dvwa之 Insecure CAPTCHA
一、简介
CAPTCHA是谷歌提供的一种用户验证服务，全称为：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)，简单来说就是用来区分人类和计算机的一种手段，可以很有效的防止恶意软件、工具人大量调用系统功能，比如注册、登录等。
二、Low
1、服务器端代码
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 .= "
The CAPTCHA was incorrect. Please try again.
";
$hide_form = false;
return;
}
else {
// CAPTCHA was correct. Do both new passwords match?
if( $pass_new == $pass_conf ) {
// Show next stage for the user
echo "
You passed the CAPTCHA! Click the button to confirm your changes.
=\"hidden\" name=\"step\" value=\"2\" />
type=\"hidden\" name=\"password_new\" value=\"{$pass_new}\" />
type=\"hidden\" name=\"password_conf\" value=\"{$pass_conf}\" />
type=\"submit\" name=\"Change\" value=\"Change\" />
</form>";
}
else {
// Both new passwords do not match.
$html .= "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"], $insert ) or die( '' . ((is_object($GLOBALS["___mysqli_ston"])) ? mysqli_error($GLOBALS["___mysqli_ston"]) : (($___mysqli_res = mysqli_connect_error()) ? $___mysqli_res : false)) . '
' );
// Feedback for the end user
echo "Password Changed.</pre>";
}
else {
// Issue with the passwords matching
echo "Passwords did not match.</pre>";
$hide_form = false;
}
((is_null($___mysqli_res = mysqli_close($GLOBALS["___mysqli_ston"]))) ? false : $___mysqli_res);
}
?>
2、漏洞分析
可以看到，修改密码包含两个步骤：校验验证码和修改密码，而且第一个步骤并不是一定要执行，可以直接修改step=2直接进行第二步。
3、漏洞利用
- 输入“New password”和“Confirm new password”，点击“change”
- 修改step为2，重放请求，密码修改成功。
三、Medium
1、服务器端代码
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 .= "
The CAPTCHA was incorrect. Please try again.
";
$hide_form = false;
return;
}
else {
// CAPTCHA was correct. Do both new passwords match?
if( $pass_new == $pass_conf ) {
// Show next stage for the user
echo "
You passed the CAPTCHA! Click the button to confirm your changes.
\" />
{$pass_conf}\" />
";
}
else {
// Both new passwords do not match.
$html .= "Both passwords must match.
";
$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 .= "
You have not passed the CAPTCHA.
";
$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"], $insert ) or die( ''
. ((is_object($GLOBALS["___mysqli_ston"])) ? mysqli_error($GLOBALS["___mysqli_ston"]) : (($___mysqli_res = mysqli_connect_error()) ? $___mysqli_res : false)) . 'Password Changed."; } else { // Issue with the passwords matching echo "
Passwords did not match."; $hide_form = false; } ((is_null($___mysqli_res = mysqli_close($GLOBALS["___mysqli_ston"]))) ? false : $___mysqli_res); } ?>
2、漏洞分析
可以看到，Medium级别的代码在第二步处理逻辑上加入了对验证码是否通过(passed_captcha)的判定，如果passed_captcha为true，则认为验证码正确，通过可以使用burpsuite修改参数绕过。
3、漏洞利用
- 输入“New password”和“Confirm new password”，点击“change”，修改“step=2”，添加“passed_captcha=true”，重放请求，密码修改成功。
四、High
1、服务器端代码
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"], $insert ) or die( ''
. ((is_object($GLOBALS["___mysqli_ston"])) ? mysqli_error($GLOBALS["___mysqli_ston"]) : (($___mysqli_res = mysqli_connect_error()) ? $___mysqli_res : false)) . 'Password Changed."; } else { // Ops. Password mismatch $html .= "
Both passwords must match."; $hide_form = false; } } else { // What happens when the CAPTCHA was entered incorrectly $html .= "
"; $hide_form = false; return; } ((is_null($___mysqli_res = mysqli_close($GLOBALS["___mysqli_ston"]))) ? false : $___mysqli_res); } // Generate Anti-CSRF token generateSessionToken(); ?>
The CAPTCHA was incorrect. Please try again.
2、漏洞分析
如果KaTeX parse error: Expected 'EOF', got '&' at position 47: …==hidd3n_valu3 &̲& HTTP_USER_AGE…resp是我们要绕过的，所以需要使得后面的条件成立。
3、漏洞利用
- 输入“New password”和“Confirm new password”，点击“change”，修改g-recaptcha-response ==hidd3n_valu3，User-Agent=reCAPTCHA，重放请求，密码修改成功。
五、Impossible
1、服务器端代码
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 );
$pass_new = ((isset($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 "
The CAPTCHA was incorrect. Please try again.
";
$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 "Password Changed.
";
}
else {
// Feedback for the end user - failed!
echo "Either your current password is incorrect or the new passwords did not match.
Please try again.
";
$hide_form = false;
}
}
}
// Generate Anti-CSRF token
generateSessionToken();
?>
2、漏洞分析
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。