DVWA 黑客攻防演练(六)不安全的验证码 Insecure CAPTCHA

之前在 CSRF 攻击 的那篇文章的最后,我觉得可以用验证码提高攻击的难度。

若有验证码的话,就比较难被攻击者利用 XSS 漏洞进行的 CSRF 攻击了,因为要识别验证码起码要调用api,跨域会被浏览器拦截,再者一些验证码很难被识别,比如知乎点击倒立的汉字,拖动拼图、百度的汉字验证码,谷歌的神奇的勾勾。。。 觉得这篇文章像是 CSRF 攻击 的一种补充(更像是谷歌验证码的使用教程,而且正常人的逻辑也不会犯这个问题的,其实可以跳过这篇文章)

话说回来,你会发现页面是这样的

DVWA 黑客攻防演练(六)不安全的验证码 Insecure CAPTCHA

其实也不用*,直接在/var/www/html/config/config.inc.php 中 乱写个字符串就行了。

DVWA 黑客攻防演练(六)不安全的验证码 Insecure CAPTCHA

之后就会变成这样。

DVWA 黑客攻防演练(六)不安全的验证码 Insecure CAPTCHA

反正验证码加载不出来,要加载出来服务端也要*,而我们需要做的是,绕过验证

而*后,验证码是这样的,这肯定是破解不出来的。

DVWA 黑客攻防演练(六)不安全的验证码 Insecure CAPTCHA

初级

看代码,这里修改密码是分成二个部分的,一个部分是用来判断验证码的正确性,如果正确了就再返回密码的界面,这个界面就不再需要输入验证码的,按提交就可以修改密码了。这两个部分的用 form 表单的 step 字段区分。。。(大家都应该能猜到如何攻击了) 代码如下

<?php
//步骤1
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;
}
}
}
//步骤2
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();
} ?>

攻击方式也很简单,用 BurpSuite 直接改请求

DVWA 黑客攻防演练(六)不安全的验证码 Insecure CAPTCHA

或者打开火狐的审查元素(F12)直接改 form 表单

DVWA 黑客攻防演练(六)不安全的验证码 Insecure CAPTCHA

所以还是会有 CSRF 攻击漏洞的。

中级

中级篇主要区别是如果第一步验证成功了,会有一个 passed_captcha 字段

...
if( isset( $_POST[ 'Change' ] ) && ( $_POST[ 'step' ] == '2' ) ) {
// Hide the CAPTCHA form
$hide_form = true; // Get input
$pass_new = $_POST[ 'password_new' ];
$pass_conf = $_POST[ 'password_conf' ]; //如果有做第一步 Check to see if they did stage 1
if( !$_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();
} ?>

所以,同样可以用 burp suite 或者火狐添加一个参数吧。用火狐先打开审查元素,发一个失败的请求,然后修改请求如下。

DVWA 黑客攻防演练(六)不安全的验证码 Insecure CAPTCHA

高级

中级、高级不一样,不分步骤了。。。都放在一块了,关注点在请求失败的条件,代码如下。

<?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();
} //神·生成token,服务器又不验证 Generate Anti-CSRF token
generateSessionToken(); ?>

关键是判断验证码是否失败的那句

// 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;
}

而验证是否失败的条件是,谷歌服务器返回的结果是 false ,而且请求属性 recaptcha_response_field 的值不能是 hidd3n_valu3 或者* 请求头部的 HTTP_USER_AGENT 不能是 reCAPTCHA



也就是当 recaptcha_response_field 会是 hidd3n_valu3,头部的 HTTP_USER_AGENT 是 reCAPTCHA ,就能绕过了。

DVWA 黑客攻防演练(六)不安全的验证码 Insecure CAPTCHA

应该是逻辑错误的,估计是如果验证成功 recaptcha_response_field 会是 hidd3n_valu3,头部的 HTTP_USER_AGENT 会变成 reCAPTCHA 吧。 所以应该是

!($resp->is_valid && $_POST[ 'recaptcha_response_field' ] == 'hidd3n_valu3' && $_SERVER[ 'HTTP_USER_AGENT' ] == 'reCAPTCHA' )

再换算

!($resp->is_valid && ($_POST[ 'recaptcha_response_field' ] == 'hidd3n_valu3' && $_SERVER[ 'HTTP_USER_AGENT' ] == 'reCAPTCHA' ))

然后在换算

!($resp->is_valid) || !($_POST[ 'recaptcha_response_field' ] == 'hidd3n_valu3' && $_SERVER[ 'HTTP_USER_AGENT' ] == 'reCAPTCHA' )

不可能

与前面的相比

  • 判断验证码的方式只有 $resp->is_valid
  • 有验证生成的 token
  • 使用 prepare 预防 sql 注入
  • 要输入原密码
Insecure CAPTCHA Source
<?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(); ?>
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