一、前言

第五关主要是对源代码的一个分析，然后进行绕过上传文件。

二、环境

靶场：upload-labs

工具：burpsuit

三、思路

首先我们查看提示，显示“上传目录存在PHP文件”，说明本关可以直接上传php文件，但估计没这么简单，还是分析一下源码。

查看源码：

$is_upload = false;
$msg = null;
if (isset($_POST['submit'])) {
    if (file_exists(UPLOAD_PATH)) {
        $deny_ext = array(".php",".php5",".php4",".php3",".php2",".html",".htm",".phtml",".pht",".pHp",".pHp5",".pHp4",".pHp3",".pHp2",".Html",".Htm",".pHtml",".jsp",".jspa",".jspx",".jsw",".jsv",".jspf",".jtml",".jSp",".jSpx",".jSpa",".jSw",".jSv",".jSpf",".jHtml",".asp",".aspx",".asa",".asax",".ascx",".ashx",".asmx",".cer",".aSp",".aSpx",".aSa",".aSax",".aScx",".aShx",".aSmx",".cEr",".sWf",".swf",".htaccess");
        $file_name = trim($_FILES['upload_file']['name']);
        $file_name = deldot($file_name);//删除文件名末尾的点
        $file_ext = strrchr($file_name, '.');
        $file_ext = strtolower($file_ext); //转换为小写
        $file_ext = str_ireplace('::$DATA', '', $file_ext);//去除字符串::$DATA
        $file_ext = trim($file_ext); //首尾去空
        
        if (!in_array($file_ext, $deny_ext)) {
            $temp_file = $_FILES['upload_file']['tmp_name'];
            $img_path = UPLOAD_PATH.'/'.$file_name;
            if (move_uploaded_file($temp_file, $img_path)) {
                $is_upload = true;
            } else {
                $msg = '上传出错！';
            }
        } else {
            $msg = '此文件类型不允许上传！';
        }
    } else {
        $msg = UPLOAD_PATH . '文件夹不存在,请手工创建！';
    }
}

通过源代码可知，本关对上传文件做了检测，而且还对文件名大小写做了转换。在上传文件后先是删除文件末尾的点，然后首尾去空，去除字符串::$DATA，但是这些操作只执行一次。所以我们可以抓取数据包来修改绕过。我们在数据包中把后缀名改为.php. .，首先他发现有一个点，这时会把他去掉，又发现有一个空格，也会把它去掉，我们这时还有一个点，也就是.php. 由于他只是验证一次，所以不会在去掉我们的点。

上传php文件然后burpsuit抓包绕过。