Refactor 'evil attributes' sanitization logic

Turned out pretty much impossible to do remove 'evil attributes'
with just one pattern - it either breaks something else, hits
pcre.backtrack_limit or causes PHP to segfault.

No benchmarks made, but there shouldn't be any performance
regressions since we're now trying to strip attributes only
after it is determined that they are inside a tag; up until
now this was done seprately for _sanitize_naughty_html()
and _remove_evil_attributes().

1 files changed, 66 insertions, 92 deletions

diff --git a/system/core/Security.php b/system/core/Security.php
index 4b42ed448..08cfcbe8f 100644
--- a/system/core/Security.php
+++ b/system/core/Security.php
@@ -492,16 +492,16 @@ class CI_Security {
 		 * Becomes: <blink>
 		 */
 		$pattern = '#'
-			.'<((/*\s*)([a-z0-9]+)(?=[^a-z0-9])' // tag start and name, followed by a non-tag character
-			.'[^>a-z0-9]*' // a valid attribute character immediately after the tag would count as a separator
+			.'<((?<closeTag>/*\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
-			.'([\s\042\047/=]+' // non-attribute characters, excluding > (tag close) for obvious reasons
+			.'(?<attributes>(?:[\s\042\047/=]*' // non-attribute characters, excluding > (tag close) for obvious reasons
 			.'[^\s\042\047>/=]+' // attribute characters
-			// optional attribue-value
-			.'(\s*=\s*' // attribute-value separator
-			.'(\042[^\042]*\042|\047[^\047]*\047|[^\s\042\047=><`]*)' // single, double or non-quoted value
-			.')?' // end optional attribute-value group
-			.')*' // end optional attributes group
+			// optional attribute-value
+				.'(?:\s*=\s*' // attribute-value separator
+					.'(?:\042[^\042]*\042|\047[^\047]*\047|[^\s\042\047=><`]*)' // single, double or non-quoted value
+				.')?' // end optional attribute-value group
+			.')*)' // end optional attributes group
 			.'[^>]*)>#isS';
 
 		// Note: It would be nice to optimize this for speed, BUT
@@ -515,9 +515,6 @@ class CI_Security {
 		while ($old_str !== $str);
 		unset($old_str);
 
-		// Remove evil attributes such as style, onclick and xmlns
-		$str = $this->_remove_evil_attributes($str, $is_image);
-
 		/*
 		 * Sanitize naughty scripting elements
 		 *
@@ -530,9 +527,11 @@ class CI_Security {
 		 * 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);
+		$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
@@ -770,72 +769,6 @@ class CI_Security {
 	// --------------------------------------------------------------------
 
 	/**
-	 * 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)
-	{
-		$evil_attributes = array('on\w*', 'style', 'xmlns', 'formaction', 'form', 'xlink:href', 'FSCommand', 'seekSegmentTime');
-
-		if ($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)]);
-		}
-
-		$pattern = '#(' // catch everything in the tag preceeding the evil attribute
-			.'<[a-z0-9]+(?=[^>a-z0-9])' // tag start and name, followed by a non-tag character
-			.'[^>a-z0-9]*' // a valid attribute character immediately after the tag would count as a separator
-			// optional attributes
-			.'([\s\042\047/=]+' // non-attribute characters, excluding > (tag close) for obvious reasons
-			.'[^\s\042\047>/=]+' // attribute characters
-			// optional attribue-value
-			.'(\s*=\s*' // attribute-value separator
-			.'(\042[^\042]*\042|\047[^\047]*\047|[^\s\042\047=><`]*)' // single, double or non-quoted value
-			.')?' // end optional attribute-value group
-			.')*' // end optional attributes group
-			.')' // end catching evil attribute prefix
-			// evil attribute starts here
-			.'([\s\042\047/=]+' // non-attribute characters (we'll replace that with a single space), again excluding '>'
-			.'('.implode('|', $evil_attributes).')'
-			.'\s*=\s*' // attribute-value separator
-			.'(\042[^\042]+\042|\047[^\047]+\047|[^\s\042\047=><`]+)' // attribute value; single, double or non-quotes
-			.')' // end evil attribute
-			.'#isS';
-
-		do
-		{
-			$count = 0;
-			$str = preg_replace($pattern, '$1 [removed]', $str, -1, $count);
-		}
-		while ($count);
-
-		return $str;
-	}
-
-	// --------------------------------------------------------------------
-
-	/**
 	 * Sanitize Naughty HTML
 	 *
 	 * Callback method for xss_clean() to remove naughty HTML elements.
@@ -846,21 +779,59 @@ class CI_Security {
 	 */
 	protected function _sanitize_naughty_html($matches)
 	{
-		static $naughty = array(
+		static $naughty_tags    = array(
 			'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'
 		);
 
-		// Is the element that we caught naughty?
-		// If not, just return it back.
-		if ( ! in_array(strtolower($matches[3]), $naughty, TRUE))
+		static $evil_attributes = array(
+			'on\w+', 'style', 'xmlns', 'formaction', 'form', 'xlink:href', 'FSCommand', 'seekSegmentTime'
+		);
+
+		// Is the element that we caught naughty? If so, escape it
+		if (in_array(strtolower($matches['tagName']), $naughty_tags, TRUE))
 		{
-			return $matches[0];
+			return '&lt;'.$matches[1].'&gt;';
 		}
+		// For other tags, see if their attributes are "evil" and strip those
+		elseif (isset($matches['attributes']))
+		{
+			// We'll need to catch all attributes separately first
+			$pattern = '#'
+				.'([\s\042\047/=]*)' // non-attribute characters, excluding > (tag close) for obvious reasons
+				.'(?<name>[^\s\042\047>/=]+)' // attribute characters
+				// optional attribute-value
+				.'(?:\s*=\s*\042[^\042]+\042|\s*=\s*\047[^\047]+\047|\s*=\s*[^\s\042\047=><`]+)?' // attribute-value separator
+				.'#i';
+
+			if ($count = preg_match_all($pattern, $matches['attributes'], $attributes, PREG_SET_ORDER | PREG_OFFSET_CAPTURE))
+			{
+				// Since we'll be using substr_replace() below, we
+				// need to handle the attributes in reverse order,
+				// so we don't damage the string.
+				for ($i = $count - 1; $i > -1; $i--)
+				{
+					// Is it indeed an "evil" attribute?
+					if (preg_match('#^('.implode('|', $evil_attributes).')$#i', $attributes[$i]['name'][0]))
+					{
+						$matches['attributes'] = substr_replace(
+							$matches['attributes'],
+							' [removed]',
+							$attributes[$i][0][1],
+							strlen($attributes[$i][0][0])
+						);
+					}
+				}
 
-		return '&lt;'.$matches[1].'&gt;';
+				// Note: This will strip some non-space characters and/or
+				//       reduce multiple spaces between attributes.
+				return '<'.$matches['closeTag'].$matches['tagName'].' '.trim($matches['attributes']).'>';
+			}
+		}
+
+		return $matches[0];
 	}
 
 	// --------------------------------------------------------------------
@@ -880,12 +851,15 @@ class CI_Security {
 	 */
 	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]);
+		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]
+		);
 	}
 
 	// --------------------------------------------------------------------