system/core/Security.php

<?php
/**
 * CodeIgniter
 *
 * An open source application development framework for PHP
 *
 * This content is released under the MIT License (MIT)
 *
 * Copyright (c) 2014 - 2015, British Columbia Institute of Technology
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 *
 * @package	CodeIgniter
 * @author	EllisLab Dev Team
 * @copyright	Copyright (c) 2008 - 2014, EllisLab, Inc. (http://ellislab.com/)
 * @copyright	Copyright (c) 2014 - 2015, British Columbia Institute of Technology (http://bcit.ca/)
 * @license	http://opensource.org/licenses/MIT	MIT License
 * @link	http://codeigniter.com
 * @since	Version 1.0.0
 * @filesource
 */
defined('BASEPATH') OR exit('No direct script access allowed');

/**
 * Security Class
 *
 * @package		CodeIgniter
 * @subpackage	Libraries
 * @category	Security
 * @author		EllisLab Dev Team
 * @link		http://codeigniter.com/user_guide/libraries/security.html
 */
class CI_Security {

	/**
	 * List of sanitize filename strings
	 *
	 * @var	array
	 */
	public $filename_bad_chars =	array(
		'../', '<!--', '-->', '<', '>',
		"'", '"', '&', '$', '#',
		'{', '}', '[', ']', '=',
		';', '?', '%20', '%22',
		'%3c',		// <
		'%253c',	// <
		'%3e',		// >
		'%0e',		// >
		'%28',		// (
		'%29',		// )
		'%2528',	// (
		'%26',		// &
		'%24',		// $
		'%3f',		// ?
		'%3b',		// ;
		'%3d'		// =
	);

	/**
	 * Character set
	 *
	 * Will be overridden by the constructor.
	 *
	 * @var	string
	 */
	public $charset = 'UTF-8';

	/**
	 * XSS Hash
	 *
	 * Random Hash for protecting URLs.
	 *
	 * @var	string
	 */
	protected $_xss_hash;

	/**
	 * CSRF Hash
	 *
	 * Random hash for Cross Site Request Forgery protection cookie
	 *
	 * @var	string
	 */
	protected $_csrf_hash;

	/**
	 * CSRF Expire time
	 *
	 * Expiration time for Cross Site Request Forgery protection cookie.
	 * Defaults to two hours (in seconds).
	 *
	 * @var	int
	 */
	protected $_csrf_expire =	7200;

	/**
	 * CSRF Token name
	 *
	 * Token name for Cross Site Request Forgery protection cookie.
	 *
	 * @var	string
	 */
	protected $_csrf_token_name =	'ci_csrf_token';

	/**
	 * CSRF Cookie name
	 *
	 * Cookie name for Cross Site Request Forgery protection cookie.
	 *
	 * @var	string
	 */
	protected $_csrf_cookie_name =	'ci_csrf_token';

	/**
	 * List of never allowed strings
	 *
	 * @var	array
	 */
	protected $_never_allowed_str =	array(
		'document.cookie'	=> '[removed]',
		'document.write'	=> '[removed]',
		'.parentNode'		=> '[removed]',
		'.innerHTML'		=> '[removed]',
		'-moz-binding'		=> '[removed]',
		'<!--'				=> '&lt;!--',
		'-->'				=> '--&gt;',
		'<![CDATA['			=> '&lt;![CDATA[',
		'<comment>'			=> '&lt;comment&gt;'
	);

	/**
	 * List of never allowed regex replacements
	 *
	 * @var	array
	 */
	protected $_never_allowed_regex = array(
		'javascript\s*:',
		'(document|(document\.)?window)\.(location|on\w*)',
		'expression\s*(\(|&\#40;)', // CSS and IE
		'vbscript\s*:', // IE, surprise!
		'wscript\s*:', // IE
		'jscript\s*:', // IE
		'vbs\s*:', // IE
		'Redirect\s+30\d',
		"([\"'])?data\s*:[^\\1]*?base64[^\\1]*?,[^\\1]*?\\1?"
	);

	/**
	 * Class constructor
	 *
	 * @return	void
	 */
	public function __construct()
	{
		// Is CSRF protection enabled?
		if (config_item('csrf_protection'))
		{
			// CSRF config
			foreach (array('csrf_expire', 'csrf_token_name', 'csrf_cookie_name') as $key)
			{
				if (NULL !== ($val = config_item($key)))
				{
					$this->{'_'.$key} = $val;
				}
			}

			// Append application specific cookie prefix
			if ($cookie_prefix = config_item('cookie_prefix'))
			{
				$this->_csrf_cookie_name = $cookie_prefix.$this->_csrf_cookie_name;
			}

			// Set the CSRF hash
			$this->_csrf_set_hash();
		}

		$this->charset = strtoupper(config_item('charset'));

		log_message('info', 'Security Class Initialized');
	}

	// --------------------------------------------------------------------

	/**
	 * CSRF Verify
	 *
	 * @return	CI_Security
	 */
	public function csrf_verify()
	{
		// If it's not a POST request we will set the CSRF cookie
		if (strtoupper($_SERVER['REQUEST_METHOD']) !== 'POST')
		{
			return $this->csrf_set_cookie();
		}

		// Check if URI has been whitelisted from CSRF checks
		if ($exclude_uris = config_item('csrf_exclude_uris'))
		{
			$uri = load_class('URI', 'core');
			foreach ($exclude_uris as $excluded)
			{
				if (preg_match('#^'.$excluded.'$#i'.(UTF8_ENABLED ? 'u' : ''), $uri->uri_string()))
				{
					return $this;
				}
			}
		}

		// Do the tokens exist in both the _POST and _COOKIE arrays?
		if ( ! isset($_POST[$this->_csrf_token_name], $_COOKIE[$this->_csrf_cookie_name])
			OR $_POST[$this->_csrf_token_name] !== $_COOKIE[$this->_csrf_cookie_name]) // Do the tokens match?
		{
			$this->csrf_show_error();
		}

		// We kill this since we're done and we don't want to polute the _POST array
		unset($_POST[$this->_csrf_token_name]);

		// Regenerate on every submission?
		if (config_item('csrf_regenerate'))
		{
			// Nothing should last forever
			unset($_COOKIE[$this->_csrf_cookie_name]);
			$this->_csrf_hash = NULL;
		}

		$this->_csrf_set_hash();
		$this->csrf_set_cookie();

		log_message('info', 'CSRF token verified');
		return $this;
	}

	// --------------------------------------------------------------------

	/**
	 * CSRF Set Cookie
	 *
	 * @codeCoverageIgnore
	 * @return	CI_Security
	 */
	public function csrf_set_cookie()
	{
		$expire = time() + $this->_csrf_expire;
		$secure_cookie = (bool) config_item('cookie_secure');

		if ($secure_cookie && ! is_https())
		{
			return FALSE;
		}

		setcookie(
			$this->_csrf_cookie_name,
			$this->_csrf_hash,
			$expire,
			config_item('cookie_path'),
			config_item('cookie_domain'),
			$secure_cookie,
			config_item('cookie_httponly')
		);
		log_message('info', 'CRSF cookie sent');

		return $this;
	}

	// --------------------------------------------------------------------

	/**
	 * Show CSRF Error
	 *
	 * @return	void
	 */
	public function csrf_show_error()
	{
		show_error('The action you have requested is not allowed.', 403);
	}

	// --------------------------------------------------------------------

	/**
	 * Get CSRF Hash
	 *
	 * @see		CI_Security::$_csrf_hash
	 * @return 	string	CSRF hash
	 */
	public function get_csrf_hash()
	{
		return $this->_csrf_hash;
	}

	// --------------------------------------------------------------------

	/**
	 * Get CSRF Token Name
	 *
	 * @see		CI_Security::$_csrf_token_name
	 * @return	string	CSRF token name
	 */
	public function get_csrf_token_name()
	{
		return $this->_csrf_token_name;
	}

	// --------------------------------------------------------------------

	/**
	 * 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.  Nothing is ever 100% foolproof,
	 * of course, but I haven't been able to get anything passed
	 * the filter.
	 *
	 * Note: Should only be used to deal with data upon submission.
	 *	 It's not something that should be used for general
	 *	 runtime processing.
	 *
	 * @link	http://channel.bitflux.ch/wiki/XSS_Prevention
	 * 		Based in part on some code and ideas from Bitflux.
	 *
	 * @link	http://ha.ckers.org/xss.html
	 * 		To help develop this script I used this great list of
	 *		vulnerabilities along with a few other hacks I've
	 *		harvested from examining vulnerabilities in other programs.
	 *
	 * @param	string|string[]	$str		Input data
	 * @param 	bool		$is_image	Whether the input is an image
	 * @return	string
	 */
	public function xss_clean($str, $is_image = FALSE)
	{
		// Is the string an array?
		if (is_array($str))
		{
			while (list($key) = each($str))
			{
				$str[$key] = $this->xss_clean($str[$key]);
			}

			return $str;
		}

		// Remove Invisible Characters
		$str = remove_invisible_characters($str);

		/*
		 * URL Decode
		 *
		 * Just in case stuff like this is submitted:
		 *
		 * <a href="http://%77%77%77%2E%67%6F%6F%67%6C%65%2E%63%6F%6D">Google</a>
		 *
		 * Note: Use rawurldecode() so it does not remove plus signs
		 */
		do
		{
			$str = rawurldecode($str);
		}
		while (preg_match('/%[0-9a-f]{2,}/i', $str));

		/*
		 * Convert character entities to ASCII
		 *
		 * This permits our tests below to work reliably.
		 * We only convert entities that are within tags since
		 * these are the ones that will pose security problems.
		 */
		$str = preg_replace_callback("/[^a-z0-9>]+[a-z0-9]+=([\'\"]).*?\\1/si", array($this, '_convert_attribute'), $str);
		$str = preg_replace_callback('/<\w+.*/si', array($this, '_decode_entity'), $str);

		// Remove Invisible Characters Again!
		$str = remove_invisible_characters($str);

		/*
		 * Convert all tabs to spaces
		 *
		 * This prevents strings like this: ja	vascript
		 * NOTE: we deal with spaces between characters later.
		 * NOTE: preg_replace was found to be amazingly slow here on
		 * large blocks of data, so we use str_replace.
		 */
		$str = str_replace("\t", ' ', $str);

		// Capture converted string for later comparison
		$converted_string = $str;

		// Remove Strings that are never allowed
		$str = $this->_do_never_allowed($str);

		/*
		 * Makes PHP tags safe
		 *
		 * Note: XML tags are inadvertently replaced too:
		 *
		 * <?xml
		 *
		 * But it doesn't seem to pose a problem.
		 */
		if ($is_image === TRUE)
		{
			// Images have a tendency to have the PHP short opening and
			// closing tags every so often so we skip those and only
			// do the long opening tags.
			$str = preg_replace('/<\?(php)/i', '&lt;?\\1', $str);
		}
		else
		{
			$str = str_replace(array('<?', '?'.'>'), array('&lt;?', '?&gt;'), $str);
		}

		/*
		 * Compact any exploded words
		 *
		 * This corrects words like:  j a v a s c r i p t
		 * These words are compacted back to their correct state.
		 */
		$words = array(
			'javascript', 'expression', 'vbscript', 'jscript', 'wscript',
			'vbs', 'script', 'base64', 'applet', 'alert', 'document',
			'write', 'cookie', 'window', 'confirm', 'prompt'
		);

		foreach ($words as $word)
		{
			$word = implode('\s*', str_split($word)).'\s*';

			// We only want to do this when it is followed by a non-word character
			// That way valid stuff like "dealer to" does not become "dealerto"
			$str = preg_replace_callback('#('.substr($word, 0, -3).')(\W)#is', array($this, '_compact_exploded_words'), $str);
		}

		/*
		 * Remove disallowed Javascript in links or img tags
		 * We used to do some version comparisons and use of stripos(),
		 * but it is dog slow compared to these simplified non-capturing
		 * preg_match(), especially if the pattern exists in the string
		 *
		 * Note: It was reported that not only space characters, but all in
		 * the following pattern can be parsed as separators between a tag name
		 * and its attributes: [\d\s"\'`;,\/\=\(\x00\x0B\x09\x0C]
		 * ... however, remove_invisible_characters() above already strips the
		 * hex-encoded ones, so we'll skip them below.
		 */
		do
		{
			$original = $str;

			if (preg_match('/<a/i', $str))
			{
				$str = preg_replace_callback('#<a[^a-z0-9>]+([^>]*?)(?:>|$)#si', array($this, '_js_link_removal'), $str);
			}

			if (preg_match('/<img/i', $str))
			{
				$str = preg_replace_callback('#<img[^a-z0-9]+([^>]*?)(?:\s?/?>|$)#si', array($this, '_js_img_removal'), $str);
			}

			if (preg_match('/script|xss/i', $str))
			{
				$str = preg_replace('#</*(?:script|xss).*?>#si', '[removed]', $str);
			}
		}
		while ($original !== $str);

		unset($original);

		// Remove evil attributes such as style, onclick and xmlns
		$str = $this->_remove_evil_attributes($str, $is_image);

		/*
		 * Sanitize naughty HTML elements
		 *
		 * If a tag containing any of the words in the list
		 * below is found, the tag gets converted to entities.
		 *
		 * So this: <blink>
		 * Becomes: &lt;blink&gt;
		 */
		$naughty = 'alert|prompt|confirm|applet|audio|basefont|base|behavior|bgsound|blink|body|embed|expression|form|frameset|frame|head|html|ilayer|iframe|input|button|select|isindex|layer|link|meta|keygen|object|plaintext|style|script|textarea|title|math|video|svg|xml|xss';
		$str = preg_replace_callback('#<(/*\s*)('.$naughty.')([^><]*)([><]*)#is', array($this, '_sanitize_naughty_html'), $str);

		/*
		 * Sanitize naughty scripting elements
		 *
		 * Similar to above, only instead of looking for
		 * tags it looks for PHP and JavaScript commands
		 * that are disallowed. Rather than removing the
		 * code, it simply converts the parenthesis to entities
		 * rendering the code un-executable.
		 *
		 * For example:	eval('some code')
		 * Becomes:	eval&#40;'some code'&#41;
		 */
		$str = preg_replace('#(alert|prompt|confirm|cmd|passthru|eval|exec|expression|system|fopen|fsockopen|file|file_get_contents|readfile|unlink)(\s*)\((.*?)\)#si',
					'\\1\\2&#40;\\3&#41;',
					$str);

		// Final clean up
		// This adds a bit of extra precaution in case
		// something got through the above filters
		$str = $this->_do_never_allowed($str);

		/*
		 * Images are Handled in a Special Way
		 * - Essentially, we want to know that after all of the character
		 * conversion is done whether any unwanted, likely XSS, code was found.
		 * If not, we return TRUE, as the image is clean.
		 * However, if the string post-conversion does not matched the
		 * string post-removal of XSS, then it fails, as there was unwanted XSS
		 * code found and removed/changed during processing.
		 */
		if ($is_image === TRUE)
		{
			return ($str === $converted_string);
		}

		return $str;
	}

	// --------------------------------------------------------------------

	/**
	 * XSS Hash
	 *
	 * Generates the XSS hash if needed and returns it.
	 *
	 * @see		CI_Security::$_xss_hash
	 * @return	string	XSS hash
	 */
	public function xss_hash()
	{
		if ($this->_xss_hash === NULL)
		{
			$rand = $this->get_random_bytes(16);
			$this->_xss_hash = ($rand === FALSE)
				? md5(uniqid(mt_rand(), TRUE))
				: bin2hex($rand);
		}

		return $this->_xss_hash;
	}

	// --------------------------------------------------------------------

	/**
	 * Get random bytes
	 *
	 * @param	int	$length	Output length
	 * @return	string
	 */
	public function get_random_bytes($length)
	{
		if (empty($length) OR ! ctype_digit((string) $length))
		{
			return FALSE;
		}

		// Unfortunately, none of the following PRNGs is guaranteed to exist ...
		if (defined('MCRYPT_DEV_URANDOM') && ($output = mcrypt_create_iv($length, MCRYPT_DEV_URANDOM)) !== FALSE)
		{
			return $output;
		}


		if (is_readable('/dev/urandom') && ($fp = fopen('/dev/urandom', 'rb')) !== FALSE)
		{
			// Try not to waste entropy ...
			is_php('5.4') && stream_set_chunk_size($fp, $length);
			$output = fread($fp, $length);
			fclose($fp);
			if ($output !== FALSE)
			{
				return $output;
			}
		}

		if (function_exists('openssl_random_pseudo_bytes'))
		{
			return openssl_random_pseudo_bytes($length);
		}

		return FALSE;
	}

	// --------------------------------------------------------------------

	/**
	 * HTML Entities Decode
	 *
	 * A replacement for html_entity_decode()
	 *
	 * The reason we are not using html_entity_decode() by itself is because
	 * while it is not technically correct to leave out the semicolon
	 * at the end of an entity most browsers will still interpret the entity
	 * correctly. html_entity_decode() does not convert entities without
	 * semicolons, so we are left with our own little solution here. Bummer.
	 *
	 * @link	http://php.net/html-entity-decode
	 *
	 * @param	string	$str		Input
	 * @param	string	$charset	Character set
	 * @return	string
	 */
	public function entity_decode($str, $charset = NULL)
	{
		if (strpos($str, '&') === FALSE)
		{
			return $str;
		}

		static $_entities;

		isset($charset) OR $charset = $this->charset;
		$flag = is_php('5.4')
			? ENT_COMPAT | ENT_HTML5
			: ENT_COMPAT;

		do
		{
			$str_compare = $str;

			// Decode standard entities, avoiding false positives
			if ($c = preg_match_all('/&[a-z]{2,}(?![a-z;])/i', $str, $matches))
			{
				if ( ! isset($_entities))
				{
					$_entities = array_map(
						'strtolower',
						is_php('5.3.4')
							? get_html_translation_table(HTML_ENTITIES, $flag, $charset)
							: get_html_translation_table(HTML_ENTITIES, $flag)
					);

					// If we're not on PHP 5.4+, add the possibly dangerous HTML 5
					// entities to the array manually
					if ($flag === ENT_COMPAT)
					{
						$_entities[':'] = '&colon;';
						$_entities['('] = '&lpar;';
						$_entities[')'] = '&rpar';
						$_entities["\n"] = '&newline;';
						$_entities["\t"] = '&tab;';
					}
				}

				$replace = array();
				$matches = array_unique(array_map('strtolower', $matches[0]));
				for ($i = 0; $i < $c; $i++)
				{
					if (($char = array_search($matches[$i].';', $_entities, TRUE)) !== FALSE)
					{
						$replace[$matches[$i]] = $char;
					}
				}

				$str = str_ireplace(array_keys($replace), array_values($replace), $str);
			}

			// Decode numeric & UTF16 two byte entities
			$str = html_entity_decode(
				preg_replace('/(&#(?:x0*[0-9a-f]{2,5}(?![0-9a-f;])|(?:0*\d{2,4}(?![0-9;]))))/iS', '$1;', $str),
				$flag,
				$charset
			);
		}
		while ($str_compare !== $str);
		return $str;
	}

	// --------------------------------------------------------------------

	/**
	 * Sanitize Filename
	 *
	 * @param	string	$str		Input file name
	 * @param 	bool	$relative_path	Whether to preserve paths
	 * @return	string
	 */
	public function sanitize_filename($str, $relative_path = FALSE)
	{
		$bad = $this->filename_bad_chars;

		if ( ! $relative_path)
		{
			$bad[] = './';
			$bad[] = '/';
		}

		$str = remove_invisible_characters($str, FALSE);

		do
		{
			$old = $str;
			$str = str_replace($bad, '', $str);
		}
		while ($old !== $str);

		return stripslashes($str);
	}

	// ----------------------------------------------------------------

	/**
	 * Strip Image Tags
	 *
	 * @param	string	$str
	 * @return	string
	 */
	public function strip_image_tags($str)
	{
		return preg_replace(array('#<img[\s/]+.*?src\s*=\s*["\'](.+?)["\'].*?\>#', '#<img[\s/]+.*?src\s*=\s*(.+?).*?\>#'), '\\1', $str);
	}

	// ----------------------------------------------------------------

	/**
	 * Compact Exploded Words
	 *
	 * Callback method for xss_clean() to remove whitespace from
	 * things like 'j a v a s c r i p t'.
	 *
	 * @used-by	CI_Security::xss_clean()
	 * @param	array	$matches
	 * @return	string
	 */
	protected function _compact_exploded_words($matches)
	{
		return preg_replace('/\s+/s', '', $matches[1]).$matches[2];
	}

	// --------------------------------------------------------------------

	/**
	 * Remove Evil HTML Attributes (like event handlers and style)
	 *
	 * It removes the evil attribute and either:
	 *
	 *  - Everything up until a space. For example, everything between the pipes:
	 *
	 *	<code>
	 *		<a |style=document.write('hello');alert('world');| class=link>
	 *	</code>
	 *
	 *  - Everything inside the quotes. For example, everything between the pipes:
	 *
	 *	<code>
	 *		<a |style="document.write('hello'); alert('world');"| class="link">
	 *	</code>
	 *
	 * @param	string	$str		The string to check
	 * @param	bool	$is_image	Whether the input is an image
	 * @return	string	The string with the evil attributes removed
	 */
	protected function _remove_evil_attributes($str, $is_image)
	{
		$evil_attributes = array('on\w*', 'style', 'xmlns', 'formaction', 'form', 'xlink:href');

		if ($is_image === TRUE)
		{
			/*
			 * Adobe Photoshop puts XML metadata into JFIF images,
			 * including namespacing, so we have to allow this for images.
			 */
			unset($evil_attributes[array_search('xmlns', $evil_attributes)]);
		}

		do {
			$count = 0;
			$attribs = array();

			// find occurrences of illegal attribute strings with quotes (042 and 047 are octal quotes)
			preg_match_all('/(?<!\w)('.implode('|', $evil_attributes).')\s*=\s*(\042|\047)([^\\2]*?)(\\2)/is', $str, $matches, PREG_SET_ORDER);

			foreach ($matches as $attr)
			{
				$attribs[] = preg_quote($attr[0], '/');
			}

			// find occurrences of illegal attribute strings without quotes
			preg_match_all('/(?<!\w)('.implode('|', $evil_attributes).')\s*=\s*([^\s>]*)/is', $str, $matches, PREG_SET_ORDER);

			foreach ($matches as $attr)
			{
				$attribs[] = preg_quote($attr[0], '/');
			}

			// replace illegal attribute strings that are inside an html tag
			if (count($attribs) > 0)
			{
				$str = preg_replace('/(<?)(\/?[^><]+?)([^A-Za-z<>\-])(.*?)('.implode('|', $attribs).')(.*?)([\s><]?)([><]*)/i', '$1$2 $4$6$7$8', $str, -1, $count);
			}
		}
		while ($count);

		return $str;
	}

	// --------------------------------------------------------------------

	/**
	 * Sanitize Naughty HTML
	 *
	 * Callback method for xss_clean() to remove naughty HTML elements.
	 *
	 * @used-by	CI_Security::xss_clean()
	 * @param	array	$matches
	 * @return	string
	 */
	protected function _sanitize_naughty_html($matches)
	{
		return '&lt;'.$matches[1].$matches[2].$matches[3] // encode opening brace
			// encode captured opening or closing brace to prevent recursive vectors:
			.str_replace(array('>', '<'), array('&gt;', '&lt;'), $matches[4]);
	}

	// --------------------------------------------------------------------

	/**
	 * JS Link Removal
	 *
	 * Callback method for xss_clean() to sanitize links.
	 *
	 * This limits the PCRE backtracks, making it more performance friendly
	 * and prevents PREG_BACKTRACK_LIMIT_ERROR from being triggered in
	 * PHP 5.2+ on link-heavy strings.
	 *
	 * @used-by	CI_Security::xss_clean()
	 * @param	array	$match
	 * @return	string
	 */
	protected function _js_link_removal($match)
	{
		return str_replace($match[1],
					preg_replace('#href=.*?(?:(?:alert|prompt|confirm)(?:\(|&\#40;)|javascript:|livescript:|mocha:|charset=|window\.|document\.|\.cookie|<script|<xss|data\s*:)#si',
							'',
							$this->_filter_attributes(str_replace(array('<', '>'), '', $match[1]))
					),
					$match[0]);
	}

	// --------------------------------------------------------------------

	/**
	 * JS Image Removal
	 *
	 * Callback method for xss_clean() to sanitize image tags.
	 *
	 * This limits the PCRE backtracks, making it more performance friendly
	 * and prevents PREG_BACKTRACK_LIMIT_ERROR from being triggered in
	 * PHP 5.2+ on image tag heavy strings.
	 *
	 * @used-by	CI_Security::xss_clean()
	 * @param	array	$match
	 * @return	string
	 */
	protected function _js_img_removal($match)
	{
		return str_replace($match[1],
					preg_replace('#src=.*?(?:(?:alert|prompt|confirm)(?:\(|&\#40;)|javascript:|livescript:|mocha:|charset=|window\.|document\.|\.cookie|<script|<xss|base64\s*,)#si',
							'',
							$this->_filter_attributes(str_replace(array('<', '>'), '', $match[1]))
					),
					$match[0]);
	}

	// --------------------------------------------------------------------

	/**
	 * Attribute Conversion
	 *
	 * @used-by	CI_Security::xss_clean()
	 * @param	array	$match
	 * @return	string
	 */
	protected function _convert_attribute($match)
	{
		return str_replace(array('>', '<', '\\'), array('&gt;', '&lt;', '\\\\'), $match[0]);
	}

	// --------------------------------------------------------------------

	/**
	 * Filter Attributes
	 *
	 * Filters tag attributes for consistency and safety.
	 *
	 * @used-by	CI_Security::_js_img_removal()
	 * @used-by	CI_Security::_js_link_removal()
	 * @param	string	$str
	 * @return	string
	 */
	protected function _filter_attributes($str)
	{
		$out = '';
		if (preg_match_all('#\s*[a-z\-]+\s*=\s*(\042|\047)([^\\1]*?)\\1#is', $str, $matches))
		{
			foreach ($matches[0] as $match)
			{
				$out .= preg_replace('#/\*.*?\*/#s', '', $match);
			}
		}

		return $out;
	}

	// --------------------------------------------------------------------

	/**
	 * HTML Entity Decode Callback
	 *
	 * @used-by	CI_Security::xss_clean()
	 * @param	array	$match
	 * @return	string
	 */
	protected function _decode_entity($match)
	{
		// Protect GET variables in URLs
		// 901119URL5918AMP18930PROTECT8198
		$match = preg_replace('|\&([a-z\_0-9\-]+)\=([a-z\_0-9\-/]+)|i', $this->xss_hash().'\\1=\\2', $match[0]);

		// Decode, then un-protect URL GET vars
		return str_replace(
			$this->xss_hash(),
			'&',
			$this->entity_decode($match, $this->charset)
		);
	}

	// --------------------------------------------------------------------

	/**
	 * Do Never Allowed
	 *
	 * @used-by	CI_Security::xss_clean()
	 * @param 	string
	 * @return 	string
	 */
	protected function _do_never_allowed($str)
	{
		$str = str_replace(array_keys($this->_never_allowed_str), $this->_never_allowed_str, $str);

		foreach ($this->_never_allowed_regex as $regex)
		{
			$str = preg_replace('#'.$regex.'#is', '[removed]', $str);
		}

		return $str;
	}

	// --------------------------------------------------------------------

	/**
	 * Set CSRF Hash and Cookie
	 *
	 * @return	string
	 */
	protected function _csrf_set_hash()
	{
		if ($this->_csrf_hash === NULL)
		{
			// If the cookie exists we will use its value.
			// We don't necessarily want to regenerate it with
			// each page load since a page could contain embedded
			// sub-pages causing this feature to fail
			if (isset($_COOKIE[$this->_csrf_cookie_name]) && is_string($_COOKIE[$this->_csrf_cookie_name])
				&& preg_match('#^[0-9a-f]{32}$#iS', $_COOKIE[$this->_csrf_cookie_name]) === 1)
			{
				return $this->_csrf_hash = $_COOKIE[$this->_csrf_cookie_name];
			}

			$rand = $this->get_random_bytes(16);
			$this->_csrf_hash = ($rand === FALSE)
				? md5(uniqid(mt_rand(), TRUE))
				: bin2hex($rand);
		}

		return $this->_csrf_hash;
	}

}