SecureESFS: Sharing Android External Storage Files in a Securer Way

As an essential component on Android devices, External Storage is frequently used for sharing files between different apps. Therefore, compared to Internal Storage, the access control on the External Storage is usually very loose. However, a lot of sensitive files might be stored on the External Storage, which makes it an attractive target for the attackers. Since Android 10, a security mechanism named Scoped Storage has been introduced to protect the sensitive files on the External Storage. However, this mechanism is mainly used to protect the app-specific files, and can't support the sharing of sensitive files between trusted apps in a secure and flexible way. In this paper, we present a secure External Storage sensitive file sharing solution named SecureESFS. It first extends a Linux kernel security mechanism named ACL on the SDCardFS filesystem to protect the External Storage. With different ACL policy settings, the user can dynamically share sensitive files between trusted apps according to specific business needs. We also enforce the integrity protection on the ACL policies by checking the hash message authentication codes (HMAC) of these policies. Moreover, we design a transparent encryption mechanism in SecureESFS to protect the sensitive files on the External Storage, when the Android devices are physically accessed by the attackers, such as removing the SD card. For versions lower than Android 10, SecureESFS can provide independent protection and secure sharing for the sensitive files on the External Storage. For versions higher than Android 10, SecureESFS can achieve the secure sharing of sensitive files while Scoped Storage provides protection for the app-specific files. SecureESFS may also be used to enhance the security of the Scoped Storage mechanism. Experiments conducted on a prototype show that SecureESFS works well and incurs acceptable overhead.