Still Vulnerable Out of the Box: Revisiting the Security of Prepaid Android Carrier Devices

Prepaid Android smartphones present an attractive option since they can be used and discarded at will without significant financial cost. The reasons for their use are manifold, although some people may use them to dissemble their true identity. Prepaid smartphones offer value, but there may be an additional "cost" for their cheap price. We present an examination of the local attack surface of 21 prepaid Android smartphones sold by American carriers (and 11 unlocked smartphones). While examining these devices, we discovered instances of arbitrary command execution in the context of a "system" user app, arbitrary AT command execution, arbitrary file write in the context of the Android System (i.e., "system_server"), arbitrary file read/write in the context of a "system" user app, programmatic factory reset, leakage of GPS coordinates to a loopback port, numerous exposures of non-resettable device identifiers to system properties, and more. The only user interaction that our threat model assumes is that the user installs and runs a third-party app that has no permissions or only a single "normal" level permission that is automatically granted to the third-party app upon installation. The installed third-party app can leverage flaws in pre-loaded software to escalate privileges to indirectly perform actions or obtain data while lacking the necessary privileges to do so directly. Due to a wide range of local interfaces with missing access control checks and inadequate input validation, a third-party app’s behavior is not truly circumscribed by the permissions that it requests. Due to the common inclusion of pre-loaded software from Android vendors, chipset manufacturers, carriers, and vendor partners, exploit code can have significant breadth. The inter-app communication used to exploit these vulnerabilities may be difficult to classify as inherently malicious in general since it uses the standard communication channels employed by non-malicious apps. We pick up again where we left off from our DEF CON 26 talk … raiding the prepaid Android smartphone aisles at Walmart. We provide another snapshot on the state of security for Android carrier devices. In this talk, we examine 21 different prepaid Android smartphones being sold by the major American carriers, and we also cover 11 unlocked Android devices, which are primarily ZTE smartphones. We identified vulnerabilities in multiple layers of the Android software stack. For each discovered vulnerability, we step through the attack requirements, access vector, and attack workflow in order to help developers and bug hunters identify common software flaws going forward.