Beneath the Surface: An Analysis of OEM Customizations on the Android TLS Protocol Stack

The open-source nature of the Android Open Source Project (AOSP) allows Original Equipment Manufacturers (OEMs) to customize the Android operating system, contributing to what is known as Android fragmentation. Google has implemented the Compatibility Definition Document (CDD) and the Compatibility Test Suite (CTS) to ensure the integrity and security of the Android ecosystem. However, the effectiveness of these policies and measures in warranting OEM compliance remains uncertain. This paper empirically studies for the first time the nature of OEM customizations in the Android TLS protocol stack, and their security implications on user-installed mobile apps across thousands of Android models. We find that approximately 80% of the analyzed Android models deviate from the standard AOSP TLS codebase and that OEM customizations often involve code changes in functions used by app developers for enhancing TLS security like end-point and certificate verification. Our analysis suggests that these customizations are likely influenced by factors such as manufacturers’ supply chain dynamics and patching prioritization tactics, including the need to support legacy components. We conclude by identifying potential root causes and emphasizing the need for stricter policy enforcement, better supply chain controls, and improved patching processes across the ecosystem.