How to Start a Forum, Part C

vrijvlinder

This is a continuation of Part B

File uploads

Allowing users to upload files to your website can be a big website security risk, even if it’s simply to change their avatar. The risk is that any file uploaded however innocent it may look, could contain a script that when executed on your server completely opens up your website.

If you have a file upload form then you need to treat all files with great suspicion. If you are allowing users to upload images, you cannot rely on the file extension or the mime type to verify that the file is an image as these can easily be faked. Even opening the file and reading the header, or using functions to check the image size are not full proof. Most images formats allow storing a comment section which could contain PHP code that could be executed by the server.

So what can you do to prevent this? Ultimately you want to stop users from being able to execute any file they upload. By default web servers won't attempt to execute files with image extensions, but it isn't recommended to rely solely on checking the file extension as a file with the name image.jpg.php has been known to get through.

Some options are to rename the file on upload to ensure the correct file extension, or to change the file permissions, for example, chmod 0666 so it can't be executed. If using *nix you could create a .htaccess file (see below) that will only allow access to set files preventing the double extension attack mentioned earlier.

    deny from all
    <Files ~ "^\w+\.(gif|jpe?g|png)$">
    order deny,allow
    allow from all
    </Files>

Ultimately, the recommended solution is to prevent direct access to uploaded files all together. This way, any files uploaded to your website are stored in a folder outside of the webroot or in the database as a blob. If your files are not directly accessible you will need to create a script to fetch the files from the private folder (or an HTTP handler in .NET) and deliver them to the browser. Image tags support an src attribute that is not a direct URL to an image, so your src attribute can point to your file delivery script providing you set the correct content type in the HTTP header. For example:

<img src="/imageDelivery.php?id=1234" />

<?php
     // imageDelivery.php

     // Fetch image filename from database based on $_GET["id"]
     ...

     // Deliver image to browser
      Header('Content-Type: image/gif');
     readfile('images/'.$fileName);  

?>

Most hosting providers deal with the server configuration for you, but if you are hosting your website on your own server then there are few things you will want to check.

Ensure you have a firewall setup, and are blocking all non essential ports. If possible setting up a DMZ (Demilitarised Zone) only allowing access to port 80 and 443 from the outside world. Although this might not be possible if you don't have access to your server from an internal network as you would need to open up ports to allow uploading files and to remotely log in to your server over SSH or RDP.

If you are allowing files to be uploaded from the Internet only use secure transport methods to your server such as SFTP or SSH.

If possible have your database running on a different server to that of your web server. Doing this means the database server cannot be accessed directly from the outside world, only your web server can access it, minimising the risk of your data being exposed.

Finally, don't forget about restricting physical access to your server.

SSL

SSL is a protocol used to provide security over the Internet. It is a good idea to use a security certificate whenever you are passing personal information between the website and web server or database. Attackers could sniff for this information and if the communication medium is not secure could capture it and use this information to gain access to user accounts and personal data.

Website security tools

Once you think you have done all you can then it's time to test your website security. The most effective way of doing this is via the use of some website security tools, often referred to as penetration testing or pen testing for short.

There are many commercial and free products to assist you with this. They work on a similar basis to scripts hackers will use in that they test all know exploits and attempt to compromise your site using some of the previous mentioned methods such as SQL injection.

Some free tools that are worth looking at:

Netsparker (Free community edition and trial version available). Good for testing SQL injection and XSS
OpenVAS. Claims to be the most advanced open source security scanner. Good for testing known vulnerabilities, currently scans over 25,000. But it can be difficult to setup and requires a OpenVAS server to be installed which only runs on *nix. OpenVAS is fork of a Nessus before it became a closed-source commercial product.
The results from automated tests can be daunting, as they present a wealth of potential issues. The important thing is to focus on the critical issues first. Each issue reported normally comes with a good explanation of the potential vulnerability. You will probably find that some of the medium/low issues aren't a concern for your site.

If you wish to take things a step further then there are some further steps you can take to manually try to compromise your site by altering POST/GET values. A debugging proxy can assist you here as it allows you to intercept the values of an HTTP request between your browser and the server. A popular freeware application called Fiddler is a good starting point.

So what should you be trying to alter on the request? If you have pages which should only be visible to a logged in user then I would try changing URL parameters such as user id, or cookie values in an attempt to view details of another user. Another area worth testing are forms, changing the POST values to attempt to submit code to perform XSS or to upload a server side script.

Hopefully these tips will help keep your site and information safe. Thankfully most CMSes have a lot of inbuilt website security features, but it is a still a good idea to have knowledge of the most common security exploits so you can ensure you are covered.

There are also some helpful modules available for CMSes to check your installation for common security flaws such as Security Review for Drupal and WP Security Scan for WordPress.