DVWA——Insecure CAPTCHA (不安全的验证码)
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。
这一模块的验证码使用的是Google提供reCAPTCHA服务,
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 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。
Low
源码
<?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'], $_POST['g-recaptcha-response'] ); // Did the CAPTCHA fail? if( !$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 == $pass_conf ) { // Show next stage for the user echo " <pre><br />You passed the CAPTCHA! Click the button to confirm your changes.<br /></pre> <form action=\"#\" method=\"POST\"> <input type=\"hidden\" name=\"step\" value=\"2\" /> <input type=\"hidden\" name=\"password_new\" value=\"{$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 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); } ?>
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
直接输入密码提交,利用Burpsuite抓包
因为我们没有FQ,所以没能成功显示验证码,发送的请求包中也就没recaptcha_challenge_field、recaptcha_response_field两个参数,将step改为2即可
Medium
源码
<?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' ], $_POST['g-recaptcha-response'] ); // Did the CAPTCHA fail? if( !$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 == $pass_conf ) { // Show next stage for the user echo " <pre><br />You passed the CAPTCHA! Click the button to confirm your changes.<br /></pre> <form action=\"#\" method=\"POST\"> <input type=\"hidden\" name=\"step\" value=\"2\" /> <input type=\"hidden\" name=\"password_new\" value=\"{$pass_new}\" /> <input type=\"hidden\" name=\"password_conf\" value=\"{$pass_conf}\" /> <input type=\"hidden\" name=\"passed_captcha\" value=\"true\" /> ////提交passed_captcha参数 <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' ] ) {//验证step1中提交的参数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); } ?>
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。
关键代码已经注释标红。
漏洞利用
我们通过Burpsuite抓包,将其中的step=1更改为step=2,将Change=Change更改为Change=Change&passed_captcha=true即可绕过。
High
源码
<?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' ) ) { //当验证码错误并且提交的参数有一个不正确时,就会提示验证不通过 // 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 ) { $pass_new = ((isset($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>"; $hide_form = false; } } ((is_null($___mysqli_res = mysqli_close($GLOBALS["___mysqli_ston"]))) ? false : $___mysqli_res); } // Generate Anti-CSRF token generateSessionToken(); ?>
可以看到,服务器的验证逻辑是当$resp(这里是指谷歌返回的验证结果)是对的,或者参数recaptcha_response_field等于hidd3n_valu3并且http包头的User-Agent参数等于reCAPTCHA时,就认为验证码输入正确,满足其一即可,验证码这里肯定是不对的,所以我们只要构造参数即可。
漏洞利用
由于$resp参数我们无法控制,所以来修改recaptcha_response_field、User-Agent两个参数上。
更改参数recaptcha_response_field以及http包头的User-Agent如下
Impossible
源码
<?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(); ?>
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。