=== WordPress Charset SQL Injection Vulnerability ===

Release date: 2007-12-10
Last modified: 2007-12-12
Source: Abel Cheung <abelcheung at gmail dot com>
Affected version: WordPress <= 2.3.1
Exploit type: Remote
Risk: Moderate
CVE: pending
Reference: http://www.abelcheung.org/advisory/20071210-wordpress-charset.txt


1. Summary
2. Detail
3. Proof of concept
4. Workaround


1. Summary

  Quoting from http://wordpress.org/:
    WordPress is a state-of-the-art semantic personal publishing platform
    with a focus on aesthetics, web standards, and usability.
    What a mouthful. WordPress is both free and priceless at the same time.

  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.


2. Detail

  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
  ......
}


3. 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


4. 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.
     Even if database charset conversion is inconvenient or impossible, use UTF-8
     as DB_CHARSET setting to avoid sending query using vulnerable multibyte charset.
  b. Alternatively, modify WordPress core (query.php) to remove search capability.


ChangeLog:
- 2007-12-12
* Modify workaround (thanks to Florian Sander for suggestion)

# milw0rm.com [2007-12-11]