01. Keep software up to date
It may seem obvious, but ensuring you keep all software up to date is vital in keeping your site secure. This applies to both the server operating system and any software you may be running on your website such as a CMS or forum. When website security holes are found in software, hackers are quick to attempt to abuse them.
If you are using a managed hosting solution then you don't need to worry so much about applying security updates for the operating system as the hosting company should take care of this.
If you are using third-party software on your website such as a CMS or forum, you should ensure you are quick to apply any security patches. Most vendors have a mailing list or RSS feed detailing any website security issues. WordPress, Umbraco and many other CMSes notify you of available system updates when you log in.
02. SQL injection
SQL injection attacks are when an attacker uses a web form field or URL parameter to gain access to or manipulate your database. When you use standard Transact SQL it is easy to unknowingly insert rogue code into your query that could be used to change tables, get information and delete data. You can easily prevent this by always using parameterised queries, most web languages have this feature and it is easy to implement.
Consider this query:
"SELECT * FROM table WHERE column = '" + parameter + "';"
If an attacker changed the URL parameter to pass in ' or '1'='1 this will cause the query to look like this:
"SELECT * FROM table WHERE column = '' OR '1'='1';"
Since '1' is equal to '1' this will allow the attacker to add an additional query to the end of the SQL statement which will also be executed.
04. Error messages
Be careful with how much information you give away in your error messages. For example if you have a login form on your website you should think about the language you use to communicate failure when attempting logins. You should use generic messages like “Incorrect username or password” as not to specify when a user got half of the query right. If an attacker tries a brute force attack to get a username and password and the error message gives away when one of the fields are correct then the attacker knows he has one of the fields and can concentrate on the other field.
Keep your error messages vague
05. Server side validation/form validation
Validation should always be done both on the browser and server side. The browser can catch simple failures like mandatory fields that are empty and when you enter text into a numbers only field. These can however be bypassed, and you should make sure you check for these validation and deeper validation server side as failing to do so could lead to malicious code or scripting code being inserted into the database or could cause undesirable results in your website.
Everyone knows they should use complex passwords, but that doesn’t mean they always do. It is crucial to use strong passwords to your server and website admin area, but equally also important to insist on good password practices for your users to protect the security of their accounts.
As much as users may not like it, enforcing password requirements such as a minimum of around eight characters, including an uppercase letter and number will help to protect their information in the long run.
Passwords should always be stored as encrypted values, preferably using a one way hashing algorithm such as SHA. Using this method means when you are authenticating users you are only ever comparing encrypted values. For extra website security it is a good idea to salt the passwords, using a new salt per password.
In the event of someone hacking in and stealing your passwords, using hashed passwords could help damage limitation, as decrypting them is not possible. The best someone can do is a dictionary attack or brute force attack, essentially guessing every combination until it finds a match. When using salted passwords the process of cracking a large number of passwords is even slower as every guess has to be hashed separately for every salt + password which is computationally very expensive.
Thankfully, many CMSes provide user management out of the box with a lot of these website security features built in, although some configuration or extra modules might be required to use salted passwords (pre Drupal 7) or to set the minimum password strength. If you are using .NET then it's worth using membership providers as they are very configurable, provide inbuilt website security and include readymade controls for login and password reset.
07. 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)$">
allow from all
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.
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 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.
Use an SSL certificate
10. 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.