WordPress Charset SQL Injection Vulnerability

WordPress Charset SQL Injection Vulnerability

11 Dec. 2007

 Summary

WordPress is "a state-of-the-art semantic personal publishing platform with a focus on aesthetics, web standards, and usability".

It is found that the search function provided within WordPress fails to sanitize input based on different character sets. So if WordPress tries to query MySQL database using certain specific character sets, WordPress search function is exploitable using charset-based SQL injection.

Currently known character sets exploitable include Big5 and GBK. All of them may use backslash ('\') as part of multibyte character. WordPress with MySQL database created any other character sets fulfilling such property may also be exploitable.

Executing this attack alone results in exposure of all database content on web interface without need of authentication. However, if combined with other exploits (such as cookie authentication vulnerability in http://www.cl.cam.ac.uk/~sjm217/advisories/wordpress-cookie-auth.txt), any remote user can obtain WordPress admin privilege, resulting in server compromise.

Credit:
The information has been provided by Abel Cheung.
The original article can be found at: http://www.abelcheung.org/advisory/20071210-wordpress-charset.txt 

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 Details

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Vulnerable Systems:
 * WordPress version 2.3.1 and prior

Most database query in WordPress uses escape() method to sanitize SQL string, which is essentially filtering input via addslashes() function. However addslashes() fails to consider character set used in SQL string, and blindly inserts backslash before any single quote, regardless of whether such backslashes will form another valid character or not.

In proof of concept used in this advisory, two bytes 0xB327 is injected into search variable. After escaping string with escape(), a backslash (0x5C) is inserted before single quote (0x27), thus becoming 0xB35C27. However 0xB35C is a valid Big5 multibyte character, leaving the single quote behind, so SQL injection occurs. The same multibyte character is also valid under GBK encoding.

Inside SQL statement used within proof of concept, MD5 hashes of all users' passwords are selected from database, and presented as post title. With suitable SQL statement, any database field can be dumped in similar way.

Currently it is known that WordPress search function uses this insufficient method to sanitize database query. Possibly other database queries utilizing same method to filter user input can be equally susceptible.

However, note that WordPress sites using such character sets is not very common, since most default installation uses either latin1 or utf8 character set. Asian sites, in particular Chinese ones, are more likely vulnerable.

Although all WordPress versions before 2.3.1 are vulnerable, only WordPress 2.2 or above allows changing database query character set via WordPress configuration file (wp-config.php). For all versions below 2.2, modifying MySQL configuration to use those character sets is needed for exploit to be functional. The setting of WordPress HTML character set (adjustable within WordPress admin page) is irrelevant.

Relevant code is listed below. In wp-includes/query.php:

// If a search pattern is specified, load the posts that match
if ( !empty($q['s']) ) {
  ......
  foreach((array)$q['search_terms'] as $term) {
    $term = addslashes_gpc($term);
  ......
}

addslashes_gpc() is defined in wp-includes/formatting.php:

function addslashes_gpc($gpc) {
  ......
  return $wpdb->escape($gpc);
}


Finally, escape() method belongs to wp-includes/wp-db.php:

function escape($string) {
  return addslashes( $string ); // Disable rest for now, causing problems
  ......
}

Proof of concept
a. After WordPress installation, modify wp-config.php to make sure it uses certain character set for database connection (Big5 can also be used):
     define('DB_CHARSET', 'GBK');

b. http://localhost/wordpress/index.php?exact=1&sentence=1&s=%b3%27))) /**/AND/**/ID=-1/**/UNION/**/SELECT/**/1,2,3,4,5,user_pass,7,8,9,10, 11,12,13,14,15,16,17,18,19,20,21,22,23,24/**/FROM/**/wp_users%23

Workaround
Note: This vulnerability only exists for database queries performed using certain character sets. For databases created in most other character sets no remedy is needed.

a. It is recommended to convert WordPress database to use character sets not vulnerable to such SQL exploit. One such charset is UTF-8, which does not use backslash ('\') as part of character and it supports various languages.

b. Alternatively, edit WordPress theme to remove search capability.