DVWA-6.1 Insecure CAPTCHA（不安全的验证码）-Low

2023-10-02 11:39:22

Insecure CAPTCHA

Insecure CAPTCHA，意思是不安全的验证码，CAPTCHA是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。但个人觉得，这一模块的内容叫做不安全的验证流程更妥当些，因为这块主要是验证流程出现了逻辑漏洞，谷歌的验证码表示不背这个锅。

reCAPTCHA验证流程

这一模块的验证码使用的是Google提供reCAPTCHA服务，下图是验证的具体流程。

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$privkey是服务器申请的private key ，$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。

ps:有人也许会问，那这个模块的实验是不是需要访问谷歌呢？答案是不用，因为我们可以绕过验证码。

Low Level

查看源码

<?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);
}

?>

可以看到，服务器将改密操作分成了两步，第一步检查用户输入的验证码，验证通过后，服务器返回表单，第二步客户端提交post请求，服务器完成更改密码的操作。但是，这其中存在明显的逻辑漏洞，服务器仅仅通过检查Change、step 参数来判断用户是否已经输入了正确的验证码。

漏洞利用

1.通过构造参数绕过验证过程的第一步

首先输入密码，点击Change按钮，抓包：