Sunday, 31 July 2011

PHP htmlentities Function

PHP htmlentities Function

Whenever you allow your users to submit text to your website, you need to be careful that you don't leave any security holes open for malicious users to exploit. If you are ever going to allow user submitted text to be visible by the public you should consider using the htmlentities function to prevent them from running html code and scripts that may be harmful to your visitors.

PHP - Converting HTML into Entities

The htmlentities function takes a string and returns the same string with
HTML converted into HTML entities.
For example, the string "<script>" would be converted to "&lt;script&gt;".

By converting the < and > into entities, it prevents the browser from using it
as an HTML element and it prevents the code from running if you were to display some user's
input on your website.

This may seem a little complicated, but if you think of the way a browser works, in separate stages,
it becomes a little easier. Let's look at the way the function htmlentities changes the
data at three different levels: in PHP, in raw HTML and in the web browser. The sample
string is a bad script that will redirect visitors to the malicious user's own website.

PHP Code:

// An imaginary article submission from a bad user
//  it will redirect anyone to example.com if the code is run in a browser
$userInput = "I am going to hax0r your site, hahaha!
 <script type='text/javascript'>

 window.location = 'http://www.example.com/'
 </script>'";
 
//Lets make it safer before we use it
$userInputEntities = htmlentities($userInput);

//Now we can display it
echo $userInputEntities;

The HTML output of the above script would be as follows:

Safe Raw HTML Code:

I am going to hax0r your site, hahaha!
 &lt;script type='text/javascript'&gt;
 window.location = 'http://www.example.com/'
 &lt;/script&gt;'

If we had not used htmlentities to convert any HTML code into safe entities, this is
what the raw HTML code would be and it would have redirect a visitor to example.com.

Dangerous Raw HTML Code:

I am going to hax0r your site, hahaha!
 <script type='text/javascript'>
 window.location = 'http://www.example.com/'
 </script>'

Those two HTML code examples are what you would see if you were to view source on the web page.
However, if you were just viewing the output normally in your browser you would
see the following.

Safe Display:

I am going to hax0r your site, hahaha!
<script type='text/javascript'>
window.location = 'http://www.example.com/'
</script>'

Dangerous Display:

You'd see whatever spammer site that the malicious user had sent you to. Probably
some herbal supplement site or weight loss pills would be displayed.

When Would You Use htmlentities?

Anytime you allow users to submit content to your website, that other visitors can see, you should
consider removing the ability to let them use HTML. Although this will remove a lot of cool things that
your users can do, like making heavily customized content, it will prevent your site from
a lot of common attacks. With some custom coding you can just remove specific tags
from running, but that is beyond the scope of this lesson.

Just remember, that when allowing users to submit content to your site you are also
giving them access to your website. Be sure you take the proper precautions.

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