How Hackers Are Bypassing Nonce-Based CSPs Using Browser Caching Tricks

A Wake-Up Call for Web Security Professionals

A groundbreaking security discovery has revealed a novel method to bypass nonce-based Content Security Policies (CSP), striking at the heart of one of the most trusted defenses against cross-site scripting (XSS) attacks. This technique cleverly leverages the interplay between browser caching—specifically the disk cache and back/forward cache (bfcache)—and the fragile assumptions developers often make about nonce handling. It shows that even CSPs configured with nonces, long considered a gold standard, can fall short when browser quirks and injection vectors come into play. By combining CSS injection, clever nonce exfiltration, and the manipulation of cached content, attackers can execute unauthorized scripts that appear perfectly legitimate to the browser.

Breaking Down the Attack: A Technical Overview

Bypassing Trust: The Hidden Weakness in Nonce CSP

This newly exposed technique breaks through nonce-based CSPs by abusing how browsers store and serve cached content. Under normal conditions, CSPs with nonces protect web pages by allowing only inline scripts that carry a valid, freshly generated token (nonce). These nonces are designed to change with every page load, making them difficult for attackers to guess or reuse. However, researchers have shown that when an attacker can inject HTML via <style> or <link rel="stylesheet">, they can exploit CSS selectors to extract the nonce from the page’s meta tags.

Exfiltration Through Style Abuse

By crafting a malicious CSS injection, the attacker leverages background image URLs to exfiltrate the nonce one character at a time to their server. This stealthy process occurs without executing any script, staying within the confines of allowed styles. Once the attacker knows the full nonce, they can construct a malicious script and insert it into a dynamic portion of the page—such as a user profile or session—using Cross-Site Request Forgery (CSRF) or a fetch-based update.

Hijacking the Browser Cache

Here’s where things get even more dangerous. The attacker uses browser navigation tricks to load an older, cached version of the page from disk cache—one that still uses the previously leaked nonce. Because the new script payload now includes the correct nonce, the CSP treats it as legitimate and allows it to execute. This combination of stale cached HTML and freshly injected dynamic content results in successful XSS that completely bypasses the nonce-based protection.

Why This Matters

This discovery isn’t just theoretical—it has serious implications for any web application relying solely on nonce-based CSPs. It reveals that caching mechanisms, particularly disk cache and bfcache, can undermine what many developers consider secure. Even a well-intentioned security policy becomes a liability when developers don’t tightly couple nonce generation with dynamic content or when they expose nonce values in ways that CSS selectors can access. The research underscores the importance of understanding how every browser feature—from styling to navigation history—can be weaponized in the wrong hands.

What Undercode Say:

The Illusion of Perfect CSP

For years, nonce-based CSPs have been the go-to defense against inline script attacks. They work under the premise that if you generate a new, unique token for each page load, unauthorized scripts can’t run. But what this research exposes is a critical flaw in that logic—one that arises from the complex, often opaque behaviors of browser caching.

CSS Injection as a Stealth Tool

CSS has long been underestimated as a security risk. But this bypass shows that CSS can not only leak sensitive data, it can do so in a way that flies under the radar. By encoding the nonce into background image requests, attackers exploit CSS purely as a data exfiltration tool. It’s elegant, efficient, and invisible to most logging systems.

Cache: The Double-Edged Sword

Browser caching improves performance, but in this case, it also reintroduces old HTML and scripts back into the execution environment. If the attacker times it right, they can load a version of the page that still accepts a previously valid nonce, while dynamically inserting a new, malicious script with that same value. Since both elements are technically valid within CSP rules, the browser doesn’t raise any alarms.

The Critical Role of CSRF and Dynamic Updates

This exploit also depends on the attacker’s ability to update user-facing content without reloading the full page. Using CSRF vulnerabilities or fetch-based asynchronous requests, they inject a script bearing the stolen nonce. It only needs to appear once in a dynamic container (like a profile or comment section) to activate when the cached page is revisited.

Lessons for Developers

This attack reinforces a key lesson in web security: defenses must be holistic, not just token-based. Developers should avoid placing CSP nonces in places accessible via CSS selectors, tightly couple dynamic content and CSP headers, and implement robust CSRF protections. Furthermore, relying on browser behavior as a layer of security is a fragile proposition—browsers prioritize speed and experience, not always airtight security.

A Reminder of Browser Quirks

Browsers are built for performance and flexibility, not inherently for secure-by-default behavior. Their caching mechanisms—especially disk cache and bfcache—are optimized to speed up navigation, but they can betray developers by retaining and serving stale yet valid pages. When developers aren’t aware of this, their applications become vulnerable in subtle but devastating ways.

The Need for Continuous CSP Testing

Security teams must rethink how they test CSP. It’s not enough to see if the policy blocks unauthorized scripts; they must also evaluate how caching, asynchronous updates, and navigational quirks affect real-world execution. Threat models should account for the lifecycle of the page—not just the initial load.

Moving Forward with Better Practices

Emerging best practices include disabling bfcache for sensitive pages, invalidating cache aggressively, using strict-referrer policies to prevent leakage, and embedding nonce values in ways that are inaccessible to selectors. Tools that monitor and simulate browser caching behavior could also become essential for modern web security testing.

🔍 Fact Checker Results:

✅ Valid Technique: The described method has been independently verified by security researchers
✅ Real-World Feasibility: Requires specific conditions (CSS injection + CSRF), but is achievable
❌ Not a Browser Bug: This is exploitation of expected browser behavior, not a vulnerability

📊 Prediction:

More real-world attacks will emerge exploiting similar interactions between caching and CSP.
Expect browser vendors to introduce new cache controls or CSP flags to reduce this risk.
Security tools will evolve to simulate and detect nonce-based bypass scenarios more effectively.

References:

Reported By: cyberpress.org
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