Second Security Audit Results

The second full security audit of F-Droid is complete. We are satisfied with the results, which confirmed again that the core security model and standard operations are solid. The audit pointed out issues in the core build process where we currently rely on manual review by trusted contributors to protect us. This audit also did show that we still have work to do to achieve our goal of keeping the Android client secure even when connected to a malicious server, for example, if an untrusted repository is manually added that was created by its operator to exploit. The full audit report is available: report_otf_fdroid.pdf (cached copy).

The audit was conducted by Radically Open Security, which is a natural partner for F-Droid since they share a focus on free software and open processes. Thanks to Open Tech Fund for finding the auditor and covering the costs of hiring them.

For more information about F-Droid’s security practices, see the documentation about the Security Model.

Reducing trust requirements

One key part of this audit was to push us further into reducing the level of trust required of the various pieces and participants in all the processes in F-Droid. Client/server systems are usually built around the assumption that the server operator is fully trusted. There are many trusted contributors working on F-Droid, and the people working on the core parts have a long and good track record. Nevertheless, it is very valuable to have a system that does not require trusting people and their computers. We aim to push all pieces of the F-Droid ecosystem to work according to the principle of least privilege.

New apps and updates are submitted to fdroiddata via merge request and git commits. Those changes are then stored forever in the git history. In the past, the code trusted everything in fdroiddata, since the commits are reviewed by humans for any malicious bits. This model has a very good track record in F-Droid as well as many other projects developed in the open by many people. Debian is a great example of this, since almost a thousand Debian Developers have commit privileges which could give them root on every machine running Debian.

This audit helped us ask the question: what kind privileges can be restricted or removed without changing the way contributors work? It also caught a number of issues where the code trusted the inputs from fdroiddata, where it had been committed and reviewed by humans, but that could then point to other code which could contain the exploits.

Reducing the trust given to various pieces reduces the stresses of contributing, and allows us as a community to open more things up to all sorts of contributors. If a contributor feels that their review is the last line of defence against exploiting F-Droid, that adds a lot of stress to the review.

Privileged Extension

There were no issues found in the Privileged Extension code or the interaction between F-Droid client and Privileged Extension.

Client F-Droid

This finding is labeling a standard practice in Android as insecure: “It is possible to install an SSL certificate on a device and run the application to capture its traffic on the proxy.” Android does not make it easy to accidentally include a certificate on the device, and it also puts up a constant notification when one is in place. We believe that following this recommendation would not noticeably improve real world security. It would eliminate legitimate use cases of installing a certificate on the device to inspect the traffic. For example: company firewalls that must inspect traffic to comply with regulations; or, auditing real world traffic of F-Droid during normal operation.

This could only be exploited to change the URL that is shown in the TOFU prompt to the user. The user is responsible for confirming what is listed in that prompt. It should be fixed, but does not seem urgent. It will be included when we overhaul the whole “App Repo” process.

This is an issue in Android itself. If an APK is signed with SHA256, that APK will not work on Android < 4.3. Using a larger RSA signing key would also be ideal. But since migrating signing keys is not supported in Android, it would be a very difficult project. F-Droid includes other layers of protection for validating APKs, including GPG signatures on APKs as well as the whole secure F-Droid distribution mechanism.

The HTTPS pinning mechanism could definitely be improved. Since HTTPS pinning is not essential to secure operation, it has never been a high priority. The integrity of the files server remains protected by the signature on the index files. For f-droid.org and the Guardian Project repo, the keys to check those signatures are built into the client.

Nearby/Swap

4.1.10 OTF-010 was a serious issue that was fixed as soon as it was found, thanks to Radically Open Security’s open audit process. It made it possible to put up phishing prompts via the swap process, but could only be exploited from the same network that the target device is on.

The security of the nearby swap is built largely on the limited scope of attacks, since it works only via Bluetooth or via local Wi-Fi restricted to the current subnet. It is also only active for a very limited time, while the user is directly using it. Then, for file integrity, the index signing key is trusted on the first use, then all files are verified against the verified index. While the security of the process can definitely be improved, it was designed to replace other much less secure methods that are widespread.

Ideally, all Bluetooth connections would use the native Bluetooth encryption and all IP connections would use HTTPS. The usability issues related to adding support for those would probably make the nearby/swap feature almost useless without a very substantial chunk of work.

fdroidserver

Moving towards least privilege

The build metadata files are used to describe an app and it’s build process. They are usually created by trusted parties (e.g. the person running the repo) or reviewed via merge requests. These findings would only have caused problems for repos that included metadata files from untrusted sources without reviewing them. For f-droid.org, the metadata files can only be downloaded to the production servers via git commits in fdroiddata. No attempts to exploit these have been seen in the git history nor merge requests. Most of these issues only affect the app building process, e.g. fdroid build. Repos made from binary APKs and fdroid update would only be affected by 4.1.3 OTF-003.

Removing weaknesses that could be chained into exploits

The data formats that were used allow for code execution from serialized code. The build metadata files are now loaded with with code serialization disabled, the apkcache file was switched from pickle to JSON, and we will switch the config.py file to YAML with safe loading.

Denial of Service

A few issues were found that allow contributors with trusted access to stop parts of the system from working.

Experimental drozer integration

As part of the Bazaar2 funding, an experimental feature for integrating Drozer into the fdroid release project was built. This feature is still alpha and is not deployed on any production setup. These issues are real issues, but there are no plans to fix them currently since there are no plans to move forward with the Drozer integration. We welcome contributions by those who want to see that happen.

f-droid.org website

Moving the whole website from Wordpress to a statically generated site paid large security dividends, there were no actionable issues found with the website. While both findings are correct in that malicious Javascript could be injected into descriptions, both the f-droid.org website and Android client do not let Javascript run from the descriptions. The f-droid.org website protects against malicious Javascript injection with a strict HTTP Content Security Policy. F-Droid client never runs CSS, Javascript, or dangerous HTML tags since it displays the HTML via Html.fromHtml() with image loading disabled.

Ideally, the index generation process in fdroid update would have robust CSS/HTML/Javascript checks so malicious injection is prevented by both the index providers and consumers. This would increase the “herd immunity” of everyone working with data generated using any of the F-Droid tools.

The links are only displayed in the “Links” section. Since there is a human review of those URLs, and those URLs are committed to git, we have left those to be open without validation. There are also other URLs like home page, issue tracker, etc. I suppose we should show the full URL in the UI if they don’t match a well known pattern. Google Play also includes a link to a website that the app author provides.

One idea for improvement here would be to scan these URLs with the Google Safe Browsing API.

Créateur de dépôts

Repomaker is a web tool to make it really easy to set up your own F-Droid repositories. It is a new project that should still be considered beta. There was one issue with high severity (CVE-2018-7753) found in an upstream library, Mozilla’s bleach. It was reported to Mozilla and promptly fixed. There were three other low impact findings which have been fixed.

Findings that are not relevant or too pedantic

These are issues which are not technically wrong, but are not relevant in the context of how F-Droid is using the code.

The BluetoothServer is only ever used to serve files to F-Droid client’s Bluetooth handling, which is nothing like a browser and definitely does not execute HTML/CSS/Javascript or display websites. It would not be possible to ever display this information in a browser without custom code in to receive it.

The download process only downloads files to a private directory. The file validation happens during the install process for all files. After download, the install process validates the SHA-256 against what is in the signed file index.

This code is only used to make JAR signatures on the index.jar for the nearby/swap process. Android protects critical files from deletion with file permissions. Only the index.jar file is ever handled by the zipsigner code.

The NanoHTTPD built-in webserver is only ever used to serve public information. Also, the parts of the code in the NanoHTTPD library that creates temp files is likely never used in F-Droid.

Obfuscation here is used not as a security mechanism, but rather as a lightweight anti-piracy mechanism. The obscured information ultimately need to be available to the app to use it, so there would always be a way to extract the obscured information, as we have seen in the decades of software copy protection arms races.

Filenames in Java are not executed by a shell. Only the / character is problematic with filesystems and file handling code. Ideally, all files produced by F-Droid software would end up safe no matter where or how they are used, but it seems that the only manageable way to sanitize files is on the endpoints that are consuming the files (e.g. fdroidclient or f-droid.org).