六、Insecure CAPTCHA：
1．漏洞原理：
Insecure CAPTCHA（不安全的验证码），CAPTCHA全程为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。这一模块的内容叫做不安全的验证流程比较好，主要是验证流程出现了逻辑漏洞，而不是谷歌的验证码有问题。这一模块验证码使用的是Google提供的reCAPTCHA服务，以下是验证具体的流程。
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$privey是服务器申请的private key，$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。
2．注意事项：
这里打开DVWA靶场的Inseure CAPTCHA的时候，会发现上方有一行这样的报错“reCAPTCHA API key missing from config file: D:\phpstudy_pro\WWW\DVWA\config\config.inc.php”这种情况。
出现这种情况是因为使用reCAPTCHA没有申请密钥，因此需要手动填入密钥，打开提示的配置文件找到文件“config.inc.php”，然后我们找到如下：
$_DVWA[ 'recaptcha_public_key' ] = ' '; $_DVWA[ 'recaptcha_private_key' ] = ' ';
然后将他们改为如下，然后就可以保存了，最后就可以开始进行实战项目了。
$_DVWA[ 'recaptcha_public_key' ] = '6LdK7xITAAzzAAJQTfL7fu6I-0aPl8KHHieAT_yJg'; $_DVWA[ 'recaptcha_private_key' ] = '6LdK7xITAzzAAL_uw9YXVUOPoIHPZLfw2K1n5NVQ';
3.实战：
（1）Low：
代码分析：
<?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 $html .= " <pre><br />You passed the CAPTCHA! Click the button to confirm your changes.<br /></pre> <form action=\"#\" method=\"POST\"> <input type=\"hidden\" name=\"step\" value=\"2\" /> <input type=\"hidden\" name=\"password_new\" value=\"{$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! $pass_new = ((isset($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 $html .= "<pre>Password Changed.</pre>"; } else { // Issue with the passwords matching $html .= "<pre>Passwords did not match.</pre>"; $hide_form = false; } ((is_null($___mysqli_res = mysqli_close($GLOBALS["___mysqli_ston"]))) ? false : $___mysqli_res); } ?>
这里修改密码一共有两个步骤，第一步是CAPTCHA的验证环节，第二步是将参数POST到后台。由于两步操作完全是分开的，没有联系，于是我们可以忽略第一步的验证，直接提交修改申请。两个步骤对应的step参数不同，可以通过抓取报文并且修改step，来实现的验证绕过。代码没有对CSRF进行任何的防护，可以利用CSRF进行攻击。
我们开始进行攻击，我们思路就是将第一步跳过直接进行第二步。所以我们这里将输入两个一致的密码不进行验证，直接用Burpsuite进行抓包操作。
将“step1”修改为“step2”重新发送后，得到以下界面，说明修改成功。
（2）Medium：
代码分析：
<?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 $html .= " <pre><br />You passed the CAPTCHA! Click the button to confirm your changes.<br /></pre> <form action=\"#\" method=\"POST\"> <input type=\"hidden\" name=\"step\" value=\"2\" /> <input type=\"hidden\" name=\"password_new\" value=\"{$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 $html .= "<pre>Password Changed.</pre>"; } else { // Issue with the passwords matching $html .= "<pre>Passwords did not match.</pre>"; $hide_form = false; } ((is_null($___mysqli_res = mysqli_close($GLOBALS["___mysqli_ston"]))) ? false : $___mysqli_res); } ?>
我们可以看出基于Low级别，在第二步增加了第一步是否通过的验证，即判断参数passed_captcha是否为真。Passed_captcha参数是通过POST提交的，整个请求也就是POST请求，故可以人为加上此参数。
接下来我们开始攻击，一共也是分两步，但是不同于Low，我们这次使用Burpsuite开始修改报文，将步骤直接调整到第二步，第二步的验证伪造为已验证。然后我们直接加入passed_captcha参数，混入POST参数提交。将“step=1”改为“step=2”，然后加上“&passed_capcha=true”即可。
放包以后，我们可以观察到页面显示“Password Change”成功即是完成。
（3）High：
代码分析：
<?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'] ); 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 $html .= "<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(); ?>
High级别将验证流程合并，通过连续的判断将两个步骤相同的部分合并，避免第一步验证的直接改参绕过。加入了token机制，有效防止了CSRF漏洞攻击，下面不再做攻击页面。
我们开始攻击，看到了后端代码发现了，及时不验证也有机会绕过验证，于是针对g-recaptcha-response和HTTP_USER_AGENT操作。
同样不验证，直接提交请求对相关参数进行抓包修改。
（4）Impossible：
代码分析：
<?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 $html .= "<pre><br />The CAPTCHA was incorrect. Please try again.</pre>"; $hide_form = false; } 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! $html .= "<pre>Password Changed.</pre>"; } else { // Feedback for the end user - failed! $html .= "<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(); ?>
Impossible的代码作为防御模板，使用Anti-CSRF token机制防御CSRF攻击。验证步骤合并为同一步，无需分开，使得验证环节无法绕过。要求输入修改之前的密码，攻击者无法绕过。利用PDO技术输入内容过滤，防止了sql注入。