Junjie Zhang

Website fingerprinting (WFP) could infer which websites a user is accessing via an encrypted proxy by passively inspecting the traffic characteristics of accessing different websites between the user and the proxy. Designing WFP attacks is crucial for understanding potential vulnerabilities of encrypted proxies, which guides the design of defensive measures against WFP. In this paper, we design a novel WFP attack against (popular) encrypted proxies that relay connections between the user and the proxy individually (e.g., Shadowsocks, V2Ray), and accordingly implement lightweight countermeasures to effectively defend against the attack. The attack features flow-context-aware and is both accurate and immediately deployable, because it fully considers the obstacle (dubbed training-testing asymmetry) that fundamentally limits the practicability of WFP and addresses the obstacle with built-in spatial-temporal flow correlation mechanism. We implement the countermeasure as middleboxes installed on both the client and server sides of encrypted proxies, without altering any existing infrastructures for compatibility. The middleboxes can obfuscate a website’s flow regularities across different visits. Large-scale experiments in real-world scenarios demonstrate that the WFP attack can generally achieve a detection rate above 98.8% with a false positive rate below 0.2%. The countermeasure forces the attack’s false positive rate to be above 0.2 and true positive rate to be below 0.9 with just five persistent TCP connections while introducing very limited bandwidth overhead (e.g., 0.49%) and almost-zero additional network latency.

Unrestricted file upload vulnerabilities enable attackers to upload and execute malicious scripts in web servers. We have built a system, namely UChecker, to effectively and automatically detect such vulnerabilities in PHP server-side web applications. Towards this end, UChecker first interprets abstract syntax trees (AST) of program source code to perform symbolic execution. It then models vulnerabilities using SMT constraints and further leverages an SMT solver to verify the satisfiability of these constraints. UChecker features a novel vulnerability-oriented locality analysis algorithm to reduce the workload of symbolic execution, an AST-driven symbolic execution engine with compact data structures, and rules to translate PHP-based constraints into SMT-based constraints by mitigating their semantic gaps. Experiments based on real-world examples have demonstrated that UChecker has accomplished a high detection accuracy. In addition, it detected three vulnerable PHP scripts that are previously unknown.

Exfiltrating sensitive information from smartphones has become one of the most significant security threats. We have built a system to identify HTTP-based information exfiltration of malicious Android applications. In this paper, we discuss the method to track the propagation of sensitive information in Android applications using static taint analysis. We have studied the leaked information, destinations to which information is exfiltrated, and their correlations with types of sensitive information. The analysis results based on 578 malicious Android applications have revealed that a significant portion of these applications are interested in identity-related sensitive information. The vast majority of malicious applications leak multiple types of sensitive information. We have also identified servers associated with three country codes including CN, US, and SG are most active in collecting sensitive information. The analysis results have also demonstrated that a wide range of non-default ports are used by suspicious URLs.