Static XSS Vulnerability Detection via Context-Sensitive Sanitizer Analysis
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Solution Overview
Problem
Current methods for detecting missing sanitizers in web applications are insufficient to fully protect against cross-site scripting (XSS) attacks, as they are manual, error-prone, and do not account for context-sensitive sanitization requirements, leading to potential vulnerabilities.
Innovation Solution
A static analysis method that generates control flow graphs, sanitizer sequences, and output context sequences to determine if sanitizers are sufficient for the context in which data is rendered, using data flow analysis and string analysis to identify potential XSS flaws by simulating browser behavior and tracking nested contexts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If manual sanitizer placement is used in web application code, then developers can implement sanitization routines, but the process is highly error-prone and difficult to fully protect from XSS attacks
Solution Approach 1:
The system automatically performs sanitizer placement and verification without requiring manual developer intervention. The static analysis tool traverses the control flow graph, identifies output contexts, and determines appropriate sanitizer sequences autonomously, eliminating the error-prone manual process while maintaining reliable XSS protection.
Solution Approach 2:
The patent replaces the manual mechanical process of sanitizer placement with an automated computational system. The static analysis tool uses control flow graph traversal and context analysis algorithms to automatically determine where sanitizers are needed and what sequences to apply, substituting human effort with a systematic automated approach.
2Reliability
If taint analysis is used to detect missing sanitizers, then execution paths can be identified where malicious inputs reach sensitive instructions, but the analysis is insufficient to fully protect against XSS attacks
Solution Approach 1:
The patent segments the analysis into distinct phases: control flow graph construction, sanitizer sequence identification, output context sequence determination, and sufficiency verification. Each phase handles a specific aspect of the problem, making the overall complex analysis manageable and more effective at detecting context-sensitive XSS vulnerabilities.
Solution Approach 2:
The patent extends taint analysis by adding the dimension of output context analysis. Instead of only tracking data flow from input to output, the system analyzes the context in which outputs are rendered and verifies that sanitizer sequences are appropriate for each context, providing more comprehensive XSS protection.
3Measurement precision
If context-sensitive sanitization is implemented, then different contexts (HTML, JavaScript) can be sanitized appropriately, but sanitizer placement becomes more complex and error-prone
Solution Approach 1:
The system performs preliminary analysis to identify all output contexts and their required sanitizer sequences before verifying the actual sanitizer placement in the code. By determining the required sanitization for each output context in advance, the system can then systematically check whether the implemented sanitizers match the requirements, simplifying the verification process.
Solution Approach 2:
The patent implements a feedback mechanism where the static analysis tool compares the actual sanitizer sequences in the code against the required sanitizer sequences for each output context. When mismatches are found, the system reports potential XSS flaws, providing feedback to developers for correction and improving the overall sanitization accuracy.
Data Source
AI summary
A method for statically analyzing a web application program may include obtaining a control flow graph for the web application program. Each control flow graph node may correspond to a statement in the web application program. The method may further include obtaining a sanitizer sequence including one or more sanitizers followed by an output statement, obtaining a placeholder corresponding to the sanitizer sequence, and generating control flow paths including an output node that corresponds to the output statement. The method may further include generating documents for each control flow path. Each document may include a sanitized value corresponding to the output statement. The method may further include inserting the placeholder into each document at a location of the sanitized value, and reporting a potential cross-site scripting flaw when the sanitizer sequence is insufficient for the output context sequence of the sanitized value.