Introduction

What if your private dashboard got cached for the world to see? Welcome to the world of Web Cache Deception (WCD) a subtle but powerful vulnerability that can expose sensitive user data to anyone on the internet.

Modern websites use caching to improve performance and reduce server load. However, when misconfigured, this speed boost can accidentally turn into a privacy nightmare.

What Is Web Cache Deception?

Web Cache Deception is a technique where attackers trick caching servers (like CDNs or reverse proxies) into storing user-specific pages. These cached pages can then be accessed by unauthenticated users, leaking private content.

Here's the trick: caching systems often rely solely on the URL path to determine what to cache. If an attacker appends a static file extension (e.g., `.css`, `.jpg`, `.txt`) to a sensitive page like `/dashboard`, the CDN might cache the entire authenticated response — and serve it to anyone who asks.

Real-World Example (Walkthrough)

  1. Log in to a web application and visit `/account`.
  2. Modify the URL to `/account/random.jpg`.
  3. Observe the response: the page still loads your personal dashboard.
  4. If the server doesn't check the extension or apply proper cache headers, it sends the same response.
  5. The CDN caches it, associating the sensitive data with the public URL `/account/random.jpg`
  6. Now, **anyone** visiting that URL sees your cached content — no login needed.

Impact

An attacker could:

  • Access private user dashboards
  • View sensitive information (PII, emails, CSRF tokens)
  • Trigger account-related actions based on cached data
  • Leak internal URLs or user history

This type of bug is especially dangerous on multi-user platforms like SaaS tools, e-commerce, and dashboards.

How to Test for Web Cache Deception

Tools Needed: Burp Suite, browser DevTools, curl

Payloads to try:

  • /profile.css
  • /dashboard/image.jpg
  • /user/settings.txt
  • /account.html

Steps

  1. Log in to a user account.
  2. Access a sensitive endpoint (like `/account`, `/settings`, `/orders`).
  3. Append a fake static file extension.
  4. Observe the response: does it return the same content?
  5. Log out or use incognito mode and revisit the same fake-URL.
  6. Check for caching headers:
  • X-Cache: HIT
  • Cache-Control

If you see your user data exposed even after logout, you've likely found a WCD vulnerability.

How to Find WCD Bugs Efficiently

  • Look for CDN-backed websites: Use 'whois', HTTP headers, or 'dig' to detect CDN usage.
  • Try common endpoints: `/dashboard`, `/account`, `/orders`, `/cart`
  • Use a variety of extensions: .jpg, .css, .pdf, .js, .ico
  • Check HTTP responses: Use Burp to analyze headers and detect caching behavior
  • Test on multi-user platforms: These have more dynamic content and higher risk

How to Prevent Web Cache Deception

For Developers and Security Teams

  • Add proper cache headers: `Cache-Control: private, no-store
  • Avoid serving user-specific content from endpoints that look static
  • Disallow requests with suspicious extensions on sensitive paths
  • Use middleware to check for unexpected URL patterns
  • Test your cache behavior with simulated user requests

Conclusion

This \$0-cost, high-impact bug shows how subtle misconfigurations can lead to massive data exposure. With just a crafted URL and a CDN misstep, private dashboards and user data can be made public without the victim ever knowing.

Web Cache Deception is a classic example of the intersection between performance optimization and security oversight. Always treat integration layers like caches as part of your security boundary.

Whether you're a developer, a security researcher, or a bug bounty hunter — understanding WCD is essential to safeguarding user data in the caching era.

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Monika

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Monika