How to prevent XSS with HTML/PHP?

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How do I prevent XSS (cross-site scripting) using just HTML and PHP?

I've seen numerous other posts on this topic but I have not found an article that clear and concisely states how to actually prevent XSS.

This Question Has 8 Answeres | Orginal Question | TimTim

One of my favorite OWASP references is the Cross-Site Scripting explanation because while there are a large number of XSS attack vectors, the following of a few rules can defend against the majority of them greatly!

This is PHP Security Cheat Sheet

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 some very educational videos on Web Security (link broken).

In order of preference:

  1. If you are using a templating engine (e.g. Twig, Smarty, Blade), check that it offers context-sensitive escaping. I know from experience that Twig does. {{ var|e('html_attr') }}
  2. If you want to allow HTML, use HTML Purifier. Even if you think you only accept Markdown or ReStructuredText, you still want to purify the HTML these markup languages output.
  3. Otherwise, use htmlentities($var, ENT_QUOTES | ENT_HTML5, $charset) and make sure the rest of your document uses the same character set as $charset. In most cases, 'UTF-8' is the desired character set.

Also, make sure you escape on output, not on input.

Use htmlspecialchars on PHP. On HTML try to avoid using:

element.innerHTML = “…”; element.outerHTML = “…”; document.write(…); document.writeln(…);

where var is controlled by the user.

Also obviously try avoiding eval(var), if you have to use any of them then try JS escaping them, HTML escape them and you might have to do some more but for the basics this should be enough.

<?php function xss_clean($data) { // Fix &entity\n; $data = str_replace(array('&amp;','&lt;','&gt;'), array('&amp;amp;','&amp;lt;','&amp;gt;'), $data); $data = preg_replace('/(&#*\w+)[\x00-\x20]+;/u', '$1;', $data); $data = preg_replace('/(&#x*[0-9A-F]+);*/iu', '$1;', $data); $data = html_entity_decode($data, ENT_COMPAT, 'UTF-8'); // Remove any attribute starting with "on" or xmlns $data = preg_replace('#(<[^>]+?[\x00-\x20"\'])(?:on|xmlns)[^>]*+>#iu', '$1>', $data); // Remove javascript: and vbscript: protocols $data = preg_replace('#([a-z]*)[\x00-\x20]*=[\x00-\x20]*([`\'"]*)[\x00-\x20]*j[\x00-\x20]*a[\x00-\x20]*v[\x00-\x20]*a[\x00-\x20]*s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:#iu', '$1=$2nojavascript...', $data); $data = preg_replace('#([a-z]*)[\x00-\x20]*=([\'"]*)[\x00-\x20]*v[\x00-\x20]*b[\x00-\x20]*s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:#iu', '$1=$2novbscript...', $data); $data = preg_replace('#([a-z]*)[\x00-\x20]*=([\'"]*)[\x00-\x20]*-moz-binding[\x00-\x20]*:#u', '$1=$2nomozbinding...', $data); // Only works in IE: <span style="width: expression(alert('Ping!'));"></span> $data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?expression[\x00-\x20]*\([^>]*+>#i', '$1>', $data); $data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?behaviour[\x00-\x20]*\([^>]*+>#i', '$1>', $data); $data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:*[^>]*+>#iu', '$1>', $data); // Remove namespaced elements (we do not need them) $data = preg_replace('#</*\w+:\w[^>]*+>#i', '', $data); do { // Remove really unwanted tags $old_data = $data; $data = preg_replace('#</*(?:applet|b(?:ase|gsound|link)|embed|frame(?:set)?|i(?:frame|layer)|l(?:ayer|ink)|meta|object|s(?:cript|tyle)|title|xml)[^>]*+>#i', '', $data); } while ($old_data !== $data); // we are done... return $data; } 

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.

You are also able to set some XSS related HTTP response headers via header(...)

X-XSS-Protection "1; mode=block"

to be sure, the browser XSS protection mode is enabled.

Content-Security-Policy "default-src 'self'; ..."

to enable browser-side content security. See this one for Content Security Policy (CSP) details: http://content-security-policy.com/ Especially setting up CSP to block inline-scripts and external script sources is helpful against XSS.

for a general bunch of useful HTTP response headers concerning the security of you webapp, look at OWASP: https://www.owasp.org/index.php/List_of_useful_HTTP_headers

Take a look at the Writing Secure Series on AddedBytes.com

http://www.addedbytes.com/writing-secure-php/

there are a lot more things you have to do to write secure php code than just htmlspecialchars, because that doesnt protect against sql injections or anything like that.


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Sajjad Hossain

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