DVWA 黑客攻防演练（六）不安全的验证码 Insecure CAPTCHA

之前在 CSRF 攻击 的那篇文章的最后，我觉得可以用验证码提高攻击的难度。

若有验证码的话，就比较难被攻击者利用 XSS 漏洞进行的 CSRF 攻击了，因为要识别验证码起码要调用api，跨域会被浏览器拦截，再者一些验证码很难被识别，比如知乎点击倒立的汉字，拖动拼图、百度的汉字验证码，谷歌的神奇的勾勾。。。 觉得这篇文章像是 CSRF 攻击 的一种补充（更像是谷歌验证码的使用教程，而且正常人的逻辑也不会犯这个问题的，其实可以跳过这篇文章）

话说回来，你会发现页面是这样的

其实也不用科学上网，直接在/var/www/html/config/config.inc.php 中 乱写个字符串就行了。

之后就会变成这样。

反正验证码加载不出来，要加载出来服务端也要科学上网，而我们需要做的是，绕过验证

而科学上网后，验证码是这样的，这肯定是破解不出来的。

初级

看代码，这里修改密码是分成二个部分的，一个部分是用来判断验证码的正确性，如果正确了就再返回密码的界面，这个界面就不再需要输入验证码的，按提交就可以修改密码了。这两个部分的用 form 表单的 step 字段区分。。。（大家都应该能猜到如何攻击了） 代码如下

<?php
//步骤1 
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;
        }
    }
}
//步骤2 
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();
}

?>

攻击方式也很简单，用 BurpSuite 直接改请求

或者打开火狐的审查元素（F12）直接改 form 表单

所以还是会有 CSRF 攻击漏洞的。

中级

中级篇主要区别是如果第一步验证成功了，会有一个 passed_captcha 字段

...
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' ];

    //如果有做第一步  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();
}

?>

所以，同样可以用 burp suite 或者火狐添加一个参数吧。用火狐先打开审查元素，发一个失败的请求，然后修改请求如下。

高级

中级、高级不一样，不分步骤了。。。都放在一块了，关注点在请求失败的条件，代码如下。

<?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' ) ) {
        // 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 = mysql_real_escape_string( $pass_new );
            $pass_new = md5( $pass_new );

            // Update database
            $insert = "UPDATE `users` SET password = '$pass_new' WHERE user = '" . dvwaCurrentUser() . "' LIMIT 1;";
            $result = mysql_query( $insert ) or die( '<pre>' . mysql_error() . '</pre>' );

            // Feedback for user
            echo "<pre>Password Changed.</pre>";
        }
        else {
            // Ops. Password mismatch
            $html     .= "<pre>Both passwords must match.</pre>";
            $hide_form = false;
        }
    }

    mysql_close();
}

//神·生成token，服务器又不验证 Generate Anti-CSRF token 
generateSessionToken();

?>

关键是判断验证码是否失败的那句

// Did the CAPTCHA fail?
if( !$resp->is_valid && ( $_POST[ 'recaptcha_response_field' ] != 'hidd3n_valu3'
                       || $_SERVER[ 'HTTP_USER_AGENT' ] != 'reCAPTCHA' ) ) {
    // What happens when the CAPTCHA was entered incorrectly
    $html     .= "<pre><br />The CAPTCHA was incorrect. Please try again.</pre>";
    $hide_form = false;
    return;
}

而验证是否失败的条件是，谷歌服务器返回的结果是 false ，而且请求属性 recaptcha_response_field 的值不能是 hidd3n_valu3 或者* 请求头部的 HTTP_USER_AGENT 不能是 reCAPTCHA 。
​
也就是当 recaptcha_response_field 会是 hidd3n_valu3，头部的 HTTP_USER_AGENT 是 reCAPTCHA ，就能绕过了。

应该是逻辑错误的，估计是如果验证成功 recaptcha_response_field 会是 hidd3n_valu3，头部的 HTTP_USER_AGENT 会变成 reCAPTCHA 吧。 所以应该是

!($resp->is_valid && $_POST[ 'recaptcha_response_field' ] == 'hidd3n_valu3' && $_SERVER[ 'HTTP_USER_AGENT' ] == 'reCAPTCHA' )

再换算

!($resp->is_valid && ($_POST[ 'recaptcha_response_field' ] == 'hidd3n_valu3' && $_SERVER[ 'HTTP_USER_AGENT' ] == 'reCAPTCHA' ))

然后在换算

!($resp->is_valid) || !($_POST[ 'recaptcha_response_field' ] == 'hidd3n_valu3' && $_SERVER[ 'HTTP_USER_AGENT' ] == 'reCAPTCHA' )

不可能

与前面的相比

• 判断验证码的方式只有 $resp->is_valid，
• 有验证生成的 token
• 使用 prepare 预防 sql 注入
• 要输入原密码

Insecure CAPTCHA Source
<?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  = 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();

?>