这个PHP代理脚本在htdocs / public_html之外提供文件的安全性如何？

Question

OK, did a search, couldn't find anything close to this, so here goes...

I'm writing a PHP 5.3+ proxy script to serve files from a directory and subdirectories outside www, htdocs, public_html, etc., e.g. /home/sites/example.com/data

It's for a Moodle plugin module so if you're familiar with Moodle code, great, if not, I've annotated it as best I can. So far it all works as expected but I've yet to run more tests on it.

The question: How secure is this? My main concern is users gaining access outside the specified directory. If there's anything you can see that is a glaring security flaw, please let me know.

The script:

require_once('../../config.php'); // conatains $CFG object
require_once('../../lib/filelib.php'); // contains mimeinfo() and send_file() definitions
// Don't use Moodle required_param() to avoid sending any HTML messages to Flash apps

require_login(); // Users must be logged in to access files

global $CFG;

$swf_relative_path = get_file_argument(); // gets the appended URL e.g. /dir/subdir/file.jpg
$swf_ok = false;
if(strrpos($swf_relative_path,'.') > strlen($swf_relative_path) - 6) {
    // Strip out special characters, extra slashes, and parent directory stuff
    $swf_disallowed = array('../','\'','\"',':','{','}','*','&','=','!','?','\\','//','///');
    $swf_replace = array('','','','','','','','','','','','','/','/');
    $swf_relative_path = str_replace($swf_disallowed,$swf_replace,$swf_relative_path);
    $swf_full_path = $CFG->dataroot.$CFG->swf_content_dir.$swf_relative_path;
    if(file_exists($swf_full_path) && is_readable($swf_full_path)) {
        $swf_path_info = pathinfo($swf_full_path);
        $swf_mime_type = mimeinfo('type', $swf_path_info['basename']);
        send_file($swf_full_path,$swf_path_info['basename'],'default',0,false,false,$swf_mime_type,false);
        exit;
    }
}
header('HTTP/1.0 404 Not Found'); // Send back a 404 so that apps don't wait for a timeout
exit('404 Error: File not found'); // Pure text output - Flash app friendly

Thanks in advance! :)

Answer 1

This is super insecure, please do not publish this script on any server.

Do not tinker with ../ and ./ or the other patterns you seem to dislike in your script. Also, just replacing them won't prevent an attacker from inserting the replaced patterns to your script.

For example, have a look at this url:

download.php?file=..././some/file

After replacing ../ with an empty string (like you did), the path of the file is ../some/file and your script is already broken as it will make files outside of your download root accessible.

One solution to avoid this is using realpath(). However, I would strongly recommend using an existing and secure script for this purpose.