Merge pull request #495 from aidantwoods/anti-xss

Prevent various XSS attacks [rebase and update of #276]

LICENSE.txt

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2013 Emanuil Rusev, erusev.com
+Copyright (c) 2013-2018 Emanuil Rusev, erusev.com
 
 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
@@ -17,4 +17,4 @@ 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.
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Parsedown.php

@@ -75,6 +75,32 @@ class Parsedown
 
     protected $urlsLinked = true;
 
+    function setSafeMode($safeMode)
+    {
+        $this->safeMode = (bool) $safeMode;
+
+        return $this;
+    }
+
+    protected $safeMode;
+
+    protected $safeLinksWhitelist = array(
+        'http://',
+        'https://',
+        'ftp://',
+        'ftps://',
+        'mailto:',
+        'data:image/png;base64,',
+        'data:image/gif;base64,',
+        'data:image/jpeg;base64,',
+        'irc:',
+        'ircs:',
+        'git:',
+        'ssh:',
+        'news:',
+        'steam:',
+    );
+
     #
     # Lines
     #
@@ -342,8 +368,6 @@ class Parsedown
     {
         $text = $Block['element']['text']['text'];
 
-        $text = htmlspecialchars($text, ENT_NOQUOTES, 'UTF-8');
-
         $Block['element']['text']['text'] = $text;
 
         return $Block;
@@ -354,7 +378,7 @@ class Parsedown
 
     protected function blockComment($Line)
     {
-        if ($this->markupEscaped)
+        if ($this->markupEscaped or $this->safeMode)
         {
             return;
         }
@@ -457,8 +481,6 @@ class Parsedown
     {
         $text = $Block['element']['text']['text'];
 
-        $text = htmlspecialchars($text, ENT_NOQUOTES, 'UTF-8');
-
         $Block['element']['text']['text'] = $text;
 
         return $Block;
@@ -515,10 +537,10 @@ class Parsedown
                 ),
             );
 
-            if($name === 'ol') 
+            if($name === 'ol')
             {
                 $listStart = stristr($matches[0], '.', true);
-                
+
                 if($listStart !== '1')
                 {
                     $Block['element']['attributes'] = array('start' => $listStart);
@@ -678,12 +700,12 @@ class Parsedown
 
     protected function blockMarkup($Line)
     {
-        if ($this->markupEscaped)
+        if ($this->markupEscaped or $this->safeMode)
         {
             return;
         }
 
-        if (preg_match('/^<(\w*)(?:[ ]*'.$this->regexHtmlAttribute.')*[ ]*(\/)?>/', $Line['text'], $matches))
+        if (preg_match('/^<(\w[\w-]*)(?:[ ]*'.$this->regexHtmlAttribute.')*[ ]*(\/)?>/', $Line['text'], $matches))
         {
             $element = strtolower($matches[1]);
 
@@ -1074,7 +1096,6 @@ class Parsedown
         if (preg_match('/^('.$marker.'+)[ ]*(.+?)[ ]*(?<!'.$marker.')\1(?!'.$marker.')/s', $Excerpt['text'], $matches))
         {
             $text = $matches[2];
-            $text = htmlspecialchars($text, ENT_NOQUOTES, 'UTF-8');
             $text = preg_replace("/[ ]*\n/", ' ', $text);
 
             return array(
@@ -1253,8 +1274,6 @@ class Parsedown
             $Element['attributes']['title'] = $Definition['title'];
         }
 
-        $Element['attributes']['href'] = str_replace(array('&', '<'), array('&amp;', '&lt;'), $Element['attributes']['href']);
-
         return array(
             'extent' => $extent,
             'element' => $Element,
@@ -1263,12 +1282,12 @@ class Parsedown
 
     protected function inlineMarkup($Excerpt)
     {
-        if ($this->markupEscaped or strpos($Excerpt['text'], '>') === false)
+        if ($this->markupEscaped or $this->safeMode or strpos($Excerpt['text'], '>') === false)
         {
             return;
         }
 
-        if ($Excerpt['text'][1] === '/' and preg_match('/^<\/\w*[ ]*>/s', $Excerpt['text'], $matches))
+        if ($Excerpt['text'][1] === '/' and preg_match('/^<\/\w[\w-]*[ ]*>/s', $Excerpt['text'], $matches))
         {
             return array(
                 'markup' => $matches[0],
@@ -1284,7 +1303,7 @@ class Parsedown
             );
         }
 
-        if ($Excerpt['text'][1] !== ' ' and preg_match('/^<\w*(?:[ ]*'.$this->regexHtmlAttribute.')*[ ]*\/?>/s', $Excerpt['text'], $matches))
+        if ($Excerpt['text'][1] !== ' ' and preg_match('/^<\w[\w-]*(?:[ ]*'.$this->regexHtmlAttribute.')*[ ]*\/?>/s', $Excerpt['text'], $matches))
         {
             return array(
                 'markup' => $matches[0],
@@ -1343,14 +1362,16 @@ class Parsedown
 
         if (preg_match('/\bhttps?:[\/]{2}[^\s<]+\b\/*/ui', $Excerpt['context'], $matches, PREG_OFFSET_CAPTURE))
         {
+            $url = $matches[0][0];
+
             $Inline = array(
                 'extent' => strlen($matches[0][0]),
                 'position' => $matches[0][1],
                 'element' => array(
                     'name' => 'a',
-                    'text' => $matches[0][0],
+                    'text' => $url,
                     'attributes' => array(
-                        'href' => $matches[0][0],
+                        'href' => $url,
                     ),
                 ),
             );
@@ -1363,7 +1384,7 @@ class Parsedown
     {
         if (strpos($Excerpt['text'], '>') !== false and preg_match('/^<(\w+:\/{2}[^ >]+)>/i', $Excerpt['text'], $matches))
         {
-            $url = str_replace(array('&', '<'), array('&amp;', '&lt;'), $matches[1]);
+            $url = $matches[1];
 
             return array(
                 'extent' => strlen($matches[0]),
@@ -1401,6 +1422,11 @@ class Parsedown
 
     protected function element(array $Element)
     {
+        if ($this->safeMode)
+        {
+            $Element = $this->sanitiseElement($Element);
+        }
+
         $markup = '<'.$Element['name'];
 
         if (isset($Element['attributes']))
@@ -1412,7 +1438,7 @@ class Parsedown
                     continue;
                 }
 
-                $markup .= ' '.$name.'="'.$value.'"';
+                $markup .= ' '.$name.'="'.self::escape($value).'"';
             }
         }
 
@@ -1426,7 +1452,7 @@ class Parsedown
             }
             else
             {
-                $markup .= $Element['text'];
+                $markup .= self::escape($Element['text'], true);
             }
 
             $markup .= '</'.$Element['name'].'>';
@@ -1485,10 +1511,77 @@ class Parsedown
         return $markup;
     }
 
+    protected function sanitiseElement(array $Element)
+    {
+        static $goodAttribute = '/^[a-zA-Z0-9][a-zA-Z0-9-_]*+$/';
+        static $safeUrlNameToAtt  = array(
+            'a'   => 'href',
+            'img' => 'src',
+        );
+
+        if (isset($safeUrlNameToAtt[$Element['name']]))
+        {
+            $Element = $this->filterUnsafeUrlInAttribute($Element, $safeUrlNameToAtt[$Element['name']]);
+        }
+
+        if ( ! empty($Element['attributes']))
+        {
+            foreach ($Element['attributes'] as $att => $val)
+            {
+                # filter out badly parsed attribute
+                if ( ! preg_match($goodAttribute, $att))
+                {
+                    unset($Element['attributes'][$att]);
+                }
+                # dump onevent attribute
+                elseif (self::striAtStart($att, 'on'))
+                {
+                    unset($Element['attributes'][$att]);
+                }
+            }
+        }
+
+        return $Element;
+    }
+
+    protected function filterUnsafeUrlInAttribute(array $Element, $attribute)
+    {
+        foreach ($this->safeLinksWhitelist as $scheme)
+        {
+            if (self::striAtStart($Element['attributes'][$attribute], $scheme))
+            {
+                return $Element;
+            }
+        }
+
+        $Element['attributes'][$attribute] = str_replace(':', '%3A', $Element['attributes'][$attribute]);
+
+        return $Element;
+    }
+
     #
     # Static Methods
     #
 
+    protected static function escape($text, $allowQuotes = false)
+    {
+        return htmlspecialchars($text, $allowQuotes ? ENT_NOQUOTES : ENT_QUOTES, 'UTF-8');
+    }
+
+    protected static function striAtStart($string, $needle)
+    {
+        $len = strlen($needle);
+
+        if ($len > strlen($string))
+        {
+            return false;
+        }
+        else
+        {
+            return strtolower(substr($string, 0, $len)) === strtolower($needle);
+        }
+    }
+
     static function instance($name = 'default')
     {
         if (isset(self::$instances[$name]))

README.md

@@ -1,4 +1,4 @@
-> You might also like [Caret](https://caret.io?ref=parsedown) - our Markdown editor for Mac / Windows / Linux.
+> I also make [Caret](https://caret.io?ref=parsedown) - a Markdown editor for Mac and PC.
 
 ## Parsedown
 
@@ -38,7 +38,7 @@ More examples in [the wiki](https://github.com/erusev/parsedown/wiki/) and in [t
 
 ### Security
 
-Parsedown does not sanitize the HTML that it generates. When you deal with untrusted content (ex: user commnets) you should also use a HTML sanitizer like [HTML Purifier](http://htmlpurifier.org/).
+Parsedown does not sanitize the HTML that it generates. When you deal with untrusted content (ex: user comments) you should also use a HTML sanitizer like [HTML Purifier](http://htmlpurifier.org/).
 
 ### Questions
 

test/ParsedownTest.php

@@ -48,6 +48,8 @@ class ParsedownTest extends TestCase
         $expectedMarkup = str_replace("\r\n", "\n", $expectedMarkup);
         $expectedMarkup = str_replace("\r", "\n", $expectedMarkup);
 
+        $this->Parsedown->setSafeMode(substr($test, 0, 3) === 'xss');
+
         $actualMarkup = $this->Parsedown->text($markdown);
 
         $this->assertEquals($expectedMarkup, $actualMarkup);

test/data/xss_attribute_encoding.html

@@ -0,0 +1,6 @@
+<p><a href="https://www.example.com&quot;">xss</a></p>
+<p><img src="https://www.example.com&quot;" alt="xss" /></p>
+<p><a href="https://www.example.com&#039;">xss</a></p>
+<p><img src="https://www.example.com&#039;" alt="xss" /></p>
+<p><img src="https://www.example.com" alt="xss&quot;" /></p>
+<p><img src="https://www.example.com" alt="xss&#039;" /></p>

test/data/xss_attribute_encoding.md

@@ -0,0 +1,11 @@
+[xss](https://www.example.com")
+
+![xss](https://www.example.com")
+
+[xss](https://www.example.com')
+
+![xss](https://www.example.com')
+
+![xss"](https://www.example.com)
+
+![xss'](https://www.example.com)

test/data/xss_bad_url.html

@@ -0,0 +1,16 @@
+<p><a href="javascript%3Aalert(1)">xss</a></p>
+<p><a href="javascript%3Aalert(1)">xss</a></p>
+<p><a href="javascript%3A//alert(1)">xss</a></p>
+<p><a href="javascript&amp;colon;alert(1)">xss</a></p>
+<p><img src="javascript%3Aalert(1)" alt="xss" /></p>
+<p><img src="javascript%3Aalert(1)" alt="xss" /></p>
+<p><img src="javascript%3A//alert(1)" alt="xss" /></p>
+<p><img src="javascript&amp;colon;alert(1)" alt="xss" /></p>
+<p><a href="data%3Atext/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==">xss</a></p>
+<p><a href="data%3Atext/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==">xss</a></p>
+<p><a href="data%3A//text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==">xss</a></p>
+<p><a href="data&amp;colon;text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==">xss</a></p>
+<p><img src="data%3Atext/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==" alt="xss" /></p>
+<p><img src="data%3Atext/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==" alt="xss" /></p>
+<p><img src="data%3A//text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==" alt="xss" /></p>
+<p><img src="data&amp;colon;text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==" alt="xss" /></p>

test/data/xss_bad_url.md

@@ -0,0 +1,31 @@
+[xss](javascript:alert(1))
+
+[xss]( javascript:alert(1))
+
+[xss](javascript://alert(1))
+
+[xss](javascript:alert(1))
+
+![xss](javascript:alert(1))
+
+![xss]( javascript:alert(1))
+
+![xss](javascript://alert(1))
+
+![xss](javascript:alert(1))
+
+[xss](data:text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==)
+
+[xss]( data:text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==)
+
+[xss](data://text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==)
+
+[xss](data:text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==)
+
+![xss](data:text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==)
+
+![xss]( data:text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==)
+
+![xss](data://text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==)
+
+![xss](data:text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==)

test/data/xss_text_encoding.html

@@ -0,0 +1,7 @@
+<p>&lt;script&gt;alert(1)&lt;/script&gt;</p>
+<p>&lt;script&gt;</p>
+<p>alert(1)</p>
+<p>&lt;/script&gt;</p>
+<p>&lt;script&gt;
+alert(1)
+&lt;/script&gt;</p>

test/data/xss_text_encoding.md

@@ -0,0 +1,12 @@
+<script>alert(1)</script>
+
+<script>
+
+alert(1)
+
+</script>
+
+
+<script>
+alert(1)
+</script>