Researchers Exploit Parameter Pollution to Evade Web Application Firewalls via JS Injection

In a striking demonstration of security research ingenuity, a team of penetration testers has revealed a novel technique that exploits HTTP parameter pollution to evade stringent Web Application Firewalls (WAFs) and achieve cross-site scripting in ASP.NET applications.

By leveraging ASP.NET’s query-string concatenation behavior alongside JavaScript’s comma operator, researchers distributed payload fragments across multiple parameters, concealing malicious operations from conventional WAF detection rules.

Researchers Exploit Parameter Pollution to Evade Web Application Firewalls via JS Injection

The core bypass hinges on how ASP.NET handles duplicate query parameters. When identical parameter names appear more than once, the framework merges values into a single entry separated by commas.

By placing fragments that break out of a quoted context, inserting function calls, and then resuming the original quoted format into separate parameters, the final merged value becomes a valid execution sequence.

When this merged value is placed in a client-side assignment, the comma operator evaluates each fragment in order, triggering the malicious call without ever matching traditional cross-site scripting signatures.

Comprehensive Evaluation of Modern WAF Configurations

Seventeen leading WAF solutions spanning major cloud providers and security vendors—were assessed against four distinct payload variants:

• A basic quoted-break injection that clearly attempts to execute an alert operation
• A parameter-pollution payload using semicolon separators for statement chaining
• Pollution with line-feed obfuscation to bypass filters that ignore newline variations
• Advanced payloads generated by the team’s heuristic detection engine

Testing results exposed a widespread inability among traditional defenses to detect this technique.

Over 70 percent of configurations were bypassed by the most complex pollution payload, with only Google Cloud Armor under ModSecurity rules, Azure WAF using Microsoft’s default rule set 2.1, and all open-appsec tiers blocking three out of four attempts.

Conversely, three AWS-managed rule sets, along with regulations from several other vendors, failed to block any tested vector.

Autonomous Hackbot Uncovers Additional Bypasses

To further explore the remaining resilient configurations, researchers deployed an autonomous hackbot designed to mutate and test payload variants in real time. This bot discovered previously unnoticed bypasses in both Azure WAF and open-appsec:

• Azure WAF was defeated using a double-escaped backslash combined with a quote delimiter, exploiting a discrepancy between the firewall’s pattern analysis and the browser’s interpreter.
• open-appsec fell within seconds to a variant that replaced common function calls with alternative built-in calls. When machine-learning filters adapted to block known functions, the hackbot introduced dynamic function constructor usage to maintain execution.

Notably, Google Cloud Armor remained undefeated throughout automated testing, highlighting the strength of its combined signature and anomaly-based defenses.

Implications for Web Security Practices

This research underscores a critical lesson: defensive gateways that do not fully emulate application-side parsing are intrinsically vulnerable to pollution-based attacks.

While machine-learning–driven defenses offer enhanced anomaly detection, they can be swiftly outmaneuvered by automated agents capable of generating novel payload variants.

These findings reinforce the principle that firewalls cannot substitute for secure development practices, and that automation complements manual testing by uncovering edge-case vulnerabilities that human analysts might overlook.

Continuous integration of robust input validation, context-aware encoding, and rigorous code reviews remains essential to closing the gap that parameter pollution techniques exploit.

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