DVWA学习记录 PartⅥ

Insecure CAPTCHA

1. 题目

Insecure CAPTCHA(全自动区分计算机和人类的图灵测试)，意思是不安全的验证码。

指在进行验证的过程中，出现了逻辑漏洞，导致验证码没有发挥其应有的作用。

Google reCAPTCHA服务验证流程：

2. Low

a. 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' ], 
        $_SERVER[ 'REMOTE_ADDR' ], 
        $_POST[ 'recaptcha_challenge_field' ], 
        $_POST[ 'recaptcha_response_field' ] ); 

    // Did the CAPTCHA fail? 
    if( !$resp->is_valid ) { 
        // 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 = mysql_real_escape_string( $pass_new ); 
        $pass_new = md5( $pass_new ); 

        // Update database 
        $insert = "UPDATE `users` SET password = '$pass_new' WHERE user = '" . dvwaCurrentUser() . "';"; 
        $result = mysql_query( $insert ) or die( '<pre>' . mysql_error() . '</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; 
    } 

    mysql_close(); 
} 

?> 

代码将修改密码的过程分为两步，通过step参数确定。第一步时，代码调用第三方验证码，进行人机检测，通过测试后，将数据包中step参数修改为2，重新发送数据。

这里可以在发送数据的时候，直接将step改为2，绕过第一步中的人机检测。

b. 漏洞利用

将step1改为2，可不通过验证修改密码。

3. Medium

a. 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' ], 
        $_SERVER[ 'REMOTE_ADDR' ], 
        $_POST[ 'recaptcha_challenge_field' ], 
        $_POST[ 'recaptcha_response_field' ] ); 

    // Did the CAPTCHA fail? 
    if( !$resp->is_valid ) { 
        // 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\" /> 
                    <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 = mysql_real_escape_string( $pass_new ); 
        $pass_new = md5( $pass_new ); 

        // Update database 
        $insert = "UPDATE `users` SET password = '$pass_new' WHERE user = '" . dvwaCurrentUser() . "';"; 
        $result = mysql_query( $insert ) or die( '<pre>' . mysql_error() . '</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; 
    } 
    mysql_close(); 
} 
?> 

与low级别相比，增加了一个参数passed_captcha。

b. 漏洞利用

在发送数据包的时候，手动添加passed_captcha参数。

4. High

a. 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
            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();
?>

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。

b. 漏洞利用

手动添加所需要的参数：

5. impossible

a. 代码分析

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  = mysql_real_escape_string( $pass_new ); 
    $pass_new  = md5( $pass_new ); 

    $pass_conf = $_POST[ 'password_conf' ]; 
    $pass_conf = stripslashes( $pass_conf ); 
    $pass_conf = mysql_real_escape_string( $pass_conf ); 
    $pass_conf = md5( $pass_conf ); 

    $pass_curr = $_POST[ 'password_current' ]; 
    $pass_curr = stripslashes( $pass_curr ); 
    $pass_curr = mysql_real_escape_string( $pass_curr ); 
    $pass_curr = md5( $pass_curr ); 

    // 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 ) { 
        // 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(); 
?> 

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，验证不再分成两部分，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。