DVWA靶场通关----(6) Insecure CAPTCHA教程
Insecure CAPTCHA（不安全的验证码）
Insecure CAPTCHA（不安全的验证码），CAPTCHA全称为Completely Automated Public Turing Test to Tell Computers and Humans Apart，中文名字是全自动区分计算机和人类的图灵测试。关于这一项，其实是在验证流程出现了逻辑漏洞。
验证流程：
用户首先访问网页，触发页面的验证码的js模块，向谷歌服务器发起请求，谷歌服务器将验证码发给用户。用户输入验证码发送数据回去，这里发给的是访问网站的服务器，网站的服务器拿到验证码后，再去访问谷歌的服务器，谷歌的服务器会判断验证码是否正确，再将结果返回给网站服务器。
Insecure CAPTCHA主题：
Low
源码解析
<?php //第一阶段，验证身份，验证阶段为step if( isset( $_POST[ 'Change' ] ) && ( $_POST[ 'step' ] == '1' ) ) { // Hide the CAPTCHA form //隐藏验证码表单 $hide_form = true; // Get input //得到用户的新密码及确认新密码 $pass_new = $_POST[ 'password_new' ]; $pass_conf = $_POST[ 'password_conf' ]; // Check CAPTCHA from 3rd party //使用第三方进行身份验证 //recaptcha_check_answer($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 class的实例，ReCaptchaResponse类有2个属性 ： $is_valid是布尔型的，表示校验是否有效， $error是返回的错误代码。 $resp = recaptcha_check_answer( $_DVWA[ 'recaptcha_private_key'], $_POST['g-recaptcha-response'] ); // Did the CAPTCHA fail? if( !$resp ) { // What happens when the CAPTCHA was entered incorrectly //验证失败时 $html .= "<pre><br />The CAPTCHA was incorrect. Please try again.</pre>"; $hide_form = false; return; } else { // CAPTCHA was correct. Do both new passwords match? //验证通过时，匹配两次密码是否一致 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; } } } //第二阶段，检查两次密码是否一致，并更新密码 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! $pass_new = ((isset($GLOBALS["___mysqli_ston"]) && is_object($GLOBALS["___mysqli_ston"])) ? mysqli_real_escape_string($GLOBALS["___mysqli_ston"], $pass_new ) : ((trigger_error("[MySQLConverterToo] Fix the mysql_escape_string() call! This code does not work.", E_USER_ERROR)) ? "" : "")); $pass_new = md5( $pass_new ); // Update database $insert = "UPDATE `users` SET password = '$pass_new' WHERE user = '" . dvwaCurrentUser() . "';"; $result = mysqli_query($GLOBALS["___mysqli_ston"], $insert ) or die( '<pre>' . ((is_object($GLOBALS["___mysqli_ston"])) ? mysqli_error($GLOBALS["___mysqli_ston"]) : (($___mysqli_res = mysqli_connect_error()) ? $___mysqli_res : false)) . '</pre>' ); // Feedback for the end user echo "<pre>Password Changed.</pre>"; } else { // Issue with the passwords matching echo "<pre>Passwords did not match.</pre>"; $hide_form = false; } ((is_null($___mysqli_res = mysqli_close($GLOBALS["___mysqli_ston"]))) ? false : $___mysqli_res); } ?>
漏洞复现
按照源码分析，总共分为两个阶段：
第一阶段：对用户的身份进行验证，step为1，验证成功后才能进行密码修改
第二阶段：step为2，两次输入的密码一致，可以进行修改
考虑：直接跳过第一阶段，输入的密码一致，是否可以？
（1）输入两个一致的密码，直接使用burp抓包
（2）将step改为2，进行测试
（3）修改成功
这种可以参考CSRF模块，都可以去构造一个Web站点，做一个诱导页面，将step设置为2，诱导用户去点击。
Medium
源码解析
<?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' ], $_POST['g-recaptcha-response'] ); // Did the CAPTCHA fail? if( !$resp ) { // 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 == $pass_conf ) { // Show next stage for the user echo " // 对参数passed_captcha进行验证，如果通过身份验证，该参数就为true <pre><br />You passed the CAPTCHA! Click the button to confirm your changes.<br /></pre> <form action=\"#\" method=\"POST\"> <input type=\"hidden\" name=\"step\" value=\"2\" /> <input type=\"hidden\" name=\"password_new\" value=\"{$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! $pass_new = ((isset($GLOBALS["___mysqli_ston"]) && is_object($GLOBALS["___mysqli_ston"])) ? mysqli_real_escape_string($GLOBALS["___mysqli_ston"], $pass_new ) : ((trigger_error("[MySQLConverterToo] Fix the mysql_escape_string() call! This code does not work.", E_USER_ERROR)) ? "" : "")); $pass_new = md5( $pass_new ); // Update database $insert = "UPDATE `users` SET password = '$pass_new' WHERE user = '" . dvwaCurrentUser() . "';"; $result = mysqli_query($GLOBALS["___mysqli_ston"], $insert ) or die( '<pre>' . ((is_object($GLOBALS["___mysqli_ston"])) ? mysqli_error($GLOBALS["___mysqli_ston"]) : (($___mysqli_res = mysqli_connect_error()) ? $___mysqli_res : false)) . '</pre>' ); // Feedback for the end user echo "<pre>Password Changed.</pre>"; } else { // Issue with the passwords matching echo "<pre>Passwords did not match.</pre>"; $hide_form = false; } ((is_null($___mysqli_res = mysqli_close($GLOBALS["___mysqli_ston"]))) ? false : $___mysqli_res); } ?>
漏洞复现
与Low难度相比，增加了一个passed_capt，当passed_capt为true时就可以修改密码了
（1）同样使用burp进行抓包
（2）修改step为2，后面增加参数passed_captcha=true
（3）修改成
High
源码解析
<?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' ], $_POST['g-recaptcha-response'] ); //（通过身份验证条件）或者 （参数g-recaptcha-respon为hidd3n_valu3并且参数 HTTP_USER_AGE为 reCAPTC）就算是验证通过了 if ( $resp || ( $_POST[ 'g-recaptcha-response' ] == 'hidd3n_valu3' && $_SERVER[ 'HTTP_USER_AGENT' ] == 'reCAPTCHA' ) ){ // CAPTCHA was correct. Do both new passwords match? if ($pass_new == $pass_conf) { $pass_new = ((isset($GLOBALS["___mysqli_ston"]) && is_object($GLOBALS["___mysqli_ston"])) ? mysqli_real_escape_string($GLOBALS["___mysqli_ston"], $pass_new ) : ((trigger_error("[MySQLConverterToo] Fix the mysql_escape_string() call! This code does not work.", E_USER_ERROR)) ? "" : "")); $pass_new = md5( $pass_new ); // Update database $insert = "UPDATE `users` SET password = '$pass_new' WHERE user = '" . dvwaCurrentUser() . "' LIMIT 1;"; $result = mysqli_query($GLOBALS["___mysqli_ston"], $insert ) or die( '<pre>' . ((is_object($GLOBALS["___mysqli_ston"])) ? mysqli_error($GLOBALS["___mysqli_ston"]) : (($___mysqli_res = mysqli_connect_error()) ? $___mysqli_res : false)) . '</pre>' ); // Feedback for user echo "<pre>Password Changed.</pre>"; } else { // Ops. Password mismatch $html .= "<pre>Both passwords must match.</pre>"; $hide_form = false; } } else { // What happens when the CAPTCHA was entered incorrectly $html .= "<pre><br />The CAPTCHA was incorrect. Please try again.</pre>"; $hide_form = false; return; } ((is_null($___mysqli_res = mysqli_close($GLOBALS["___mysqli_ston"]))) ? false : $___mysqli_res); } // Generate Anti-CSRF token generateSessionToken(); ?>
漏洞复现
可以看到，验证过程已经不分步走了，都合在一个阶段里面了
服务器的验证逻辑是当$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。
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。
（1）burp抓包
（2）修改参数 $_POST[ 'g-recaptcha-response' ] == 'hidd3n_valu3' $_SERVER[ 'HTTP_USER_AGENT' == 'reCAPTCHA'
（3）修改成功
Impossible
源码解析
<?php 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 ); $pass_new = ((isset($GLOBALS["___mysqli_ston"]) && is_object($GLOBALS["___mysqli_ston"])) ? mysqli_real_escape_string($GLOBALS["___mysqli_ston"], $pass_new ) : ((trigger_error("[MySQLConverterToo] Fix the mysql_escape_string() call! This code does not work.", E_USER_ERROR)) ? "" : "")); $pass_new = md5( $pass_new ); $pass_conf = $_POST[ 'password_conf' ]; $pass_conf = stripslashes( $pass_conf ); $pass_conf = ((isset($GLOBALS["___mysqli_ston"]) && is_object($GLOBALS["___mysqli_ston"])) ? mysqli_real_escape_string($GLOBALS["___mysqli_ston"], $pass_conf ) : ((trigger_error("[MySQLConverterToo] Fix the mysql_escape_string() call! This code does not work.", E_USER_ERROR)) ? "" : "")); $pass_conf = md5( $pass_conf ); $pass_curr = $_POST[ 'password_current' ]; $pass_curr = stripslashes( $pass_curr ); $pass_curr = ((isset($GLOBALS["___mysqli_ston"]) && is_object($GLOBALS["___mysqli_ston"])) ? mysqli_real_escape_string($GLOBALS["___mysqli_ston"], $pass_curr ) : ((trigger_error("[MySQLConverterToo] Fix the mysql_escape_string() call! This code does not work.", E_USER_ERROR)) ? "" : "")); $pass_curr = md5( $pass_curr ); // Check CAPTCHA from 3rd party $resp = recaptcha_check_answer( $_DVWA[ 'recaptcha_private_key' ], $_POST['g-recaptcha-response'] ); // Did the CAPTCHA fail? if( !$resp ) { // 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(); ?>
漏洞复现
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，验证过程不再分成两部分了，同时要求用户输入之前的密码，进一步加强了身份认证。
