DVWA靶场实战(六)——Insecure CAPTCHA

DVWA靶场实战（六）

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。