PHP 5 Form Validation
PHP
Form Validation
Think
SECURITY when processing PHP forms!
These pages
will show how to process PHP forms with security in mind. Proper validation of
form data is important to protect your form from hackers and spammers!
The HTML
form we will be working at in these chapters, contains various input fields:
required and optional text fields, radio buttons, and a submit button:
The
validation rules for the form above are as follows:
Field Validation Rules
Name Required. + Must only contain letters and
whitespace
E-mail Required. + Must contain a valid email address
(with @ and .)
Website Optional. If present, it must contain a valid
URL
Comment Optional. Multi-line input field (textarea)
Gender Required. Must select one
First we
will look at the plain HTML code for the form:
Text
Fields
The name,
email, and website fields are text input elements, and the comment field is a
textarea. The HTML code looks like this:
Name:
<input type="text" name="name">
E-mail:
<input type="text" name="email">
Website:
<input type="text" name="website">
Comment:
<textarea name="comment" rows="5"
cols="40"></textarea>
Radio
Buttons
The gender
fields are radio buttons and the HTML code looks like this:
Gender:
<input
type="radio" name="gender" value="female">Female
<input
type="radio" name="gender" value="male">Male
The Form
Element
The HTML
code of the form looks like this:
<form
method="post" action="<?php echo
htmlspecialchars($_SERVER["PHP_SELF"]);?>">
When the
form is submitted, the form data is sent with method="post".
What is the
$_SERVER["PHP_SELF"] variable?
The
$_SERVER["PHP_SELF"] is a super global variable that returns the
filename of the currently executing script.
So, the
$_SERVER["PHP_SELF"] sends the submitted form data to the page
itself, instead of jumping to a different page. This way, the user will get
error messages on the same page as the form.
What is the
htmlspecialchars() function?
The
htmlspecialchars() function converts special characters to HTML entities. This
means that it will replace HTML characters like < and > with < and
>. This prevents attackers from exploiting the code by injecting HTML or
Javascript code (Cross-site Scripting attacks) in forms.
Big Note on
PHP Form Security
The
$_SERVER["PHP_SELF"] variable can be used by hackers!
If PHP_SELF
is used in your page then a user can enter a slash (/) and then some Cross Site
Scripting (XSS) commands to execute.
Cross-site
scripting (XSS) is a type of computer security vulnerability typically found in
Web applications. XSS enables attackers to inject client-side script into Web
pages viewed by other users.
Assume we
have the following form in a page named "test_form.php":
<form
method="post" action="<?php echo
$_SERVER["PHP_SELF"];?>">
Now, if a
user enters the normal URL in the address bar like
"http://www.example.com/test_form.php", the above code will be
translated to:
<form
method="post" action="test_form.php">
So far, so
good.
However,
consider that a user enters the following URL in the address bar:
In this
case, the above code will be translated to:
<form
method="post"
action="test_form.php/"><script>alert('hacked')</script>
This code
adds a script tag and an alert command. And when the page loads, the JavaScript
code will be executed (the user will see an alert box). This is just a simple
and harmless example how the PHP_SELF variable can be exploited.
Be aware of
that any JavaScript code can be added inside the <script> tag! A hacker
can redirect the user to a file on another server, and that file can hold
malicious code that can alter the global variables or submit the form to
another address to save the user data, for example.
How To Avoid
$_SERVER["PHP_SELF"] Exploits?
$_SERVER["PHP_SELF"]
exploits can be avoided by using the htmlspecialchars() function.
The form
code should look like this:
<form
method="post" action="<?php echo
htmlspecialchars($_SERVER["PHP_SELF"]);?>">
The
htmlspecialchars() function converts special characters to HTML entities. Now
if the user tries to exploit the PHP_SELF variable, it will result in the
following output:
<form
method="post"
action="test_form.php/"><script>alert('hacked')</script>">
The exploit
attempt fails, and no harm is done!
Validate
Form Data With PHP
The first
thing we will do is to pass all variables through PHP's htmlspecialchars()
function.
When we use
the htmlspecialchars() function; then if a user tries to submit the following
in a text field:
<script>location.href('http://www.hacked.com')</script>
- this would
not be executed, because it would be saved as HTML escaped code, like this:
<script>location.href('http://www.hacked.com')</script>
The code is
now safe to be displayed on a page or inside an e-mail.
We will also
do two more things when the user submits the form:
Strip
unnecessary characters (extra space, tab, newline) from the user input data
(with the PHP trim() function)
Remove
backslashes (\) from the user input data (with the PHP stripslashes() function)
The next
step is to create a function that will do all the checking for us (which is
much more convenient than writing the same code over and over again).
We will name
the function test_input().
Now, we can
check each $_POST variable with the test_input() function, and the script looks
like this:
Example
<?php
//
define variables and set to empty values
$name
= $email = $gender = $comment = $website = "";
if
($_SERVER["REQUEST_METHOD"] == "POST") {
$name = test_input($_POST["name"]);
$email =
test_input($_POST["email"]);
$website =
test_input($_POST["website"]);
$comment =
test_input($_POST["comment"]);
$gender =
test_input($_POST["gender"]);
}
function
test_input($data) {
$data = trim($data);
$data = stripslashes($data);
$data = htmlspecialchars($data);
return $data;
}
?>
Notice that
at the start of the script, we check whether the form has been submitted using
$_SERVER["REQUEST_METHOD"]. If the REQUEST_METHOD is POST, then the
form has been submitted - and it should be validated. If it has not been submitted,
skip the validation and display a blank form.
However, in
the example above, all input fields are optional. The script works fine even if
the user does not enter any data.
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