5 Helpful Tips for Creating Secure PHP Applications

5 Helpful Tips for Creating Secure PHP Applications

PHP is one of the most popular programming languages for the web. Sometimes a feature-friendly language can help the programmer too much, and security holes can creep in, creating roadblocks in the development path. In this tutorial, we will take a look at 5 tips to help you avoid some common PHP security pitfalls and development glitches.
During the development process, application error reporting is your best friend. Error reports can help you find spelling mistakes in your variables, detect incorrect function usage and much more. However, once the site goes live the same reporting that was an ally during development can turn traitor and tell your users much more about your site than you may want them to know (the software you run, your folder structure, etc).
Once your site goes live, you should make sure to hide all error reporting. This can be done by invoking the following simple function at the top of your application file(s).

Get rid of those public errors!
If something does go wrong, you still want and need to know about it. Therefore, you should always make sure to log your errors to a protected file. This can be done with the PHP function set_error_handler.

Sample Error Log
From its earliest days, PHP's designers have always included some features to make development easier. Or so they thought! Some of these helpful features can have unintended consequences. I call these "bad features" because they have allowed data validation nightmares and created a pathway for bugs to finding their way into scripts. One of the first things you should do when the development process begins is disable certain of these features.
Note: Depending on your host, these may or may not be turned off for you. If you are developing on your own computer or other similar local environment, they probably won't be turned off. Some of these features have also been removed in the upcoming PHP6, but are ubiquitous in PHP4 applications and are only deprecated in PHP5 applications.

Register Globals (register_globals)
In short, register_globals was meant to help rapid application development. Take for example this URL, http://yoursite.tld/index.php?var=1, which includes a query string. The register_globals statement allows us to access the value with $var instead of $_GET['var'] automatically. This might sound useful to you, but unfortunately all variables in the code now have this property, and we can now easily get into PHP applications that do not protect against this unintended consequence. The following code snippet is just one common example you will see in PHP scripts:

If the application is running with register_globals ON, a user could just place access=1 into a query string, and would then have access to whatever the script is running.

Unfortunately, we cannot disable register_globals from the script side (using ini_set, like we normally might), but we can use an .htaccess files to do this. Some hosts also allow you to have a php.ini file on the server.

Disabling with .htaccess
Disabling with php.ini
Note: If you use a custom php.ini file that is not applicable to the entire server, you must include these declarations in every sub folder that has PHP.

Flow of register global
Magic Quotes (magic_quotes_gpc, magic_quotes_runtime, magic_quotes_sybase)
Magic Quotes was a feature meant to save programmers the trouble of using addslashes() and other similar security features in their code. There are at least three problems associated with magic quotes. One problem with this helpful feature is if both magic quotes and addslashes() are used. If this is the case, then you end up with multiple slashes being added, causing errors. The second problem is if you make the assumption magic quotes is turned on and it actually is not. Then all the input goes unchecked. The third problem is that magic quotes only escapes single and double quotes, but if you are using a database engine, there are also many database-specific characters that also need to be escaped. It is recommended use that you disable this feature and use proper variable validation instead (see below).

Unfortunately, we also cannot disable magic quotes from the script side using ini_set. As with register_globals, we can use .htaccess or php.ini files to do this.

Disabling with .htaccess
Disabling with php.ini
Note: If you use a custom php.ini file that is not applicable to the entire server, you must include these declarations in every sub folder that has PHP.

Example htaccess file
In addition to escaping characters, another great to way to protect input is to validate it. With many applications, you actually already know what kind of data you are expecting on input. So the simplest way to protect yourself against attacks is to make sure your users can only enter the appropriate data.

For example, say we are creating an application that lists users birthdays and allows users to add their own. We will be wanting to accept a month as a digit between 1-12, a day between 1-31 and a year in the format of YYYY.

Having this kind of logic in your application is simple and regular expressions (regex) are the perfect way to handle input validation. Take the following example:

In this example, we simply checked (in the first two if statements) for integers [0-9] with a length of one or two {1,2} and we did the same in the third if statement, but checked for a strict length of 4 characters {4}.

In all instances, if the data doesn't match the format we want, we return some kind of error. This type of validation leaves very little room for any type of SQL attack.

Regex expressions like those above can be a little difficult to grasp at first, but explaining them is out of the scope of this article. The php manual has some additional resources to help you with validation. The PEAR database also has a few packages such as the Validate package to help with emails, dates, and URLS.

Below is an example of the above script in action using 200 as an input for a month, abc for the day and just 09 for the year.
Example of a validation script running
A web application usually accepts input from users and displays it in some way. This can, of course, be in a wide variety of forms including comments, threads or blog posts that are in the form of HTML code. When accepting input, allowing HTML can be a dangerous thing, because that allows for JavaScript to be executed in unintended ways. If even one hole is left open, JavasScript can be executed and cookies could be hijacked. This cookie data could then be used to fake a real account and give an illegal user access to the website's data.

There are a few ways you can protect yourself from such attacks. One way is to disallow HTML altogether, because then there is no possible way to allow any JavaScript to execute. However, if you do this then formatting is also disallowed, which is not always an option for forum and blog software.

If you want HTML mostly disabled, but still want to allow simple formatting, you can allow just a few selected HTML tags (without attributes) such as <strong> or <em>. Or, alternatively, you can allow a popular set of tags called "BBCode" or "BB Tags," commonly seen on forums in the format of [b]test[/b]. This can be a perfect way to allow some formatting customization while disallowing anything dangerous. You can implement BBCode using pre-existing packages such as HTML_BBCodeParser or write your own BBCode implementation with regular expressions and a series of preg_replace statements.

Example of BBCode in action
Last, but not least, is one of the most well-known security attacks on the web: SQL injection. SQL injection attacks occur when data goes unchecked, and the application doesn't escape characters used in SQL strings such as single quotes (') or double quotes (").

If these characters are not filtered out users can exploit the system by making queries always true and thus allowing them to trick login systems.
Pesky login box being hacked
Luckily, PHP does offer a few tools to help protect your database input. When you are connected to an sql server you can use these functions with a simple call, and your variables should be safe to use
in queries. Most of the major database systems offered with PHP include these protection functions.

MySQLi allows you to do this in one of two ways. Either with the mysqli_real_escape_string function when connected to a server:
Or with prepared statements.
Prepared statements are a method of separating SQL logic from the data being passed to it. The functions used within the MySQLi library filter our input for us when we bind variables to the prepared statement. This can be used like so (when connected to a server):
One thing to note when using prepared statements is the "i" in bind_param. i stands for for integer but you can use s for string, d for double, and b for blob depending on what data we are passing.
Although this will protect you in most circumstances, you should still keep in mind proper data validation as mentioned previously.

This short tutorial can only scratch the surface of web security. Ultimately, it is up to developers to ensure that the applications they build are safe by educating themselves about the dangers of the web and the most common kinds of vulnerabilities and attacks. If you wish to read more about security issues in PHP, there is a section on security in the php manual devoted to them.