Insecure CAPTCHA
1. 题目
Insecure CAPTCHA(全自动区分计算机和人类的图灵测试)，意思是不安全的验证码。
指在进行验证的过程中，出现了逻辑漏洞，导致验证码没有发挥其应有的作用。
Google reCAPTCHA服务验证流程：
2. Low
a. 代码分析
<?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();
}
?>
代码将修改密码的过程分为两步，通过step参数确定。第一步时，代码调用第三方验证码，进行人机检测，通过测试后，将数据包中step参数修改为2，重新发送数据。
这里可以在发送数据的时候，直接将step改为2，绕过第一步中的人机检测。
b. 漏洞利用
将step1改为2，可不通过验证修改密码。
3. Medium
a. 代码分析
<?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();
}
?>
与low级别相比，增加了一个参数passed_captcha。
b. 漏洞利用
在发送数据包的时候，手动添加passed_captcha参数。
4. High
a. 代码分析
<?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 : $___mysqli_res);
}
// Generate Anti-CSRF token
generateSessionToken();
?>
可以看到，服务器的验证逻辑是当$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。
b. 漏洞利用
手动添加所需要的参数：
5. impossible
a. 代码分析
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();
?>
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，验证不再分成两部分，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。