WAF Protections Bypassed via JS Injection and Parameter Pollution for XSS Attacks

A groundbreaking security research has revealed that parameter pollution techniques combined with JavaScript injection can bypass 70% of modern Web Application Firewalls (WAFs), raising serious concerns about the effectiveness of current web security defenses.

Security researchers conducting autonomous penetration testing discovered a sophisticated method to circumvent WAF protections by exploiting fundamental differences in how web applications and security systems parse HTTP parameters.

The vulnerability was initially identified in an ASP.NET application protected by a highly restrictive WAF configuration.

The attack technique leverages HTTP Parameter Pollution, a method that exploits inconsistent handling of duplicate HTTP parameters across different web technologies.

When multiple parameters with the same name are present in a request, various frameworks process them differently – some concatenate values, others take only the first or last occurrence.

How the Attack Works

The researchers found that ASP.NET’s specific behavior of concatenating duplicate parameters with commas creates a dangerous opportunity for JavaScript injection.

When processing a query string like /?q=1’&q=alert(1)&q=’2, ASP.NET combines these values into 1′,alert(1),’2, which becomes valid JavaScript code when inserted into a string context.

This technique transforms seemingly innocuous parameter combinations into executable malicious code that bypasses traditional WAF detection methods.

JavaScript’s comma operator allows multiple statements to be executed in sequence, making the resulting code both syntactically valid and functionally dangerous.

The research team tested 17 different WAF configurations from major cloud providers and security vendors, including AWS WAF, Google Cloud Armor, Azure WAF, Cloudflare, and others. The results were alarming:

• Simple injection payloads achieved a 17.6% bypass rate
• Complex parameter pollution payloads bypassed 70.6% of tested configurations
• Only 5 WAF configurations successfully blocked all test payloads: Google Cloud Armor, Azure WAF, and three open-appsec configurations
• Three AWS WAF rule sets were completely bypassed by every payload tested

The study revealed that machine learning-based WAFs significantly outperformed signature-based systems.

Traditional WAFs relying on pattern matching struggled to detect attacks that exploit framework-specific parsing behaviors, while ML-powered solutions demonstrated superior defensive capabilities.

However, even advanced systems showed vulnerabilities. The researchers’ autonomous “hackbot” discovered additional bypasses, including a surprisingly simple payload that defeated Azure WAF: test\’;alert(1);//.

These findings highlight a critical gap in web application security strategies.

Organizations investing in expensive WAF technologies may remain vulnerable to attacks exploiting basic implementation differences between security systems and web applications.

The research emphasizes that WAFs should not be considered a complete solution for insecure code and that comprehensive security strategies must address vulnerabilities at multiple layers.

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