# Chrome Cache to XSS
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Technique taken [**from this writeup**](https://blog.arkark.dev/2022/11/18/seccon-en/#web-spanote)**.**
There are two important types of cache:
* **back/forward cache (bfcache)**
* ref. [https://web.dev/i18n/en/bfcache/](https://web.dev/i18n/en/bfcache/)
* It stores a complete snapshot of a page **including the JavaScript heap**.
* The cache is used for back/forward navigations.
* it has preference over disk cache
* **disk cache**
* ref. [https://www.chromium.org/developers/design-documents/network-stack/disk-cache/](https://www.chromium.org/developers/design-documents/network-stack/disk-cache/)
* It stores a resource fetched from the web. The cache **doesn't include the JavaScript heap**.
* The cache is also used for back/forward navigations to skip communication costs.
As a interesting point of disk cache, the **cache includes** not only the HTTP response rendered to a web page, but also **those fetched with `fetch`**. In other words, if you a**ccess the URL for a fetched** resource, the **browser will render the resource** on the page.
There is another important point. If both disk cache and bfcache are valid for an accessed page at back/forward navigations, the **bfcache has priority over the disk cache**. So, if you need to access a page stored in both caches but you want to use the one from the disk, you need to somehow **disable bfcache.**
### Disable bfcache
bfcache is disabled by [default options](https://github.com/puppeteer/puppeteer/blob/v19.2.0/packages/puppeteer-core/src/node/ChromeLauncher.ts#L175) of puppeteer.
Let's try the interesting behavior in this challenge.
Firstly, you have to disable bfcache[\[2\]](https://blog.arkark.dev/2022/11/18/seccon-en/#fn2). There are many conditions where bfcache is disabled, the list is:
* [https://source.chromium.org/chromium/chromium/src/+/main:out/mac-Debug/gen/third\_party/blink/renderer/core/inspector/protocol/page.cc?q=BackForwardCacheNotRestoredReasonEnum \&ss=chromium](https://source.chromium.org/chromium/chromium/src/+/main:out/mac-Debug/gen/third\_party/blink/renderer/core/inspector/protocol/page.cc?q=BackForwardCacheNotRestoredReasonEnum%20\&ss=chromium)
The easy way is to use `RelatedActiveContentsExist`.
* `RelatedActiveContentsExist`: The page opend with `window.open()` and it has a reference of `window.opener`.
* ref. [https://web.dev/i18n/en/bfcache/#avoid-windowopener-references](https://web.dev/i18n/en/bfcache/#avoid-windowopener-references)
Therefore, the following procedure reproduces the behavior:
1. Access a web page (E.g. `https://example.com`)
2. Execute `open("http://spanote.seccon.games:3000/api/token")`
* 
* The server returns a response with 500 status code.
3. In the opend tab, access `http://spanote.seccon.games:3000/`
* 
* Then, the response of `http://spanote.seccon.games:3000/api/token` is cached as a disk cache.
4. Execute `history.back()`
* 
* The cached JSON response is rendered on the page!
You can confirm that disk cache is used using DevTools in Google Chrome:\
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