fix: update html cleaning rules (#444)

* fix: update html cleaning rules

* chore: add docstrings and code cleaning

* chore: fixes test cases and adds few new tests

* chore: simplify test cases and refactor docstrings

openedx/core/djangolib/markup.py

@@ -5,6 +5,7 @@
 
 import markupsafe
 import bleach
+import re
 from lxml.html.clean import Cleaner
 from mako.filters import decode
 
@@ -14,6 +15,39 @@
 Text = markupsafe.escape                        # pylint: disable=invalid-name
 
 
+class HTMLCleaner(Cleaner):
+    """
+    HTMLCleaner extends lxml.html.clean.Cleaner to sanitize HTML content while preserving
+    valid URLs and removing unsafe JavaScript links.
+    """
+    def _remove_javascript_link(self, link: str):
+        """
+        Overrides the parent class's method to preserve valid URLs.
+
+        This method uses a regular expression to identify valid URLs that start with 'http', 'https',
+        'ftp', or 'file' schemes. If the link is a valid URL, it is returned unchanged. Otherwise,
+        the parent class's method is used to remove the JavaScript link.
+
+        Args:
+            link (str): The hyperlink (href attribute value) to be checked and potentially sanitized.
+
+        Returns:
+            str: The original link or empty string.
+
+        Examples:
+            Valid URLs:
+                'https://www.example.com/javascript:something'
+                'file://localhost/path/to/file'
+            Invalid URLs:
+                'javascript:alert("hello")'
+                'javascript:alert("hello") https://www.example.com/page'
+        """
+        is_url = re.compile(r"^(?:https?|ftp|file)://", re.I).search(link.strip())
+        if is_url:
+            return link
+        return super()._remove_javascript_link(link)
+
+
 def HTML(html):                                 # pylint: disable=invalid-name
     """
     Mark a string as already HTML, so that it won't be escaped before output.
@@ -70,6 +104,6 @@ def clean_dangerous_html(html):
     """
     if not html:
         return html
-    cleaner = Cleaner(style=True, inline_style=False, safe_attrs_only=False)
+    cleaner = HTMLCleaner(style=True, inline_style=False, safe_attrs_only=False)
     html = cleaner.clean_html(html)
     return HTML(html)

openedx/core/djangolib/tests/test_markup.py

@@ -11,7 +11,7 @@
 from django.utils.translation import ngettext
 from mako.template import Template
 
-from openedx.core.djangolib.markup import HTML, Text, strip_all_tags_but_br
+from openedx.core.djangolib.markup import HTML, HTMLCleaner, Text, strip_all_tags_but_br
 
 
 @ddt.ddt
@@ -157,3 +157,36 @@ def test_clean_dengers_html_filter(self):
         assert not html_soup.find('form')
         assert not html_soup.find('blink')
         assert not html_soup.find('object')
+
+@ddt.ddt
+class TestHTMLCleaner(unittest.TestCase):
+    """
+    Tests that Url links are being cleaned properly and no useful link is removed.
+    """
+
+    def setUp(self):
+        self.cleaner = HTMLCleaner()
+
+    @ddt.data(
+        "https://example.com/data:something", # Javascript cannot be executed this way so these urls are safe.
+        "http://example.com/path/to/page",
+        "ftp://ftp.example.com/resource",
+        "file://localhost/path/to/file",
+    )
+    def test_valid_urls(self, url):
+        """
+        Test that valid URLs are preserved.
+        """
+        cleaned_url = self.cleaner._remove_javascript_link(url)
+        self.assertEqual(cleaned_url, url)
+
+    @ddt.data(
+        "javascript:alert('Hello')",
+        "mocha:some_code https://example.com", # Javascript can be executed this way so this code should be removed.
+    )
+    def test_javascript_is_removed(self, url):
+        """
+        Test that malicious Javascript is removed.
+        """
+        cleaned_url = self.cleaner._remove_javascript_link(url)
+        self.assertEqual(cleaned_url, '')

openedx/features/wikimedia_features/messenger/apps.py

@@ -20,7 +20,6 @@ class MessengerConfig(AppConfig):
         PluginSettings.CONFIG: {
             ProjectType.LMS: {
                 SettingsType.COMMON: {PluginSettings.RELATIVE_PATH: 'settings.common'},
-                SettingsType.TEST: {PluginSettings.RELATIVE_PATH: 'settings.test'},
             }
         }
     }