From 2925687e1e86dc113116330efd547b9db5c0f1a6 Mon Sep 17 00:00:00 2001
From: ArthurHoaro <arthur@hoa.ro>
Date: Fri, 19 Feb 2016 19:37:13 +0100
Subject: Markdown: don't escape content + sanitize sensible tags

Instead of trying to fix broken content for Markdown parsing, parse it unescaped, then sanatize sensible tags such as scripts, etc.
---
 application/Utils.php                | 18 +++++++++++++++++-
 plugins/markdown/markdown.php        | 37 ++++++++++++++++++++++++++++--------
 tests/plugins/PluginMarkdownTest.php | 19 +++++++++++-------
 3 files changed, 58 insertions(+), 16 deletions(-)

diff --git a/application/Utils.php b/application/Utils.php
index 10d60698..868946df 100644
--- a/application/Utils.php
+++ b/application/Utils.php
@@ -62,13 +62,29 @@ function endsWith($haystack, $needle, $case=true)
 }
 
 /**
- * htmlspecialchars wrapper
+ * Htmlspecialchars wrapper
+ *
+ * @param string $str the string to escape.
+ *
+ * @return string escaped.
  */
 function escape($str)
 {
     return htmlspecialchars($str, ENT_COMPAT, 'UTF-8', false);
 }
 
+/**
+ * Reverse the escape function.
+ *
+ * @param string $str the string to unescape.
+ *
+ * @return string unescaped string.
+ */
+function unescape($str)
+{
+    return htmlspecialchars_decode($str);
+}
+
 /**
  * Link sanitization before templating
  */
diff --git a/plugins/markdown/markdown.php b/plugins/markdown/markdown.php
index 3630ef14..a45b6574 100644
--- a/plugins/markdown/markdown.php
+++ b/plugins/markdown/markdown.php
@@ -117,23 +117,43 @@ function reverse_space2nbsp($description)
 }
 
 /**
- * Remove '&gt;' at start of line auto generated by Shaarli core system
- * to allow markdown blockquotes.
+ * Remove dangerous HTML tags (tags, iframe, etc.).
+ * Doesn't affect <code> content (already escaped by Parsedown).
  *
  * @param string $description input description text.
  *
- * @return string $description without HTML links.
+ * @return string given string escaped.
  */
-function reset_quote_tags($description)
+function sanitize_html($description)
 {
-    return preg_replace('/^( *)&gt; /m', '$1> ', $description);
+    $escapeTags = array(
+        'script',
+        'style',
+        'link',
+        'iframe',
+        'frameset',
+        'frame',
+    );
+    foreach ($escapeTags as $tag) {
+        $description = preg_replace_callback(
+            '#<\s*'. $tag .'[^>]*>(.*</\s*'. $tag .'[^>]*>)?#is',
+            function ($match) { return escape($match[0]); },
+            $description);
+    }
+    $description = preg_replace(
+        '#(<[^>]+)on[a-z]*="[^"]*"#is',
+        '$1',
+        $description);
+    return $description;
 }
 
 /**
  * Render shaare contents through Markdown parser.
  *   1. Remove HTML generated by Shaarli core.
- *   2. Generate markdown descriptions.
- *   3. Wrap description in 'markdown' CSS class.
+ *   2. Reverse the escape function.
+ *   3. Generate markdown descriptions.
+ *   4. Sanitize sensible HTML tags for security.
+ *   5. Wrap description in 'markdown' CSS class.
  *
  * @param string $description input description text.
  *
@@ -147,11 +167,12 @@ function process_markdown($description)
     $processedDescription = reverse_text2clickable($processedDescription);
     $processedDescription = reverse_nl2br($processedDescription);
     $processedDescription = reverse_space2nbsp($processedDescription);
-    $processedDescription = reset_quote_tags($processedDescription);
+    $processedDescription = unescape($processedDescription);
     $processedDescription = $parsedown
         ->setMarkupEscaped(false)
         ->setBreaksEnabled(true)
         ->text($processedDescription);
+    $processedDescription = sanitize_html($processedDescription);
     $processedDescription = '<div class="markdown">'. $processedDescription . '</div>';
 
     return $processedDescription;
diff --git a/tests/plugins/PluginMarkdownTest.php b/tests/plugins/PluginMarkdownTest.php
index 455f5ba7..8e1a128a 100644
--- a/tests/plugins/PluginMarkdownTest.php
+++ b/tests/plugins/PluginMarkdownTest.php
@@ -100,13 +100,18 @@ class PluginMarkdownTest extends PHPUnit_Framework_TestCase
     }
 
     /**
-     * Test reset_quote_tags()
+     * Test sanitize_html().
      */
-    function testResetQuoteTags()
-    {
-        $text = '> quote1'. PHP_EOL . ' > quote2 ' . PHP_EOL . 'noquote';
-        $processedText = escape($text);
-        $reversedText = reset_quote_tags($processedText);
-        $this->assertEquals($text, $reversedText);
+    function testSanitizeHtml() {
+        $input = '< script src="js.js"/>';
+        $input .= '< script attr>alert(\'xss\');</script>';
+        $input .= '<style> * { display: none }</style>';
+        $output = escape($input);
+        $input .= '<a href="#" onmouseHover="alert(\'xss\');" attr="tt">link</a>';
+        $output .= '<a href="#"  attr="tt">link</a>';
+        $this->assertEquals($output, sanitize_html($input));
+        // Do not touch escaped HTML.
+        $input = escape($input);
+        $this->assertEquals($input, sanitize_html($input));
     }
 }
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