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Insecure CAPTCHA（不安全的驗證碼）Insecure CAPTCHA

Insecure CAPTCHA

Insecure CAPTCHA，意思是不安全的驗證碼，CAPTCHA是Completely Automated Public Turing Test to Tell Computers and Humans Apart (全自動區分計算機和人類的圖靈測試)的簡稱。驗證碼是沒有問題的，關鍵是代碼寫的有問題，可以繞過。

環境需要把config.inc.php中的recaptcha_public_key \recaptcha_private_key補充。

Insecure CAPTCHA（不安全的驗證碼）Insecure CAPTCHA
Insecure CAPTCHA（不安全的驗證碼）Insecure CAPTCHA

reCAPTCHA驗證流程

Insecure CAPTCHA（不安全的驗證碼）Insecure CAPTCHA

LOW

代碼情況

Unknown Vulnerability Source
vulnerabilities/captcha/source/low.php
<?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);
}

?>

recaptcha_check_answer( p r i v k e y , privkey, privkey,remoteip, c h a l l e n g e , challenge, challenge,response)參數 p r i v k e y 是 服 務 器 申 請 的 p r i v a t e k e y ， privkey是伺服器申請的private key ， privkey是伺服器申請的privatekey，remoteip是使用者的ip， c h a l l e n g e 是 r e c a p t c h a c h a l l e n g e f i e l d 字 段 的 值 ， 來 自 前 端 頁 面 ， challenge 是recaptcha_challenge_field 字段的值，來自前端頁面 ， challenge是recaptchac​hallengef​ield字段的值，來自前端頁面，response是 recaptcha_response_field 字段的值。

通過代碼，發現step=2，和2個密碼相同就可以直接寫入資料庫更新。不需要驗證碼正确。

step=1時需要比對驗證碼，是以驗證失敗，無法修改密碼。

Insecure CAPTCHA（不安全的驗證碼）Insecure CAPTCHA

step=2時，無需通過谷歌驗證碼驗證。2個密碼一樣即可通過驗證，修改密碼。

Insecure CAPTCHA（不安全的驗證碼）Insecure CAPTCHA

驗證漏洞時，需要開個梯子。

Medium 

Unknown Vulnerability Source
vulnerabilities/captcha/source/medium.php
<?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\" />
                    <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
        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);
}

?>

Medium級别的代碼增加了passed_captcha=ture的驗證，本質上沒有差別。依舊可以通過參數step、passed_captcha進行繞過。

Insecure CAPTCHA（不安全的驗證碼）Insecure CAPTCHA
Insecure CAPTCHA（不安全的驗證碼）Insecure CAPTCHA

step=2成功繞過passed_captcha=ture

Insecure CAPTCHA（不安全的驗證碼）Insecure CAPTCHA

High 

Unknown Vulnerability Source
vulnerabilities/captcha/source/high.php
<?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，伺服器的驗證邏輯是當$resp（這裡是指谷歌傳回的驗證結果）是false，并且參數g-recaptcha_response_field不等于hidd3n_valu3（或者http標頭的User-Agent參數不等于reCAPTCHA）時，就認為驗證碼輸入錯誤，反之則認為已經通過了驗證碼的檢查。

Insecure CAPTCHA（不安全的驗證碼）Insecure CAPTCHA

$resp不可控，修改UA頭和g-recaptcha-response繞過

Insecure CAPTCHA（不安全的驗證碼）Insecure CAPTCHA

Impossible 

Unknown Vulnerability Source
vulnerabilities/captcha/source/impossible.php
<?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;
    }
    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();

?>

Impossible級别的代碼增加了Anti-CSRF token 機制防禦CSRF攻擊，利用PDO技術防護sql注入，驗證碼無法繞過，同時要求使用者輸入舊密碼，hacker不知道舊密碼的情況下無法通過認證。

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