"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. A cross-site scripting vulnerability may be used by attackers to bypass access controls such as the same origin policy. Cross-site scripting carried out on websites accounted for roughly 84% of all security vulnerabilities documented by Symantec as of 2007." - https://en.wikipedia.org/wiki/Cross-site_scripting

https://anti-xss-demo.suckup.de/

1. Use filter_input() - don't use GLOBAL-Array (e.g. $_SESSION, $_GET, $_POST, $_SERVER) directly

2. Use html-sanitizer or HTML Purifier if you need a more configurable solution

3. Add "Content Security Policy's" -> Introduction to Content Security Policy

4. DO NOT WRITE YOUR OWN REGEX TO PARSE HTML!

5. READ THIS TEXT -> XSS (Cross Site Scripting) Prevention Cheat Sheet

6. TEST THIS TOOL -> Zed Attack Proxy (ZAP)

composer require voku/anti-xss

use voku\helper\AntiXSS;

require_once __DIR__ . '/vendor/autoload.php'; // example path

$antiXss = new AntiXSS();

Example 1: (HTML Character)

$harm_string = "Hello, i try to <script>alert('Hack');</script> your site";
$harmless_string = $antiXss->xss_clean($harm_string);

// Hello, i try to alert&#40;'Hack'&#41;; your site

Example 2: (Hexadecimal HTML Character)

$harm_string = "<IMG SRC=&#x6A&#x61&#x76&#x61&#x73&#x63&#x72&#x69&#x70&#x74&#x3A&#x61&#x6C&#x65&#x72&#x74&#x28&#x27&#x58&#x53&#x53&#x27&#x29>";
$harmless_string = $antiXss->xss_clean($harm_string);
    
// <IMG >

Example 3: (Unicode Hex Character)

$harm_string = "<a href='&#x2000;javascript:alert(1)'>CLICK</a>";
$harmless_string = $antiXss->xss_clean($harm_string);
    
// <a >CLICK</a>

Example 4: (Unicode Character)

$harm_string = "<a href=\"\u0001java\u0003script:alert(1)\">CLICK<a>";
$harmless_string = $antiXss->xss_clean($harm_string);
    
// <a >CLICK</a>

Example 5.1: (non Inline CSS)

$harm_string = '<li style="list-style-image: url(javascript:alert(0))">';
$harmless_string = $antiXss->xss_clean($harm_string);

// <li >

Example 5.2: (with Inline CSS)

$harm_string = '<li style="list-style-image: url(javascript:alert(0))">';
$antiXss->removeEvilAttributes(array('style')); // allow style-attributes
$harmless_string = $antiXss->xss_clean($harm_string);

// <li style="list-style-image: url(alert&#40;0&#41;)">

Example 6: (check if an string contains a XSS attack)

$harm_string = "\x3cscript src=https://www.example.com/malicious-code.js\x3e\x3c/script\x3e";
$harmless_string = $antiXss->xss_clean($harm_string);

// 

$antiXss->isXssFound(); 

// true

Example 7: (allow e.g. iframes)

$harm_string = "<iframe width="560" onclick="alert('xss')" height="315" src="https://www.youtube.com/embed/foobar?rel=0&controls=0&showinfo=0" frameborder="0" allowfullscreen></iframe>";

$antiXss->removeEvilHtmlTags(array('iframe'));

$harmless_string = $antiXss->xss_clean($harm_string);

// <iframe width="560"  height="315" src="https://www.youtube.com/embed/foobar?rel=0&controls=0&showinfo=0" frameborder="0" allowfullscreen></iframe>

1. Composer is a prerequisite for running the tests.

composer install

2. The tests can be executed by running this command from the root directory:

./vendor/bin/phpunit

↑ Add some strings to the "_do_not_close_html_tags"-array.

Parameters:

• string[] $strings

Return:

• $this

↑ Add some strings to the "_evil_attributes"-array.

Parameters:

• string[] $strings

Return:

• $this

↑ Add some strings to the "_evil_html_tags"-array.

Parameters:

• string[] $strings

Return:

• $this

↑ Add some strings to the "_never_allowed_call_strings"-array.

Parameters:

• string[] $strings

Return:

• $this

↑ Add some strings to the "_never_allowed_js_callback_regex"-array.

Parameters:

• string[] $strings

Return:

• $this

↑ Add some strings to the "_never_allowed_on_events_afterwards"-array.

Parameters:

• string[] $strings

Return:

• $this

↑ Add some strings to the "_never_allowed_regex"-array.

Parameters:

• string[] $strings

Return:

• $this

↑ Add some strings to the "_never_allowed_str_afterwards"-array.

Parameters:

• string[] $strings

Return:

• $this

↑ Check if the "AntiXSS->xss_clean()"-method found an XSS attack in the last run.

Parameters: nothing

Return:

• bool|null <p>Will return null if the "xss_clean()" wasn't running at all.</p>

↑ Remove some strings from the "_do_not_close_html_tags"-array.

WARNING: Use this method only if you have a really good reason.

Parameters:

• string[] $strings

Return:

• $this

↑ Remove some strings from the "_evil_attributes"-array.

WARNING: Use this method only if you have a really good reason.

Parameters:

• string[] $strings

Return:

• $this

↑ Remove some strings from the "_evil_html_tags"-array.

WARNING: Use this method only if you have a really good reason.

Parameters:

• string[] $strings

Return:

• $this

↑ Remove some strings from the "_never_allowed_call_strings"-array.

WARNING: Use this method only if you have a really good reason.

Parameters:

• string[] $strings

Return:

• $this

↑ Remove some strings from the "_never_allowed_js_callback_regex"-array.

WARNING: Use this method only if you have a really good reason.

Parameters:

• string[] $strings

Return:

• $this

↑ Remove some strings from the "_never_allowed_on_events_afterwards"-array.

WARNING: Use this method only if you have a really good reason.

Parameters:

• string[] $strings

Return:

• $this

↑ Remove some strings from the "_never_allowed_regex"-array.

WARNING: Use this method only if you have a really good reason.

Parameters:

• string[] $strings

Return:

• $this

↑ Remove some strings from the "_never_allowed_str_afterwards"-array.

WARNING: Use this method only if you have a really good reason.

Parameters:

• string[] $strings

Return:

• $this

↑ Set the replacement-string for not allowed strings.

Parameters:

• string $string

Return:

• $this

↑ Set the option to stripe 4-Byte chars.

INFO: use it if your DB (MySQL) can't use "utf8mb4" -> preventing stored XSS-attacks

Parameters:

• bool $bool

Return:

• $this

↑ XSS Clean

Sanitizes data so that "Cross Site Scripting" hacks can be prevented. This method does a fair amount of work but it is extremely thorough, designed to prevent even the most obscure XSS attempts. But keep in mind that nothing is ever 100% foolproof...

Note: Should only be used to deal with data upon submission. It's not something that should be used for general runtime processing.

Parameters:

• TXssCleanInput $str <p>input data e.g. string or array of strings</p>

Return:

• string|string[]

For support and donations please visit Github | Issues | PayPal | Patreon.

For status updates and release announcements please visit Releases | Twitter | Patreon.

For professional support please contact me.

• Thanks to GitHub (Microsoft) for hosting the code and a good infrastructure including Issues-Managment, etc.
• Thanks to IntelliJ as they make the best IDEs for PHP and they gave me an open source license for PhpStorm!
• Thanks to Travis CI for being the most awesome, easiest continous integration tool out there!
• Thanks to StyleCI for the simple but powerfull code style check.
• Thanks to PHPStan && Psalm for relly great Static analysis tools and for discover bugs in the code!