Oh-Pwn-VPN! Security Analysis of Open VPN-Based Android Apps

摘要

Free VPN apps have gained popularity among millions of users due to their convenience, and have been massively used for accessing blocked sites and preventing network eavesdropping. As a popular open-source VPN solution, OpenVPN is widely used by developers to implement their own VPN services. Despite the prevalence of OpenVPN, it can be insecurely customized and deployed by developers in lack of security guide. In this paper, we perform a systematic security analysis of 84 popular OpenVPN-based apps on the Google Play store. We analyze the deployment security of OpenVPN on Android from the aspects of client profile, code implementation, and permission management. Our experiment reveals three types of misconfigurations that exist in several apps: insecure customized protocols, weak authentication at the client side, and incorrect file permissions on Android. The misconfigurations found by us can lead to some serious attacks, such as VPN traffic decryption and Man-in-the-Middle attacks, endangering millions of users' privacy. Our work shows that, although OpenVPN protocol itself has withstood security analysis, insecure custom modification and configuration can still compromise the security of VPN apps. We then discuss potential causes of these misconfigurations and make practical recommendations for developers to securely deploy OpenVPN services.