google_forms/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) 2019 - 2022, CodeIgniter Foundation
 *
 * 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. (https://ellislab.com/)
 * @copyright   Copyright (c) 2014 - 2019, British Columbia Institute of Technology (https://bcit.ca/)
 * @copyright   Copyright (c) 2019 - 2022, CodeIgniter Foundation (https://codeigniter.com/)
 * @license https://opensource.org/licenses/MIT MIT License
 * @link    https://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        https://codeigniter.com/userguide3/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).cookie' => '[removed]',
        'document.write'  => '[removed]',
        '(document).write'  => '[removed]',
        '.parentNode'     => '[removed]',
        '.innerHTML'      => '[removed]',
        '-moz-binding'    => '[removed]',
        '<!--'            => '&lt;!--',
        '-->'             => '--&gt;',
        '<![CDATA['       => '&lt;![CDATA[',
        '<comment>'   => '&lt;comment&gt;',
        '<%'              => '&lt;&#37;'
    );

    /**
     * List of never allowed regex replacements
     *
     * @var array
     */
    protected $_never_allowed_regex = array(
        'javascript\s*:',
        '(\(?document\)?|\(?window\)?(\.document)?)\.(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((string) 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;
                }
            }
        }

        // Check CSRF token validity, but don't error on mismatch just yet - we'll want to regenerate
        $valid = isset($_POST[$this->_csrf_token_name], $_COOKIE[$this->_csrf_cookie_name])
            && is_string($_POST[$this->_csrf_token_name]) && is_string($_COOKIE[$this->_csrf_cookie_name])
            && hash_equals($_POST[$this->_csrf_token_name], $_COOKIE[$this->_csrf_cookie_name]);

        // We kill this since we're done and we don't want to pollute 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();

        if ($valid !== true) {
            $this->csrf_show_error();
        }

        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;
        }

        if (is_php('7.3')) {
            setcookie(
                $this->_csrf_cookie_name,
                $this->_csrf_hash,
                array(
                    'expires'  => $expire,
                    'path'     => config_item('cookie_path'),
                    'domain'   => config_item('cookie_domain'),
                    'secure'   => $secure_cookie,
                    'httponly' => config_item('cookie_httponly'),
                    'samesite' => 'Strict'
                )
            );
        } else {
            $domain = trim(config_item('cookie_domain'));
            header('Set-Cookie: ' . $this->_csrf_cookie_name . '=' . $this->_csrf_hash
                    . '; Expires=' . gmdate('D, d-M-Y H:i:s T', $expire)
                    . '; Max-Age=' . $this->_csrf_expire
                    . '; Path=' . rawurlencode(config_item('cookie_path'))
                    . ($domain === '' ? '' : '; Domain=' . $domain)
                    . ($secure_cookie ? '; Secure' : '')
                    . (config_item('cookie_httponly') ? '; HttpOnly' : '')
                    . '; SameSite=Strict');
        }

        log_message('info', 'CSRF 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)) {
            foreach ($str as $key => &$value) {
                $str[$key] = $this->xss_clean($value);
            }

            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
         */
        if (stripos($str, '%') !== false) {
            do {
                $oldstr = $str;
                $str = rawurldecode($str);
                $str = preg_replace_callback('#%(?:\s*[0-9a-f]){2,}#i', array($this, '_urldecodespaces'), $str);
            } while ($oldstr !== $str);
            unset($oldstr);
        }

        /*
         * 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', 'eval'
        );

        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)) {
                $str = preg_replace_callback('#<a(?:rea)?[^a-z0-9>]+([^>]*?)(?:>|$)#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);

        /*
         * 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;
         */
        $pattern = '#'
            . '<((?<slash>/*\s*)((?<tagName>[a-z0-9]+)(?=[^a-z0-9]|$)|.+)' // tag start and name, followed by a non-tag character
            . '[^\s\042\047a-z0-9>/=]*' // a valid attribute character immediately after the tag would count as a separator
            // optional attributes
            . '(?<attributes>(?:[\s\042\047/=]*' // non-attribute characters, excluding > (tag close) for obvious reasons
            . '[^\s\042\047>/=]+' // attribute characters
            // optional attribute-value
                . '(?:\s*=' // attribute-value separator
                    . '(?:[^\s\042\047=><`]+|\s*\042[^\042]*\042|\s*\047[^\047]*\047|\s*(?U:[^\s\042\047=><`]*))' // single, double or non-quoted value
                . ')?' // end optional attribute-value group
            . ')*)' // end optional attributes group
            . '[^>]*)(?<closeTag>\>)?#isS';

        // Note: It would be nice to optimize this for speed, BUT
        //       only matching the naughty elements here results in
        //       false positives and in turn - vulnerabilities!
        do {
            $old_str = $str;
            $str = preg_replace_callback($pattern, array($this, '_sanitize_naughty_html'), $str);
        } while ($old_str !== $str);
        unset($old_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
        );

        // Same thing, but for "tag functions" (e.g. eval`some code`)
        // See https://github.com/bcit-ci/CodeIgniter/issues/5420
        $str = preg_replace(
            '#(alert|prompt|confirm|cmd|passthru|eval|exec|expression|system|fopen|fsockopen|file|file_get_contents|readfile|unlink)(\s*)`(.*?)`#si',
            '\\1\\2&#96;\\3&#96;',
            $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;
        }

        if (function_exists('random_bytes')) {
            try {
                // The cast is required to avoid TypeError
                return random_bytes((int) $length);
            } catch (Exception $e) {
                // If random_bytes() can't do the job, we can't either ...
                // There's no point in using fallbacks.
                log_message('error', $e->getMessage());
                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;

        if (! isset($_entities)) {
            $_entities = array_map('strtolower', get_html_translation_table(HTML_ENTITIES, $flag, $charset));

            // 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;';
            }
        }

        do {
            $str_compare = $str;

            // Decode standard entities, avoiding false positives
            if (preg_match_all('/&[a-z]{2,}(?![a-z;])/i', $str, $matches)) {
                $replace = array();
                $matches = array_unique(array_map('strtolower', $matches[0]));
                foreach ($matches as &$match) {
                    if (($char = array_search($match . ';', $_entities, true)) !== false) {
                        $replace[$match] = $char;
                    }
                }

                $str = str_replace(array_keys($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
            );

            if ($flag === ENT_COMPAT) {
                $str = str_replace(array_values($_entities), array_keys($_entities), $str);
            }
        } while ($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*(["\'])([^\\1]+?)\\1.*?\>#i',
                '#<img[\s/]+.*?src\s*=\s*?(([^\s"\'=<>`]+)).*?\>#i'
            ),
            '\\2',
            $str
        );
    }

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

    /**
     * URL-decode taking spaces into account
     *
     * @see     https://github.com/bcit-ci/CodeIgniter/issues/4877
     * @param   array   $matches
     * @return  string
     */
    protected function _urldecodespaces($matches)
    {
        $input    = $matches[0];
        $nospaces = preg_replace('#\s+#', '', $input);
        return ($nospaces === $input)
            ? $input
            : rawurldecode($nospaces);
    }

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

    /**
     * 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];
    }

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

    /**
     * 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)
    {
        static $naughty_tags    = array(
            'alert', 'area', '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'
        );

        static $evil_attributes = array(
            'on\w+', 'style', 'xmlns', 'formaction', 'form', 'xlink:href', 'FSCommand', 'seekSegmentTime'
        );

        // First, escape unclosed tags
        if (empty($matches['closeTag'])) {
            return '&lt;' . $matches[1];
        }
        // Is the element that we caught naughty? If so, escape it
        elseif (in_array(strtolower($matches['tagName']), $naughty_tags, true)) {
            return '&lt;' . $matches[1] . '&gt;';
        }
        // For other tags, see if their attributes are "evil" and strip those
        elseif (isset($matches['attributes'])) {
            // We'll store the already filtered attributes here
            $attributes = array();

            // Attribute-catching pattern
            $attributes_pattern = '#'
                . '(?<name>[^\s\042\047>/=]+)' // attribute characters
                // optional attribute-value
                . '(?:\s*=(?<value>[^\s\042\047=><`]+|\s*\042[^\042]*\042|\s*\047[^\047]*\047|\s*(?U:[^\s\042\047=><`]*)))' // attribute-value separator
                . '#i';

            // Blacklist pattern for evil attribute names
            $is_evil_pattern = '#^(' . implode('|', $evil_attributes) . ')$#i';

            // Each iteration filters a single attribute
            do {
                // Strip any non-alpha characters that may precede an attribute.
                // Browsers often parse these incorrectly and that has been a
                // of numerous XSS issues we've had.
                $matches['attributes'] = preg_replace('#^[^a-z]+#i', '', $matches['attributes']);

                if (! preg_match($attributes_pattern, $matches['attributes'], $attribute, PREG_OFFSET_CAPTURE)) {
                    // No (valid) attribute found? Discard everything else inside the tag
                    break;
                }

                if (
                    // Is it indeed an "evil" attribute?
                    preg_match($is_evil_pattern, $attribute['name'][0])
                    // Or does it have an equals sign, but no value and not quoted? Strip that too!
                    or (trim($attribute['value'][0]) === '')
                ) {
                    $attributes[] = 'xss=removed';
                } else {
                    $attributes[] = $attribute[0][0];
                }

                $matches['attributes'] = substr($matches['attributes'], $attribute[0][1] + strlen($attribute[0][0]));
            } while ($matches['attributes'] !== '');

            $attributes = empty($attributes)
                ? ''
                : ' ' . implode(' ', $attributes);
            return '<' . $matches['slash'] . $matches['tagName'] . $attributes . '>';
        }

        return $matches[0];
    }

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

    /**
     * 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;|`|&\#96;)|javascript:|livescript:|mocha:|charset=|window\.|\(?document\)?\.|\.cookie|<script|<xss|d\s*a\s*t\s*a\s*:)#si',
                '',
                $this->_filter_attributes($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|eval)(?:\(|&\#40;|`|&\#96;)|javascript:|livescript:|mocha:|charset=|window\.|\(?document\)?\.|\.cookie|<script|<xss|base64\s*,)#si',
                '',
                $this->_filter_attributes($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;
    }
}