I've been spending a lot of time lately looking into how CDNs and Load Balancers talk to origin servers. It's honestly kind of wild how much we trust these middle-men to "just work." One of the coolest (and most dangerous) things I've run into is Web Cache Deception.

Basically, it's a way to trick a server into caching a user's private info as a public file. Here is how it works.

The Technical Flow

Let's say we have a target URL: https://example.com/user/profile-info. This page has some pretty sensitive PII (Personally Identifiable Information).

If I, as an attacker, send a request to:

GET /user/profile-info/nonexistent.css

Here is the "misunderstanding" that happens:

1. The Cache Server gets suspicious (in a good way)

The cache sees that .css extension at the end of the URL. It's programmed to think, "Hey, CSS files are static assets that don't change from user to user. I should save this to speed things up." It checks its memory, doesn't find it, and requests the file from the origin.

2. The Origin Server is too flexible

This is where the bug actually lives. The origin server gets the request but it's configured to ignore "path suffixes." It sees /user/profile-info/ and just ignores the .css part, thinking it's just a non significent parameter or a weird extra path. It sends back the actual profile data for the logged-in user.

3. The "Theft" happens in the storage

The Cache Server receives that private profile data. But remember, it still thinks it's looking at a CSS file. So, it saves that whole response (the private info!) under the key /user/profile-info/nonexistent.css.

Now, the cache is "poisoned." If I visit that exact URL, the cache server goes, "Oh, I have that CSS file right here!" and serves me the previous user's private data.

How to hunt for this

If you're out there bug hunting, this is a relativly simple but high-impact find. Here is my basic workflow:

  1. Find the sensitive stuff: Look for any page that displays account details, API keys, or settings.
  2. Add the "Static" suffix: Try adding things like /test.css, /.js, or /.jpg to the end of the URL.
  3. Check the response: Does the page still load correctly even with that extra junk in the URL? If yes, the server is ignoring the suffix.
  4. Watch the Headers: Look for X-Cache or CF-Cache-Status. You're looking for a MISS on the first try and a HIT on the second.
  5. The Final Test: Log in as "User A" and hit the weird URL. Then, open a totaly separate browser profile (incognito or another account) where you aren't logged in. Paste the URL. If you see User A's info… well, congratulations, you found a Web Cache Deception specifically a static extension cache rule misconfiguration.

Wait, so how do we fix this?

Fixing Web Cache Deception can be a bit of a headache because it usually involves coordination between the dev team and the DevOps/Infra team. You can't just flip a single switch most of the time.

Here's the best ways to stop this from happening:

  • Set your Cache-Control headers right: The most robust way to fix this is to tell the cache explicitly not to store sensitive pages. If the origin server responds with Cache-Control: no-store or private, most modern CDNs (like Cloudflare or Akamai) will respect that and won't save the data.
  • Turn off "Path Suffix" matching: On the web server side (like Nginx or Apache), you should configure it so that /profile is a valid path, but /profile/anything-else returns a 404. If the origin returns an error, the cache won't save the response.
  • Use specific "Cache-Only" directories: A lot of high-security sites put all their actual static assets in a specific folder like /static/ or /assets/. Then, they configure the CDN to only cache things that come from those specific paths.
  • Check your Framework defaults: Some older frameworks (like older Spring versions) used to do "Suffix Pattern Matching" by default. Basically, they thought they were being helpfull by making /api/users.json and /api/users do the same thing. Make sure that feature is turned off.

Wrapping up

Web Cache Deception is honestly one of my favorite bugs because it relies on two systems both doing their jobs "correctly" but failing to communicate. It's a classic logic flaw.

If you're building a web app, don't just rely on your CDN's default settings. Take five minutes to audit what's actually being cached — your users (and their PII) will thank you.

(a small note : guys if this sounds too much AI then pardon me. i was taking notes while learning about this vulnerability and my Grammar and stuff was a little too bad for a blog post so i pasted my notes which i myself wrote to Google Gemini (no sponsor) and told it to improve the writing and then i posted it if you guys want the original stuff let me know at nullifiedsec@nullifiedsec.com)