonderfall 2022-03-01 05:47:33 +01:00
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content/posts/fdroid-issues.md
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@ -28,7 +28,7 @@ F-Droid requires that the source code of the app is exempt from any proprietary
> But... can't I just trust F-Droid and be done with it?
[You don't have to take my word for it](https://forum.f-droid.org/t/is-it-as-safe-as-it-is-from-fdroid-official-repo/15956/12): they openly admit themselves it's a [very basic process](https://forum.f-droid.org/t/is-it-as-safe-as-it-is-from-fdroid-official-repo/15956/2) relying on badness enumeration (this doesn't work by the way) and a few scripts. You are not exempted from trusting upstream developers and it goes for any repository.
[You don't have to take my word for it](https://forum.f-droid.org/t/is-it-as-safe-as-it-is-from-fdroid-official-repo/15956/12): they openly admit themselves it's a [very basic process](https://forum.f-droid.org/t/is-it-as-safe-as-it-is-from-fdroid-official-repo/15956/2) relying on badness enumeration (this doesn't work by the way) which consists in a few scripts scanning the code for proprietary blobs and known trackers. You are therefore not exempted from trusting upstream developers and it goes for any repository.
> How can you be sure that the app repository can be held to account for the code it delivers?
@ -73,7 +73,7 @@ allow untrusted_app_25 proc_tty_drivers:file r_file_perms;
This is a mere sample of the [SELinux exceptions](https://android.googlesource.com/platform/system/sepolicy/+/refs/tags/android-12.0.0_r21/private) that have to be made on older API levels so that you can understand why it matters.
It turns out the official F-Droid client doesn't care much about this since it lags behind quite a bit, **[targeting the API level 25](https://gitlab.com/fdroid/fdroidclient/-/blob/2a8b16683a2dbee16d624a58e7dd3ea1da772fbd/app/build.gradle#L33)** (of which some SELinux exceptions were shown above). As a workaround, some users recommended third-party clients such as [Foxy Droid](https://f-droid.org/en/packages/nya.kitsunyan.foxydroid/) or [Aurora Droid](https://f-droid.org/en/packages/com.aurora.adroid/). While these clients might be technically better, they're poorly maintained for some, and they also introduce yet another party to the mix.
It turns out the official F-Droid client doesn't care much about this since it lags behind quite a bit, **[targeting the API level 25](https://gitlab.com/fdroid/fdroidclient/-/blob/2a8b16683a2dbee16d624a58e7dd3ea1da772fbd/app/build.gradle#L33)** (Android 7.1) of which some SELinux exceptions were shown above. As a workaround, some users recommended third-party clients such as [Foxy Droid](https://f-droid.org/en/packages/nya.kitsunyan.foxydroid/) or [Aurora Droid](https://f-droid.org/en/packages/com.aurora.adroid/). While these clients might be technically better, they're poorly maintained for some, and they also introduce yet another party to the mix. [Droid-ify](https://github.com/Iamlooker/Droid-ify) seems to be a better option that the official client in most aspects.
Furthermore, F-Droid **doesn't enforce a minimum target SDK** for the official repository. Play Store [does that quite aggressively](https://developer.android.com/google/play/requirements/target-sdk) for new apps and app updates:
@ -85,7 +85,7 @@ While it may seem bothersome, it's a necessity to keep the **app ecosystem moder
## 4. General lack of good practices
The F-Droid client allows multiple repositories to coexist within the same app. Many of the issues highlighted above were focused on the main official repository which most of the F-Droid users will use anyway. However, having **other repositories in a single app also violates the security model of Android** which was not designed for this at all. The OS expects you to trust **an app repository as a single source** of apps, yet F-Droid isn't that by design as it mixes several repositories in one single app. This is important because the OS management APIs are not meant for this and see F-Droid as a single source, so you're trusting the app client to not mess up far more than you should. It is also worth noting the repository metadata format isn't properly signed by lacking whole-file signing and key rotation.
As a matter of fact, the new unattended update API from Android 12 that allows seamless app updates for app repositories without [privileged access](https://f-droid.org/en/packages/org.fdroid.fdroid.privileged/) to the system (such an approach is not compatible with the security model) won't work with F-Droid. It should be mentioned that a third-party client called [Droid-ify](https://github.com/Iamlooker/Droid-ify/issues/20) is trying to figure out a way to make it work, although the underlying issues about the F-Droid infrastructure largely remain. Indeed, this secure API allowing for unprivileged unattended updates not only requires for the app repository client to target API level 31, but the updated apps also have to at least target API level 29.
As a matter of fact, the [new unattended update API](https://developer.android.com/reference/android/Manifest.permission#UPDATE_PACKAGES_WITHOUT_USER_ACTION) added in API level 31 (Android 12) that allows seamless app updates for app repositories without [privileged access](https://f-droid.org/en/packages/org.fdroid.fdroid.privileged/) to the system (such an approach is not compatible with the security model) won't work with F-Droid. It should be mentioned that [Droid-ify](https://github.com/Iamlooker/Droid-ify/issues/20) is trying to figure out a way to make it work, although the underlying issues about the F-Droid infrastructure largely remain. Indeed, this secure API allowing for unprivileged unattended updates not only requires for the app repository client to target API level 31, but the apps to be updated also have to at least target API level 29.
Their client also lacks **TLS certificate pinning**, unlike Play Store which does that for all connections to Google. Certificate pinning is a way for apps to increase the security of their connection to services [by providing a set of public key hashes](https://developer.android.com/training/articles/security-config#CertificatePinning) of known-good certificates for these services instead of trusting pre-installed CAs. This can avoid some cases where an interception (*man-in-the-middle* attack) could be possible and lead to various security issues considering you're trusting the app to deliver you other apps.
@ -116,11 +116,14 @@ To be fair, they've thought several times about adding certificate pinning to th
F-Droid also has a problem regarding the adoption of **[new signature schemes](https://source.android.com/security/apksigning)** as they [held out on the v1 signature scheme](https://forum.f-droid.org/t/why-f-droid-is-still-using-apk-signature-scheme-v1/10602) (which was [horrible](https://www.xda-developers.com/janus-vulnerability-android-apps/) and deprecated since 2017) until they were forced by Android 11 requirements to support the newer v2/v3 schemes. Quite frankly, this is straight-up bad, and **signing APKs with GPG** is no better considering [how bad PGP and its reference implementation GPG are](https://latacora.micro.blog/2019/07/16/the-pgp-problem.html) (even Debian [is trying to move away from it](https://wiki.debian.org/Teams/Apt/Spec/AptSign)). Ideally, F-Droid should fully move on to newer signature schemes, and should completely phase out the legacy signature schemes which are still being used for some apps and metadata.
## 5. Confusing UX
It is worth mentioning that their website has (for some reason) always been hosting an [outdated APK of F-Droid](https://forum.f-droid.org/t/why-does-the-f-droid-website-nearly-always-host-an-outdated-f-droid-apk/6234), and this is still the case today, leading to many users wondering why they can't install F-Droid on their secondary user profile (due to the downgrade prevention enforced by Android). "Stability" seems to be the main reason mentioned on their part, which doesn't make sense: either your version isn't ready to be published in a stable channel, or it is and new users should be able to access it easily.
F-Droid should enforce the approach of prefixing the package name of their alternate builds with `org.f-droid` for instance (or add a `.fdroid` suffix as some already have). Building and signing while **reusing the package name/ID** is bad practice as it causes **signature verification errors** when some users try to update/install these apps from other sources, even directly from the developer. That is again due to the security model of Android which enforces a signature check when installing app updates (or installing them again in a secondary user profile). Note that this is going to be an issue with Play App Signing as well, and developers are encouraged to follow this approach should they intend to distribute their apps through different distribution channels.
F-Droid should enforce the approach of prefixing the package name of their alternate builds with `org.f-droid` for instance (or add a `.fdroid` suffix as some already have). Building and signing while **reusing the package name** ([application ID](https://developer.android.com/studio/build/configure-app-module)) is bad practice as it causes **signature verification errors** when some users try to update/install these apps from other sources, even directly from the developer. That is again due to the security model of Android which enforces a signature check when installing app updates (or installing them again in a secondary user profile). Note that this is going to be an issue with Play App Signing as well, and developers are encouraged to follow this approach should they intend to distribute their apps through different distribution channels.
## 5. Misleading permissions approach
This results in a confusion user experience where it's hard to keep track of who signs each app, and from which repository the app should be downloaded or updated.
## 6. Misleading permissions approach
F-Droid shows a list of the [low-level permissions](https://developer.android.com/reference/android/Manifest.permission) for each app: these low-level permissions are usually grouped in the standard high-level permissions (Location, Microphone, Camera, etc.) and special toggles (nearby Wi-Fi networks, Bluetooth devices, etc.) that are explicitly based on a type of sensitive data. While showing a list of low-level permissions could be useful information for a developer, it's often a **misleading** and inaccurate approach for the end-user. Apps have to [request the standard permissions at runtime](https://developer.android.com/guide/topics/permissions/overview#runtime) and do not get them simply by being installed, so knowing all the "under the hood" permissions is not useful and makes the permission model unnecessarily confusing.
For example, the low-level permission `RECEIVE_BOOT_COMPLETED` is referred to in F-Droid as the *run at startup* description, when in fact this permission is not needed to start at boot and just refers to a specific time broadcasted by the system once it finishes booting, and is not about background usage (though power usage may be a valid concern).
@ -134,11 +137,9 @@ Play Store for instance conveys the permissions in a way less misleading way: th
Moreover, [Play Store restricts the use of highly invasive permissions](https://support.google.com/googleplay/android-developer/answer/9888170) such as `MANAGE_EXTERNAL_STORAGE` which allows apps to opt out of scoped storage if they can't work with more privacy friendly approaches (like a file explorer). Apps that can't justify their use of this permission (which again has to be granted dynamically) may be removed from Play Store. This is where an app repository can actually be useful in their review process to protect end-users from installing poorly made apps that might compromise their privacy. Not that it matters much if these apps target very old API levels that are inclined to require invasive permissions in the first place...
## Conclusion: what should you do?
F-Droid **weakens the security model of Android substantially** for all of the reasons above. While they could easily make some improvements, I don't think F-Droid is in an ideal situation to solve all of these issues because some of them are **inherent flaws** in their architecture. That being said, I can only wish for them to improve.
F-Droid **weakens the security model of Android substantially** for all of the reasons above. While they could easily make some improvements, I don't think F-Droid is in an ideal situation to solve all of these issues because some of them are **inherent flaws** in their architecture. I'd also argue that their core philosophy is not aligned with some security principles expressed in this article. In any case, I can only wish for them to improve since they're one of the most popular alternatives to commercial app repositories, and are therefore trusted by a large userbase.
F-Droid is not the only way to get and support open-source apps. Sure, it can help you in finding one that you wouldn't have known existed otherwise. Many developers also publish their FOSS apps on the **Play Store** or their website directly. Most of the time, releases are available on **GitHub**, which is great since each GitHub releases page has an Atom feed. Nonetheless, I'd still recommend using **Play Store for top-notch security** as it does additional checks, and it is even perfectly usable on GrapheneOS with their [sandboxed Play services](https://grapheneos.org/usage#sandboxed-play-services) compatibility layer.
If downloading APKs from GitHub, you can use `apksigner` to validate the authenticity by comparing the certificate fingerprint against the fingerprint from another source (it wouldn't matter otherwise). This is how you may proceed to get the certificate:
F-Droid is not the only way to get and support open-source apps. Sure, it can help you in finding one that you wouldn't have known existed otherwise. Many developers also publish their FOSS apps on the **Play Store** or their website directly. Most of the time, releases are available on **GitHub**, which is great since each GitHub releases page has an Atom feed. If downloading APKs from regular websites, you can use `apksigner` to validate the authenticity by comparing the certificate fingerprint against the fingerprint from another source (it wouldn't matter otherwise). This is how you may proceed to get the certificate:
```
apksigner verify --print-certs --verbose myCoolApp.apk
@ -146,13 +147,15 @@ apksigner verify --print-certs --verbose myCoolApp.apk
Also, as written above: the OS pins the app signature for all profiles upon installation, and enforces it for app updates. In practice, this means the source doesn't matter as much after the initial installation.
For most people, I'd recommend just using **Play Store for top-notch security** as it does additional checks. Play Store isn't quite flawless, but emphasises the adoption of modern security standards which in turn encourages better privacy practices; as strange as it may sound, Google is not always doing bad things in that regard.
> Should I really care?
It's up to your threat model, and of course your personal preferences. Most likely, your phone won't turn into a nuclear weapon if you install F-Droid on it - and this is far from the point that this article is trying to make. Still, I believe this information will be valuable for anyone who values a **practical approach to privacy** (rather than an ideological one).
It's up to your threat model, and of course your personal preferences. Most likely, your phone won't turn into a nuclear weapon if you install F-Droid on it - and this is far from the point that this article is trying to make. Still, I believe the information presented will be valuable for anyone who values a **practical approach to privacy** (rather than an ideological one).
> But there is more malware in Play Store! How can you say that it's more secure?
It doesn't matter as you shouldn't really rely on any quality control to be the sole guarantee that a software is free of malicious or exploitable code. Play Store and even the Apple App Store may have a considerable amount of malware because a full reverse-engineering of any uploaded app isn't feasible realistically. However, they fulfill their role quite well, and that is all that is expected of them.
As explained above, it doesn't matter as you shouldn't really rely on any quality control to be the sole guarantee that a software is free of malicious or exploitable code. Play Store and even the Apple App Store may have a considerable amount of malware because a full reverse-engineering of any uploaded app isn't feasible realistically. However, they fulfill their role quite well, and that is all that is expected of them.
> Aren't open-source apps more secure? Doesn't it make F-Droid safer?
@ -160,7 +163,9 @@ You can still find and get your open-source apps elsewhere. And no, open-source
> Isn't Google evil? Isn't Play Store spyware?
Some people tend to exaggerate the importance of Google in their threat model, at the cost of pragmatism and security/privacy good practices. Play Store isn't spyware and can run unprivileged like it does on GrapheneOS (including unattended updates). Unprivileged apps have always had the ability to [query installed packages](https://developer.android.com/training/package-visibility) on a given user profile.
Some people tend to exaggerate the importance of Google in their threat model, at the cost of pragmatism and security/privacy good practices. Play Store isn't spyware and can run unprivileged like it does on GrapheneOS (including with unattended updates support). Unprivileged apps have always had the ability to [query installed packages](https://developer.android.com/training/package-visibility) on a given user profile.
**Play Store evidently has some privacy issues** given it's a proprietary service which requires an account (this cannot be circumvented), and Google services have a history of nagging users to enable privacy-invasive features. Again, some of these privacy issues can be mitigated by setting up the [Play services compatibility layer from GrapheneOS](https://grapheneos.org/usage#sandboxed-google-play) which runs Play services and Play Store in the regular app sandbox (the `untrusted_app` domain). This solution could very well be ported to other Android-based operating systems. If you want to go further, consider using a properly configured account with the least amount of personally indentifiable information possible (note that the phone number requirement appears to be region-dependent).
If you don't have Play services installed, you can use a third-party Play Store client called **[Aurora Store](https://auroraoss.com/)**. Aurora Store has some issues of its own, and some of them overlap in fact with F-Droid. Aurora Store somehow still requires [the legacy storage permission](https://gitlab.com/AuroraOSS/AuroraStore/-/blob/26f5d4fd558263a89baee4c3cbe1d220913da104/app/src/main/AndroidManifest.xml#L28-32), has yet to [implement certificate pinning](https://gitlab.com/AuroraOSS/AuroraStore/-/issues/697), has been known to sometimes retrieve wrong versions of apps, and [distributed account tokens](https://gitlab.com/AuroraOSS/AuroraStore/-/issues/722) over [cleartext HTTP](https://gitlab.com/AuroraOSS/AuroraStore/-/issues/734) until fairly recently; not that it matters much since tokens were designed to be shared between users, which is already concerning. I'd recommend against using the shared "anonymous" accounts feature: you should make your own throwaway account with minimal information.