从DVWA靶场学代码审计
DVWA是较为经典的一个传统漏洞的靶场，内置了low,medium,hight,impossible四个安全级别供安全人员去研究相关漏洞。今天就来对impossible这个级别进行代码审计，从中学习一下传统漏洞的代码层面的防御措施。
#文件上传
#源代码
<?php
if( isset( $_POST[ 'Upload' ] ) ) {
// Check Anti-CSRF token
checkToken( $_REQUEST[ 'user_token' ], $_SESSION[ 'session_token' ], 'index.php' );
// File information
$uploaded_name = $_FILES[ 'uploaded' ][ 'name' ];
$uploaded_ext = substr( $uploaded_name, strrpos( $uploaded_name, '.' ) + 1);
$uploaded_size = $_FILES[ 'uploaded' ][ 'size' ];
$uploaded_type = $_FILES[ 'uploaded' ][ 'type' ];
$uploaded_tmp = $_FILES[ 'uploaded' ][ 'tmp_name' ];
// Where are we going to be writing to?
$target_path = DVWA_WEB_PAGE_TO_ROOT . 'hackable/uploads/';
//$target_file = basename( $uploaded_name, '.' . $uploaded_ext ) . '-';
$target_file = md5( uniqid() . $uploaded_name ) . '.' . $uploaded_ext;uniqid()
$temp_file = ( ( ini_get( 'upload_tmp_dir' ) == '' ) ? ( sys_get_temp_dir() ) : ( ini_get( 'upload_tmp_dir' ) ) );
$temp_file .= DIRECTORY_SEPARATOR . md5( uniqid() . $uploaded_name ) . '.' .$uploaded_ext;
// Is it an image?
if( ( strtolower( $uploaded_ext ) == 'jpg' || strtolower( $uploaded_ext ) == 'jpeg' || strtolower( $uploaded_ext ) == 'png' ) &&
( $uploaded_size < 100000 ) &&
( $uploaded_type == 'image/jpeg' || $uploaded_type == 'image/png' ) &&
getimagesize( $uploaded_tmp ) ) {
// Strip any metadata, by re-encoding image (Note, using php-Imagick is recommended over php-GD)
if( $uploaded_type == 'image/jpeg' ) {
$img = imagecreatefromjpeg( $uploaded_tmp );
imagejpeg( $img, $temp_file, 100);
}//
else {
$img = imagecreatefrompng( $uploaded_tmp );
imagepng( $img, $temp_file, 9);
}
imagedestroy( $img );
// Can we move the file to the web root from the temp folder?
if( rename( $temp_file, ( getcwd() . DIRECTORY_SEPARATOR . $target_path . $target_file ) ) ) {
// Yes!
$html .= "<pre><a href='${target_path}${target_file}'>${target_file}</a> succesfully uploaded!</pre>";
}
else {
// No
$html .= '<pre>Your image was not uploaded.</pre>';
}
// Delete any temp files
if( file_exists( $temp_file ) )
unlink( $temp_file );
}
else {
// Invalid file
$html .= '<pre>Your image was not uploaded. We can only accept JPEG or PNG images.</pre>';
}
}
// Generate Anti-CSRF token
generateSessionToken();
?>
首先是进行csrf-token检查，如果校验不通过，返回到index.php，相关代码:
function checkToken( $user_token, $session_token, $returnURL ) {
if( $user_token !== $session_token || !isset( $session_token ) ) {
dvwaMessagePush( 'CSRF token is incorrect' );
dvwaRedirect( $returnURL );
}
}
然后就是对文件信息的一个获取
// File information
$uploaded_name = $_FILES[ 'uploaded' ][ 'name' ];//文件名
$uploaded_ext = substr( $uploaded_name, strrpos( $uploaded_name, '.' ) + 1);//文件后缀
$uploaded_size = $_FILES[ 'uploaded' ][ 'size' ];//文件大小
$uploaded_type = $_FILES[ 'uploaded' ][ 'type' ];//文件类型
$uploaded_tmp = $_FILES[ 'uploaded' ][ 'tmp_name' ];//文件临时名
接着就是定义上传路径和临时路径以及对文件名进行加密
$target_path = DVWA_WEB_PAGE_TO_ROOT . 'hackable/uploads/';//写入路径
$target_file = md5( uniqid() . $uploaded_name ) . '.' . $uploaded_ext;//先调用uniqid()生成一个微秒级的当前时间的值，然后与文件名拼接，最后整体经过md5加密，再和后缀拼接构成完整的一个文件名
$temp_file = ( ( ini_get( 'upload_tmp_dir' ) == '' ) ? ( sys_get_temp_dir() ) : ( ini_get( 'upload_tmp_dir' ) ) );//定义了临时文件写入路径为默认值或系统默认目录，windows为c:windowstemp,linux为/tmp
$temp_file .= DIRECTORY_SEPARATOR . md5( uniqid() . $uploaded_name ) . '.' . $uploaded_ext;//定义了加密后的临时文件写入路径
接下来就是重新生成图片，导致图片马直接挂掉.
if( $uploaded_type == 'image/jpeg' ) {
$img = imagecreatefromjpeg( $uploaded_tmp );
imagejpeg( $img, $temp_file, 100);
}//
else {
$img = imagecreatefrompng( $uploaded_tmp );
imagepng( $img, $temp_file, 9);
}
imagedestroy( $img );
然后如果我们上传的图片都通过了校验，那么此时图片的路径会变为web根目录下的/hackable/uploads/然后回显路径.关键代码:
if( rename( $temp_file, ( getcwd() . DIRECTORY_SEPARATOR . $target_path . $target_file ) ) ) {
// Yes!
$html .= "<pre><a href='${target_path}${target_file}'>${target_file}</a> succesfully uploaded!</pre>";
}
else {
// No
$html .= '<pre>Your image was not uploaded.</pre>';
}
代码层防御思路:
后台白名单+二次验证（例如图片重新生成，过滤掉其他可疑数据）
非必要情况下不要回显文件路径到客户端
#SQL注入:
#源代码
<?php
if( isset( $_GET[ 'Submit' ] ) ) {
// Check Anti-CSRF token
checkToken( $_REQUEST[ 'user_token' ], $_SESSION[ 'session_token' ], 'index.php' );
// Get input
$id = $_GET[ 'id' ];
// Was a number entered?
if(is_numeric( $id )) {
// Check the database
$data = $db->prepare( 'SELECT first_name, last_name FROM users WHERE user_id = (:id) LIMIT 1;' );
$data->bindParam( ':id', $id, PDO::PARAM_INT );
$data->execute();
$row = $data->fetch();
// Make sure only 1 result is returned
if( $data->rowCount() == 1 ) {
// Get values
$first = $row[ 'first_name' ];
$last = $row[ 'last_name' ];
// Feedback for end user
$html .= "<pre>ID: {$id}<br />First name: {$first}<br />Surname: {$last}</pre>";
}
}
}
// Generate Anti-CSRF token
generateSessionToken();
?>
可以看到通过prepare()对sql语句进行了预编译,以及使用了bindParam()来对参数进行绑定，大大降低了sql注入的风险
$data = $db->prepare( 'SELECT first_name, last_name FROM users WHERE user_id = (:id) LIMIT 1;' );
$data->bindParam( ':id', $id, PDO::PARAM_INT );
$data->execute();
并且限制了查询结果
if( $data->rowCount() == 1 ) {
// Get values
$first = $row[ 'first_name' ];
$last = $row[ 'last_name' ];
代码层防御思路:
预处理SQL语句
参数化查询
不将不必要的信息返回给客户端
#文件包含
#源代码
<?php
// The page we wish to display
$file = $_GET[ 'page' ];
// Only allow include.php or file{1..3}.php
if( $file != "include.php" && $file != "file1.php" && $file != "file2.php" && $file != "file3.php" ) {
// This isn't the page we want!
echo "ERROR: File not found!";
exit;
}
?>
只允许包含固定文件像include.php或者file1.php
if( $file != "include.php" && $file != "file1.php" && $file != "file2.php" && $file != "file3.php" )
代码层防御思路:
包含的文件不能由用户可控
白名单校验
#RCE（远程代码/命令执行）
#<?php
if( isset( $_POST[ 'Submit' ] ) ) {
// Check Anti-CSRF token
checkToken( $_REQUEST[ 'user_token' ], $_SESSION[ 'session_token' ], 'index.php' );
// Get input
$target = $_REQUEST[ 'ip' ];
$target = stripslashes( $target );
// Split the IP into 4 octects
$octet = explode( ".", $target );
// Check IF each octet is an integer
if( ( is_numeric( $octet[0] ) ) && ( is_numeric( $octet[1] ) ) && ( is_numeric( $octet[2] ) ) && ( is_numeric( $octet[3] ) ) && ( sizeof( $octet ) == 4 ) ) {
// If all 4 octets are int's put the IP back together.
$target = $octet[0] . '.' . $octet[1] . '.' . $octet[2] . '.' . $octet[3];
// Determine OS and execute the ping command.
if( stristr( php_uname( 's' ), 'Windows NT' ) ) {
// Windows
$cmd = shell_exec( 'ping ' . $target );
}
else {
// *nix
$cmd = shell_exec( 'ping -c 4 ' . $target );
}
// Feedback for the end user
$html .= "<pre>{$cmd}</pre>";
}
else {
// Ops. Let the user name theres a mistake
$html .= '<pre>ERROR: You have entered an invalid IP.</pre>';
}
}
对输入的参数用explode()打散为ip格式的数组并进行严格校验
$octet = explode( ".", $target ); // Check IF each octet is an integer if( ( is_numeric( $octet[0] ) ) && ( is_numeric( $octet[1] ) ) && ( is_numeric( $octet[2] ) ) && ( is_numeric( $octet[3] ) ) && ( sizeof( $octet ) == 4 ) ) { // If all 4 octets are int's put the IP back together. $target = $octet[0] . '.' . $octet[1] . '.' . $octet[2] . '.' . $octet[3];
代码层防御措施：
非必要情况下不提供命令执行接口
对于输入的命令进行严格过滤（特殊字符& | ;等）
#CSRF
#源代码
<?php
if( isset( $_GET[ 'Change' ] ) ) {
// Check Anti-CSRF token
checkToken( $_REQUEST[ 'user_token' ], $_SESSION[ 'session_token' ], 'index.php' );
// Get input
$pass_curr = $_GET[ 'password_current' ];
$pass_new = $_GET[ 'password_new' ];
$pass_conf = $_GET[ 'password_conf' ];
// Sanitise current password input
$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 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 passwords match and does the current password match the user?
if( ( $pass_new == $pass_conf ) && ( $data->rowCount() == 1 ) ) {
// It does!
$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 );
// Update database with new password
$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 user
$html .= "<pre>Password Changed.</pre>";
}
else {
// Issue with passwords matching
$html .= "<pre>Passwords did not match or current password incorrect.</pre>";
}
}
// Generate Anti-CSRF token
generateSessionToken();
?>
其实关键的操作就是检查csrf-token,以及验证新密码与用户是否匹配
if( ( $pass_new == $pass_conf ) && ( $data->rowCount() == 1 ) ) {
// It does!
$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 );
checkToken()代码：
function checkToken( $user_token, $session_token, $returnURL ) { # Validate the given (CSRF) token
if( $user_token !== $session_token || !isset( $session_token ) ) {
dvwaMessagePush( 'CSRF token is incorrect' );
dvwaRedirect( $returnURL );
}
}
代码层防御思路:
页面嵌入token
token与后端用户session中的token绑定
GET和POST请求都得进行验证
#XSS-reflect
#源代码
<?php
// Is there any input?
if( array_key_exists( "name", $_GET ) && $_GET[ 'name' ] != NULL ) {
// Check Anti-CSRF token
checkToken( $_REQUEST[ 'user_token' ], $_SESSION[ 'session_token' ], 'index.php' );
// Get input
$name = htmlspecialchars( $_GET[ 'name' ] );
// Feedback for end user
$html .= "<pre>Hello ${name}</pre>";
}
// Generate Anti-CSRF token
generateSessionToken();
?>
通过htmlspecialchars()函数将预定义的字符转化为HTML实体,防止xss
$name = htmlspecialchars( $_GET[ 'name' ] );
#XSS-Stored
#源代码
<?php
if( isset( $_POST[ 'btnSign' ] ) ) {
// Check Anti-CSRF token
checkToken( $_REQUEST[ 'user_token' ], $_SESSION[ 'session_token' ], 'index.php' );
// Get input
$message = trim( $_POST[ 'mtxMessage' ] );
$name = trim( $_POST[ 'txtName' ] );
// Sanitize message input
$message = stripslashes( $message );
$message = ((isset($GLOBALS["___mysqli_ston"]) && is_object($GLOBALS["___mysqli_ston"])) ? mysqli_real_escape_string($GLOBALS["___mysqli_ston"], $message ) : ((trigger_error("[MySQLConverterToo] Fix the mysql_escape_string() call! This code does not work.", E_USER_ERROR)) ? "" : ""));
$message = htmlspecialchars( $message );
// Sanitize name input
$name = stripslashes( $name );
$name = ((isset($GLOBALS["___mysqli_ston"]) && is_object($GLOBALS["___mysqli_ston"])) ? mysqli_real_escape_string($GLOBALS["___mysqli_ston"], $name ) : ((trigger_error("[MySQLConverterToo] Fix the mysql_escape_string() call! This code does not work.", E_USER_ERROR)) ? "" : ""));
$name = htmlspecialchars( $name );
// Update database
$data = $db->prepare( 'INSERT INTO guestbook ( comment, name ) VALUES ( :message, :name );' );
$data->bindParam( ':message', $message, PDO::PARAM_STR );
$data->bindParam( ':name', $name, PDO::PARAM_STR );
$data->execute();
}
// Generate Anti-CSRF token
generateSessionToken();
?>
对输入输出都进行过滤，编码等操作
$message = stripslashes( $message );
$message = htmlspecialchars( $message );
$name = stripslashes( $name );
$name = htmlspecialchars( $name );
#XSS-dom-based
#源代码
<script>
if (document.location.href.indexOf("default=") >= 0) {
var lang = document.location.href.substring(document.location.href.indexOf("default=")+8);
document.write("<option value='" + lang + "'>" + decodeURI(lang) + "</option>");
document.write("<option value='' disabled='disabled'>----</option>");
}
document.write("<option value='English'>English</option>");
document.write("<option value='French'>French</option>");
document.write("<option value='Spanish'>Spanish</option>");
document.write("<option value='German'>German</option>");
</script>
$decodeURI = "decodeURI";
if ($vulnerabilityFile == 'impossible.php') {
$decodeURI = "";
}
可以看到我们输入的参数经过url编码后无法被decodeURI解码，也就无法产生dom型xss了
代码层防御思路：
对输入输出进行编码
过滤特殊字符(< > /等)
#WeakSessionId
#源代码
<?php
$html = "";
if ($_SERVER['REQUEST_METHOD'] == "POST") {
$cookie_value = sha1(mt_rand() . time() . "Impossible");
setcookie("dvwaSession", $cookie_value, time()+3600, "/vulnerabilities/weak_id/", $_SERVER['HTTP_HOST'], true, true);
}
?>
使用随机数+时间戳+固定字符串（“Impossible”）进行 sha1 运算，作为 session Id 降低了被爆破的可能
代码层防御思路
使用多种加密算法进行加密
调用安全的接口生成session id
来源：freebuf.com 2021-07-05 18:12:08 by: MrRobot9527
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