PHP Security, Part 1
by John Coggeshall
07/31/2003
In my last two columns (Common Style Mistakes,
part one and Common Style Mistakes,
part two), I discussed some common bad practices to avoid when writing
PHP scripts which can make them more difficult to read and more prone to
bugs. In today's column I'll change gears and discuss the meat of this
series: the importance of security when working with PHP.
The Importance of Thinking About Security
More than meets the eye
The most effective and often overlooked measure to prevent malicious
users from compromising your scripts is to consider the possibility it
could happen when you write them. It's s important to be mindful of the
possible security implications of your code.
Consider the following example function designed to simplify the life
of a developer who writes a great many text files from PHP scripts:
<?php
function write_text($filename, $text="") {
static $open_files = array();
// If filename is null, close all open files
if ($filename == NULL) {
foreach($open_files as $fr) {
fclose($fr);
}
return true;
}
$index = md5($filename);
if(!isset($open_files[$index])) {
$open_files[$index] = fopen($filename, "a+");
if(!$open_files[$index]) return false;
}
fputs($open_files[$index], $text);
return true;
}?>
This function takes two parameters by default, the filename and the
text to write to that file. The function will first check to see if it has
already opened that file in the past; if it has, it will reuse the old
file reference. Otherwise, it will create one if one doesn't exist. In
either case, the text is then written to the file. If the filename passed
to the function is NULL, then all the opened file references are closed.
An example usage is provided below.
If the developer is writing a number of text files in this manner, this
function will make his code look much cleaner and easier to understand. Let's
assume that this function lives in a separate file which is included in the
scripts which require the function. Here's one of the scripts where it's used,
called quotes.php:
<html><body>
<form action="<?=$_SERVER['PHP_SELF']?>" method="get">
Choose the nature of the quote:
<select name="quote" size="3">
<option value="funny">Humorous quotes</option>
<option value="political">Political quotes</option>
<option value="love">Romantic Quotes</option>
</select><br />
The quote: <input type="text" name="quote_text" size="30" />
<input type="submit" value="Save Quote" />
</form>
</body></html>
<?php
include_once('write_text.php');
$filename = "/home/web/quotes/{$_GET['quote']}";
$quote_msg = $_GET['quote_text'];
if (write_text($filename, $quote_msg)) {
echo "<center><hr><h4>Quote saved!</h4></center>";
} else {
echo "<center><hr><h4>Error writing quote</h4></center>";
}
write_text(NULL);
?>
As you can see, this developer has used the write_text()
function created previously to develop a system to allow users to submit their
favorite quotes, which are then saved to a text file. Unfortunately, though
the developer may not know it, this script could also allow a malicious user to
compromise the security of the web server.
Perhaps right now you are scratching your head and wondering exactly how
such an innocent looking script poses such a security risk. Instead of asking
you to figure it out yourself, consider the following URL, remembering that the
script itself is called quotes.php:
http://www.somewhere.com/fun/quotes.php?quote=different_file.dat"e_text=garbage+data
What will happen when this URL is presented to the web server?
Obviously the quotes.php script will be executed; but instead of
writing a quote to one of the three desired files, a completely new file
called different_file.dat will be written with a string
garbage data inside of it. Obviously, this is not desired
behavior at all. In fact, a malicious user might even be able to create an
account by accessing the Unix password file by specifying
../../../etc/passwd for the quote parameter
(although that would require the web server to be running scripts as a
superuser, and if that is the case you should stop reading this and fix
that right now). Perhaps the most serious implication of this script is
it could even be used to allow someone to write and execute arbitrary PHP
scripts, if the /home/web/quotes/ directory were accessible from
a browser. The evil possibilities are endless.
There are several solutions. If you only need to write a few files in
the directory, consider using an associative array to store the file
names. If the user input exists in the associative array, it's safe to
write. Another option is to strip out all non-alpha and non-numeric
characters, to make sure there are no directory separators. Yet another
idea is to check the file extension to make sure it won't be executed by
the web server.
The bottom line is simple. As a developer you must be aware of more
than what your scripts do under the desired circumstances. What will
happen if invalid data is entered into a form element? Is there any way a
malicious user could make your script behave in an unintended way? What
measures are being taken to prevent these attacks? Your web server and PHP
scripts are only as safe as the weakest security link, so it's important
to identify these possible weak links before they're identified for
you.
Common security-related mistakes
To give you a few pointers, here's a brief and incomplete list of
coding or administrative failures which can compromise security:
Mistake 1. Trusting data
As will be the theme throughout my discussion of security as it
relates to PHP scripts, you should never trust data provided by an outside
source. No matter if it comes from a user-submitted form, a file in the
filesystem, or an environment variable, nothing should simply be taken at
face value. All user input should be validated and formatted to make sure
it's safe.
Mistake 2. Storing sensitive data in the web tree
Any and all sensitive data should always be stored in a separate file
from the script that needs it and stored in a directory that cannot be
accessed via a web server request. When the sensitive data in question is
needed, that data can be included in the appropriate PHP script via an
include or require statement.
Mistake 3. Not implementing recommended security precautions
The PHP manual contains an entire section devoted to security
precautions when using and coding PHP scripts. The manual (almost) always
clearly notes on a case-by-case basis when a potential security risk
exists and how that risk can be minimized. Again, malicious users rely on
developers and system administrators failing to pay attention to security
concerns in order to gain access to their systems. Heeding these warnings
and acting appropriately significantly diminishes the chance of a
malicious user being able to do any real damage to your system.
More on security soon
I cannot stress enough the importance of thinking about security in
order to protect your servers from malicious users. Now you should be
looking at your scripts in a whole new light. With a little experience,
soon you'll be catching these potential security lapses before you even
write the code to create them. The next column will discuss a few more
common ways security is compromised in PHP scripts and the steps you as a
developer can do to minimize them.
John Coggeshall
is a a PHP consultant and author who started losing sleep over PHP around five years ago.
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