Pastable version of HTML Filter

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 <pre class="sunburst">#!/usr/bin/php
&lt;?php
/**
 * @author Cameron Zemek &lt;cameronz@bundaberg.qld.gov.au&gt;
 */

/**
 * Handles HTML special characters (ie. &lt; &amp; &gt; &quot;) thereby making it easier
 * for the filter to remove XSS attacks.
 */
class HtmlParser {
 /**
 * Parse HTML snippet
 *
 * @param $html string HTML snippet
 * @param $handler HtmlParserHandler Callback handler
 */
 public function parse($html, HtmlParserHandler $handler) {
 $this-&gt;input = $html; 
 $this-&gt;pos = 0;
 $this-&gt;len = strlen($html);
 $this-&gt;handler = $handler;

$text = '';
 while ($this-&gt;pos &lt; $this-&gt;len) {
 $char = $this-&gt;look();
 if ($char === '&lt;') {
 if ($this-&gt;lookMatches('&lt;!')) {
 // Handle HTML comment
 $this-&gt;commentBlock();
 } else {
 // Process tag
 $this-&gt;fireText($text);
 $text = '';
 $this-&gt;tag();
 }
 } else {
 $text .= $this-&gt;char();
 }
 }
 if ($text !== '') {
 $this-&gt;fireText($text);
 }
 }

protected function fireOpenTag($tagName, $attributes) {
 $this-&gt;handler-&gt;openTag($tagName, $attributes);
 }

protected function fireCloseTag($tagName) {
 $this-&gt;handler-&gt;closeTag($tagName);
 }

protected function fireComment($comment) {
 $this-&gt;handler-&gt;comment($comment);
 }

protected function fireText($text) {
 if (strlen($text) == '') {
 return;
 }
 $this-&gt;handler-&gt;text($text);
 }

/**
 * Parse HTML comment block
 */
 private function commentBlock() {
 $comment = '';
 $this-&gt;matches('&lt;!');
 while ($this-&gt;look() !== '&gt;' &amp;&amp; $this-&gt;pos &lt; $this-&gt;len) {
 if ($this-&gt;lookMatches('--')) {
 $comment .= $this-&gt;comment();
 } else {
 $this-&gt;matchAny(); // Ignore characters outside comment
 }
 }
 $this-&gt;match('&gt;');
 $this-&gt;fireComment($comment);
 }

// Check if close tag
 $closeTag = false;
 if ($this-&gt;look() === '/') {
 $closeTag = true;
 $output .= $this-&gt;match('/');
 }

$tagName = strtolower($this-&gt;matchWord());

// If not valid tag, escape output
 if (strlen($tagName) === 0) {
 $this-&gt;fireText(htmlspecialchars($output));
 return;
 }

if ($closeTag) {
 $this-&gt;match('&gt;');
 $this-&gt;fireCloseTag($tagName);
 return;
 }

$this-&gt;matchWhitespace();
 $attributes = array();
 while ($this-&gt;look() !== '&gt;' &amp;&amp; $this-&gt;pos &lt; $this-&gt;len) {
 $attribute = $this-&gt;attribute();

// Invalid attribute, finish tag
 if (strlen($attribute) === 0) {
 break;
 }

list($attributeName, $attributeValue) = $attribute;
 $attributes[$attributeName] = $attributeValue;
 $this-&gt;matchWhitespace();
 }

if ($this-&gt;look() === '&gt;') {
 $this-&gt;match('&gt;');
 }
 $this-&gt;fireOpenTag($tagName, $attributes);
 }

/**
 * Parse attribute value
 */
 private function attributeValue() {
 if ($this-&gt;look() === '&quot;' || $this-&gt;look() === &quot;'&quot;) {
 $quoteChar = $this-&gt;look();
 $this-&gt;match($quoteChar);
 $attributeValue = $this-&gt;matchUntil($quoteChar);
 $this-&gt;match($quoteChar);
 } else {
 $attributeValue = '';
 while (!ctype_space($this-&gt;look()) &amp;&amp; $this-&gt;look() != '&gt;' &amp;&amp; $this-&gt;pos &lt; $this-&gt;len) {
 $attributeValue .= $this-&gt;char();
 }
 }
 if (strlen($attributeValue) === 0) {
 return null;
 }
 return $attributeValue;
 }

/**
 * Match character, handling special characters and character entities
 */
 private function char() {
 if ($this-&gt;look() === '&amp;') {
 return $this-&gt;entity();
 } else {
 return htmlspecialchars($this-&gt;matchAny());
 }
 }

/**
 * Parse HTML entity in number format. Eg. &amp;#169;
 */
 private function entityNumber() {
 $entity = $this-&gt;matches('&amp;#');
 $entity .= $this-&gt;matchNumber();
 $len = strlen($entity);
 if ($len &lt;= 2 || $len &gt; 6) {
 // Invalid entity, escape &amp;
 return htmlspecialchars($entity);
 }
 if ($this-&gt;look() === ';') {
 $entity .= $this-&gt;match(';');
 } else {
 $entity .= ';';
 }
 return $entity; 
 }

/**
 * Parse HTML entity in hex format. Eg. &amp;#x6A;
 */
 private function entityHex() {
 $entity = $this-&gt;matches('&amp;#x', true);
 $entity .= $this-&gt;matchHexNumber();
 $len = strlen($entity);
 if ($len &lt;= 3 || $len &gt; 7) {
 // Invalid entity, escape &amp;
 return htmlspecialchars($entity);
 }
 if ($this-&gt;look() === ';') {
 $entity .= $this-&gt;match(';');
 } else {
 $entity .= ';';
 }
 return $entity; 
 }

private function look() {
 return $this-&gt;input[ $this-&gt;pos ];
 }

private function lookMatches($str, $ignoreCase = true) {
 $input = substr($this-&gt;input, $this-&gt;pos, strlen($str));
 if ($ignoreCase) {
 $input = strtolower($input);
 }
 return $str === $input;
 }

private function matchAny() {
 return $this-&gt;input[ $this-&gt;pos++ ];
 }

private function match($char, $ignoreCase = false) {
 $input = $this-&gt;look();
 if ($ignoreCase) {
 $input = strtolower($input); 
 }
 if ($input !== $char) {
 // This means we have a logic coding error
 throw new Exception('Invalid match');
 }
 return $this-&gt;input[ $this-&gt;pos++ ];
 }

private function matches($str, $ignoreCase = false) {
 $input = substr($this-&gt;input, $this-&gt;pos, strlen($str));
 if ($ignoreCase) {
 $input = strtolower($input);
 }
 if ($str !== $input) {
 // This means we have a logic coding error
 throw new Exception('Invalid match');
 }
 $this-&gt;pos += strlen($str);
 return $str;
 }

private function matchUntil($char) {
 $match = '';
 while ($this-&gt;look() !== $char &amp;&amp; $this-&gt;pos &lt; $this-&gt;len) {
 $match .= $this-&gt;char();
 }
 return $match;
 }

private function matchWord() {
 $word = '';
 while (ctype_alpha($this-&gt;input[ $this-&gt;pos ]) &amp;&amp; $this-&gt;pos &lt; $this-&gt;len) {
 $word .= $this-&gt;input[ $this-&gt;pos++ ];
 }
 return $word;
 }

private function matchNumber() {
 $num = '';
 while (ctype_digit($this-&gt;input[ $this-&gt;pos ]) &amp;&amp; $this-&gt;pos &lt; $this-&gt;len) {
 $num .= $this-&gt;input[ $this-&gt;pos++ ];
 }
 return $num;
 }

private function matchHexNumber() {
 $num = '';
 while (ctype_xdigit($this-&gt;input[ $this-&gt;pos ]) &amp;&amp; $this-&gt;pos &lt; $this-&gt;len) {
 $num .= $this-&gt;input[ $this-&gt;pos++ ];
 }
 return $num;
 }

private function matchWhitespace() {
 $ws = '';
 while (ctype_space($this-&gt;input[ $this-&gt;pos ]) &amp;&amp; $this-&gt;pos &lt; $this-&gt;len) {
 $ws .= $this-&gt;input[ $this-&gt;pos++ ];
 }
 return $ws;
 }

private $pos; // Position in input
 private $len; // Length of input
 private $input;
 private $handler;
}

class HtmlFilter implements HtmlParserHandler {
 /**
 * Allowed tags
 */
 private $allowedTags = array('a', 'b', 'i', 'u', 'strong', 'em',
 'sub', 'sup', 'br', 'ul', 'ol', 'li', 'table', 'colgroup', 'col',
 'thead', 'tbody', 'tr', 'td', 'img');

/**
 * Tags which should always be self-closing (eg. &quot;&lt;img /&gt;&quot;)
 */
 private $selfCloseTags = array('img', 'br', 'col');

/**
 * Attributes which contain URLs, and should be checked for valid protocols
 */
 private $protocolAttributes = array('src', 'href');

private $output = ''; // Safe HTML

public function openTag($tagName, $attributes) {
 // Ignore tags that are not white listed
 if (!in_array($tagName, $this-&gt;allowedTags)) {
 return;
 }

array_push($this-&gt;tagStack, $tagName);
 $this-&gt;output .= '&lt;' . $tagName;
 $allowedAttributes = $this-&gt;allowedAttributes[$tagName];
 if (isset($allowedAttributes) &amp;&amp; is_array($allowedAttributes)) {
 foreach ($attributes as $name =&gt; $value) {
 if (in_array($name, $allowedAttributes)) {
 // If its a protocol attribute, check if its an allowed protocol
 if (in_array($name, $this-&gt;protocolAttributes)) {
 foreach ($this-&gt;allowedProtocols as $protocol) {
 $testOn = substr($value, 0, strlen($protocol));
 if ($testOn === $protocol) {
 $this-&gt;output .= ' ' . $name . '=&quot;' . $value . '&quot;';
 }
 }
 } else {
 $this-&gt;output .= ' ' . $name . '=&quot;' . $value . '&quot;';
 }
 }
 }
 }
 if (in_array($tagName, $this-&gt;selfCloseTags)) {
 $this-&gt;output .= ' /';
 }
 $this-&gt;output .= '&gt;';
 }

public function closeTag($tagName) {
 if (!in_array($tagName, $this-&gt;tagStack)) {
 // Orphan close tag, ignore
 return;
 }
 while (true) {
 if (count($this-&gt;tagStack) === 0) {
 break;
 }
 $popTag = array_pop($this-&gt;tagStack);
 if ($popTag === $tagName) {
 break;
 }
 $this-&gt;output .= '&lt;/' . $popTag . '&gt;';
 }
 $this-&gt;output .= '&lt;/' . $tagName . '&gt;';
 }

public function comment($comment) {
 if ($this-&gt;stripComments) {
 return;
 }
 $this-&gt;output .= '&lt;!--' . $comment . '--&gt;';
 }

public function text($text) {
 $this-&gt;output .= $text;
 }

private $parser;
 
 public function __construct() {
 $this-&gt;parser = new HtmlParser;
 }

public function filter($html) {
 $this-&gt;parser-&gt;parse($html, $this);
 return $this-&gt;output;
 }
}

$filename = $argv[1];
$contents = file_get_contents($filename);

$filter = new HtmlFilter;
$safeHTML = $filter-&gt;filter($contents);

// Compress html
//$safeHTML = preg_replace('/\s+/', ' ', $safeHTML);

echo $safeHTML . &quot;\n&quot;;
</pre>
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Pastable version of HTML Filter