In our previous blog posts, we explored Android Binder’s intricacies, from exploiting a vulnerability (CVE-2023-20938) for kernel code execution to examining its inner workings. In this post, we shift our focus to finding vulnerabilities in the Binder kernel driver through fuzzing.

This post provides a practical guide to fuzzing the Binder kernel driver using the Linux Kernel Library (LKL). To demonstrate the advantages of this approach, we first explore existing fuzzing efforts using Syzkaller, a state-of-the-art kernel fuzzer, and highlight its challenges for this use case. Then, we dive into how LKL overcomes these limitations and our improvements, such as randomized scheduling.

In our last blog, we talked about Binder CVE-2023-20938 and how we exploited it to get kernel code execution. As you may have already noticed, exploiting this issue is not straightforward. While it is often true that kernel race conditions are notoriously tricky to exploit, the intricacy of the Binder driver’s implementation adds another layer of complexity.

This blog post dives deeper into the inner workings of Binder, including the lifecycles of its objects and the underpinnings that keep everything running smoothly across Android. We will also introduce the libdevbinder library we developed during our engagement. This library provides simpler interfaces for researchers interact with the Binder driver for the purpose of learning and experimentation. Binder is an incredibly complicated target! You’ll notice the length of this blog post reflects that complexity, and while we try to cover salient points from the perspective of security research here, there is always more to learn. The Android Red Team believes in empowering the security researcher community; sharing knowledge helps improve security across the entire ecosystem. This blog post aims to help security researchers (like you) learn more about Binder. If you learn enough to find some vulnerabilities, our goal has been achieved (oh and please, let us know!).

At OffensiveCon 2024, the Android Red Team gave a presentation (slides) on finding and exploiting CVE-2023-20938, a use-after-free vulnerability in the Android Binder device driver. This post will provide technical details about this vulnerability and how our team used it to achieve root privilege from an untrusted app on a fully up-to-date (at the time of exploitation) Android device. This vulnerability affected all Android devices using GKI kernel versions 5.4 and 5.10.

This vulnerability is fixed and the patches were released as part of the Android Security Bulletin–February 2023 and July 2023 (more details in the remediation section of the blog).