Advertisement
PHP

5 Helpful Tips for Creating Secure PHP Applications

by

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.

Tip 1: Use Proper Error Reporting

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).

error_reporting(0);
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

Tip 2: Disable PHP's "Bad Features"

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( !empty( $_POST['username'] ) && $_POST['username'] == 'test' && !empty( $_POST['password'] ) && $_POST['password'] == "test123" )
{
    $access = true;
}

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

php_flag register_globals 0

Disabling with php.ini

register_globals = Off

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

php_flag magic_quotes_gpc 0 php_flag magic_quotes_runtime 0

Disabling with php.ini

magic_quotes_gpc = Off
magic_quotes_runtime = Off
magic_quotes_sybase = Off

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

Tip 3: Validate Input

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:

if ( ! preg_match( "/^[0-9]{1,2}$/", $_GET['month'] ) )
{
    // handle error
}
if ( ! preg_match( "/^[0-9]{1,2}$/", $_GET['day'] ) )
{
    // handle error
}
if ( ! preg_match( "/^[0-9]{4}$/", $_GET['year'] ) )
{
    // handle error
}

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

Tip 4: Watch for Cross Site Scripting (XSS) Attacks in User Input

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

Tip 5: Protecting against SQL Injection

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:

$username = mysqli_real_escape_string( $GET['username'] );
mysql_query( "SELECT * FROM tbl_members WHERE username = '".$username."'");

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):

$id = $_GET['id'];
$statement = $connection->prepare( "SELECT * FROM tbl_members WHERE id = ?" );
$statement->bind_param( "i", $id );
$statement->execute();

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.

Closing

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.

What are your tips?

  • Subscribe to the NETTUTS RSS Feed for more daily web development tuts and articles.


Related Posts
  • Code
    Web Development
    How to Use New Relic With PHP & WordPressRelic retina preview
    Today we will look at how to monitor a PHP application using New Relic. More specifically we will set up a basic WordPress installation and get some performance data about it, in the New Relic dashboards.Read More…
  • Code
    Web Development
    Securely Handling User's Login CredentialsSecure wide retina preview
    Consider the following tips on how to properly secure your user's login credentials.Read More…
  • Code
    iOS SDK
    Securing and Encrypting Data on iOSPs8e2e preview image@2x
    Whether you're creating a mobile application or a web service, keeping sensitive data secure is important and security has become an essential aspect of every software product. In this tutorial, I will show you how to safely store user credentials using the application's keychain and we'll take a look at encrypting and decrypting user data using a third party library.Read More…
  • Code
    ASP.NET
    Preventing Code InjectionCsrf dotnet retina preview
    Often, websites seem to exist primarily to put something into a database in order to pull it out later. While other database methods, such as NoSQL, have gained popularity in recent years, data for many websites still resides in the traditional SQL database. This data often consists of valuable personal information such as credit card numbers and other personal information of interest to identity thieves and criminals. Hackers therefore always look to get this data. One of the most common targets of these attacks is the SQL databases that lie behind many web applications through a process of SQL Injection.Read More…
  • Code
    PHP
    Validation and Exception Handling: From the UI to the BackendProcedural to oop php retina preview
    Sooner or later in your programming career you will be faced with the dilemma of validation and exception handling. This was the case with me and my team also. A couple or so years ago we reached a point when we had to take architectural actions to accommodate all the exceptional cases our quite large software project needed to handle. Below is a list of practices we came to value and apply when it comes to validation and exception handling.Read More…
  • Code
    PHP
    PHP 101Code
    There's no denying that ours is an incredibly difficult industry. Ever considered learning a second language? Well, how about five? That's what will be required of you, if you intend to become a modern web developer. Considering this, if you're not careful, very quickly, you may find yourself overwhelmed, as you stare blindly at countless confusing blog articles, or techical books.Read More…