How to prevent XSS with HTML/PHP?

Basically you need to use the function htmlspecialchars() whenever you want to output something to the browser that came from the user input.

The correct way to use this function is something like this:

echo htmlspecialchars($string, ENT_QUOTES, 'UTF-8');

Google Code University also has these very educational videos on Web Security:

One of the most important steps is to sanitize any user input before it is processed and/or rendered back to the browser. PHP has some "filter" functions that can be used.

The form that XSS attacks usually have is to insert a link to some off-site javascript that contains malicious intent for the user. Read more about it here.

You'll also want to test your site - ~~I can recommend the Firefox add-on [XSS Me]~~. Looks like Easy XSS is now the way to go.

Cross-posting this as a consolidated reference from the SO Documentation beta which is going offline.

Problem

Cross-site scripting is the unintended execution of remote code by a web client. Any web application might expose itself to XSS if it takes input from a user and outputs it directly on a web page. If input includes HTML or JavaScript, remote code can be executed when this content is rendered by the web client.

For example, if a 3rd party side contains a JavaScript file:

// http://example.com/runme.js
document.write("I'm running");

And a PHP application directly outputs a string passed into it:

<?php
echo '<div>' . $_GET['input'] . '</div>';

If an unchecked GET parameter contains <script src="http://example.com/runme.js"></script> then the output of the PHP script will be:

<div><script src="http://example.com/runme.js"></script></div>

The 3rd party JavaScript will run and the user will see "I'm running" on the web page.

Solution

As a general rule, never trust input coming from a client. Every GET parameter, POST or PUT content, and cookie value could be anything at all, and should therefore be validated. When outputting any of these values, escape them so they will not be evaluated in an unexpected way.

Keep in mind that even in the simplest applications data can be moved around and it will be hard to keep track of all sources. Therefore it is a best practice to always escape output.

PHP provides a few ways to escape output depending on the context.

Filter Functions

PHPs Filter Functions allow the input data to the php script to be sanitized or validated in many ways. They are useful when saving or outputting client input.

HTML Encoding

htmlspecialchars will convert any "HTML special characters" into their HTML encodings, meaning they will then not be processed as standard HTML. To fix our previous example using this method:

<?php
echo '<div>' . htmlspecialchars($_GET['input']) . '</div>';
// or
echo '<div>' . filter_input(INPUT_GET, 'input', FILTER_SANITIZE_SPECIAL_CHARS) . '</div>';

Would output:

<div>&lt;script src=&quot;http://example.com/runme.js&quot;&gt;&lt;/script&gt;</div>

Everything inside the <div> tag will not be interpreted as a JavaScript tag by the browser, but instead as a simple text node. The user will safely see:

<script src="http://example.com/runme.js"></script>

URL Encoding

When outputting a dynamically generated URL, PHP provides the urlencode function to safely output valid URLs. So, for example, if a user is able to input data that becomes part of another GET parameter:

<?php
$input = urlencode($_GET['input']);
// or
$input = filter_input(INPUT_GET, 'input', FILTER_SANITIZE_URL);
echo '<a href="http://example.com/page?input="' . $input . '">Link</a>';

Any malicious input will be converted to an encoded URL parameter.

Using specialised external libraries or OWASP AntiSamy lists

Sometimes you will want to send HTML or other kind of code inputs. You will need to maintain a list of authorised words (white list) and un-authorized (blacklist).

You can download standard lists available at the OWASP AntiSamy website. Each list is fit for a specific kind of interaction (ebay api, tinyMCE, etc...). And it is open source.

There are libraries existing to filter HTML and prevent XSS attacks for the general case and performing at least as well as AntiSamy lists with very easy use. For example you have HTML Purifier