Splendor: Static Detection of Stored XSS in Modern Web Applications

In modern websites, stored Cross-Site Scripting (XSS) is the most dangerous XSS vulnerability, which can store payloads in the web system and be triggered directly by the victim. Database (DB) as the most commonly used storage medium for data on websites is therefore also the most common place where stored XSS occurs. Due to the modularity of modern programming architectures, the complex underlying database operations will often be encapsulated and abstracted as a Data Access Layer (DAL) to provide unified data access services to the business layer. The heavy use of Object-Oriented (OO) and dynamic language features involved in the encapsulation makes it increasingly challenging for static taint analysis tools to understand how tainted data flows between the source code and the exact locations in database.

In this paper, we propose the first static analysis framework for detecting stored XSS in modern web applications using DAL and implement a prototype Splendor for PHP code analysis. The highlight in the framework is the design of a heuristic but precise token-matching method to locate the flows of taint data between database and source code. The precisions of the identified DB read and write (R/W) locations are 91.3% and 82.6%, respectively. With the identified R/W locations, the disconnected taint paths can be statically stitched to obtain a complete taint propagation path of stored XSS. Comparisons with existing works on 5 real-world applications and large-scale experiments on PHP web applications in Github show that Splendor significantly outperforms both the state-of-the-art static and dynamic approaches on stored-XSS detection, and detects 17 zero-day vulnerabilities.