DVWA之Insecure CAPTCHA

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。

reCAPTCHA验证流程：

这一模块验证码使用的是Google提供的reCAPTCHA服务，下图为验证的具体流程

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, $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的实列，ReCaptchaResponse类有两个属性：

$is_valid是布尔型的，表示校验是否有效。$ error是返回的错误代码。
1.low
<?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);
}

?>

服务器将改密码操作分为两步，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，这种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。

可以通过用burp抓包修改参数来绕过验证：

(ps:因为没有翻墙，所以没有成功显示验证码，发送的请求包也就没有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.medium
<?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级别的代码在第二步验证时，加入了对参数passed_captcha的检查，如果参数值为true，则认为用户通过了验证码的检查，然而我们依然可以通过伪造参数绕过验证，与low也基本一致。

通过burp抓包

修改step参数，添加passed_captcha参数，绕过验证

更改成功。
3.high
<?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();

?>

这里服务器验证逻辑是当$resp(这里是指谷歌返回的验证结果)是false时，参数g-recaptcha-response不等于hidd3n_valu3且http包头的User-Agent的参数不等于reCAPTCHA时，就认为验证码输入错误，反之认为已通过验证码的检查。

仍是使用burp抓包伪造，由于$resp无法控制，所以重心放在参数g-recaptcha-response、User-Agent上。

修改g-recaptcha-response、User-Agent参数

修改成功。

impossible
<?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级别代码增加了Anti-CSRF token机制防御CSRF攻击，利用PDO结束防御SQL注入，验证过程不在分成两部分，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。

Gridea

DVWA之Insecure CAPTCHA

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