Web Application Security, Part 1

Application-level web security is of increasing concern among web developers. This article outlines some types of security threats to your web application and how to solve those threats.

This is Part 1 of the Web Application Security article, geared toward the material covered in Module 2. For material covered in Module 3 (MySQL), see Web Application Security, Part 2. For material covered in Module 6 (JavaScript), see Web Application Security, Part 3.

Introduction to Application-Level Web Security

Every day, computer hackers around the world penetrate web applications, often for personal profits. You may find it hard to believe, but even high-profile web sites (banks, social media, even computer security companies) are vulnerable to application-level attacks!

Not only is it embarrassing to be the programmer who wrote the vulnerable code, but it could also cost you your job. As a prudent web developer, it is imperative that you take precautionary measures to make your application difficult to penetrate. Indeed, most of the time, if your site is well-written, hackers will just move on.

Here's the golden rule: Anything in your site that accepts user input, whether via a form, an AJAX request, a file upload, or even malformed links, can be used as an attack vector. NEVER TRUST USER INPUT!!! This can be summarized in the acronym FIEO, or Filter Input, Escape Output.

FIEO in PHP

Filtering Input

"Filter Input" means that you should check that input data is of the format that you are expecting. For example, if you are expecting a number, you should cast it to a float or an int. If you are expecting a phone number, you should run it through a regular expression (you will learn regular expressions in module 4). For example:

<?php
// Cast a number to a float or an int:
$amount = (float) $_POST['amount'];

// Pass a phone number through a regular expression:
$phone = preg_match('/\d{3}-\d{3}-\d{4}/', $_POST['phone']) ? $_POST['phone'] : "";
?>

Escaping Output

"Escape Output" means that you need to nullify, or escape, characters that have special meaning in the markup language of interest. For example, consider the following string:

If a<b and b<c then a<c.

Since a less-than sign means the start of a tag in HTML, and b is a valid tag name, the above string will not render as you might expect in HTML. Therefore, we need to escape our less-than signs by using HTML entities:

If a<b and b<c then a<c.

The "<" is an HTML entity that will render as a less-than sign. (For more information on HTML entities, read this article on the WebPlatform wiki.)

PHP provides a function that, given a string, will convert special characters to their HTML entity equivalents.

<?php
$str = "If a<b and b<c then a<c.";

// Convert special characters to HTML entities before outputting:
echo htmlentities($str);
?>

Note: htmlentities escapes a string for use in HTML, but it does not escape a string for use in other markup languages. You need to use different methods when escaping strings for other languages.

Why Not to Escape Input

Filtering your input is important, as shown above. However, it is bad practice to escape your input. For example, don't do this:

<?php
$message = htmlentites($_POST['amount']); // bad practice
// then store $message in a database, etc.
?>

The reason this is bad practice is that it permanently ties that string to its final output format. For example, what if some time down the road you want to support display of that message in a PDF? You'd need to go back and remove all the HTML entities again.

This is why you should filter strings at the input stage but not escape them until the final output stage.

Format String Injection

If you like using functions like printf and sprintf, you may find yourself writing

printf( "%s", htmlentities($_GET['username']) ); // good example

It is tempting to reduce this to

printf( htmlentities($_GET['username']) ); // BAD example

Although the second implementation will work for most usernames, it is not correct! You are essentially making the client-provided username the format string for printf. If the username contains any percentage sign (%), it will be interpreted as the start of a parameter in the format string, causing your script to return errors. Worse yet, it is known that certain combinations of format parameters will actually reveal system information in the error log.

Solution

The solution is simple: never put dynamic input as the format string. It should always be static, either hard-coded or from a stable source like a YAML file. User-supplied input should always be fed into the string as arguments to sprintf and printf.

If you are outputting only one little string like in the example above, it suffices to use a PHP function like print or echo:

print htmlentities($_GET['username']); // good example
echo htmlentities($_GET['username']); // good example

Server Configurations

Sometimes hackers attempt to penetrate your application from the server side rather than the application side. Server-side security is beyond the realm of this course, but here are some things you should keep in mind.

• Use a highly secure root password, and it should be one that you don't use anywhere else. Seriously.
• Use a firewall system to block unnecessary ports from public access. SSH and Web Server should really be the only ports you need. You should keep the web serve on port 80, but you have the option of moving SSH to a port other than 22 to make it slightly more secure.