Markdown xss backport (#8244)

* Update helpers.py

* Update mixins.py

* format

* format

* Allow horizontal rule in markdown

* More instructive error msg

* Specify output_format to markdown.markdown

Ref: https://python-markdown.github.io/reference/markdown/serializers/

* Cleanup

* Adjust allowable markdown tags

* Add unit test for malicious markdown XSS

* Allow <pre> tag

---------

Co-authored-by: Matthias Mair <code@mjmair.com>

Author: SchrodingersGat

src/backend/InvenTree/InvenTree/helpers.py

@@ -810,6 +810,40 @@ def remove_non_printable_characters(
     return cleaned
 
 
+def clean_markdown(value: str):
+    """Clean a markdown string.
+
+    This function will remove javascript and other potentially harmful content from the markdown string.
+    """
+    import markdown
+    from markdownify.templatetags.markdownify import markdownify
+
+    try:
+        markdownify_settings = settings.MARKDOWNIFY['default']
+    except (AttributeError, KeyError):
+        markdownify_settings = {}
+
+    extensions = markdownify_settings.get('MARKDOWN_EXTENSIONS', [])
+    extension_configs = markdownify_settings.get('MARKDOWN_EXTENSION_CONFIGS', {})
+
+    # Generate raw HTML from provided markdown (without sanitizing)
+    # Note: The 'html' output_format is required to generate self closing tags, e.g. <tag> instead of <tag />
+    html = markdown.markdown(
+        value or '',
+        extensions=extensions,
+        extension_configs=extension_configs,
+        output_format='html',
+    )
+
+    # Clean the HTML content (for comparison). Ideally, this should be the same as the original content
+    clean_html = markdownify(value)
+
+    if html != clean_html:
+        raise ValidationError(_('Data contains prohibited markdown content'))
+
+    return value
+
+
 def hash_barcode(barcode_data):
     """Calculate a 'unique' hash for a barcode string.
 

src/backend/InvenTree/InvenTree/mixins.py

@@ -6,7 +6,11 @@
 from rest_framework.response import Response
 
 from InvenTree.fields import InvenTreeNotesField
-from InvenTree.helpers import remove_non_printable_characters, strip_html_tags
+from InvenTree.helpers import (
+    clean_markdown,
+    remove_non_printable_characters,
+    strip_html_tags,
+)
 
 
 class CleanMixin:
@@ -57,18 +61,20 @@ def clean_string(self, field: str, data: str) -> str:
 
         # By default, newline characters are removed
         remove_newline = True
+        is_markdown = False
 
         try:
             if hasattr(self, 'serializer_class'):
                 model = self.serializer_class.Meta.model
                 field = model._meta.get_field(field)
 
                 # The following field types allow newline characters
-                allow_newline = [InvenTreeNotesField]
+                allow_newline = [(InvenTreeNotesField, True)]
 
                 for field_type in allow_newline:
-                    if issubclass(type(field), field_type):
+                    if issubclass(type(field), field_type[0]):
                         remove_newline = False
+                        is_markdown = field_type[1]
                         break
 
         except AttributeError:
@@ -80,6 +86,9 @@ def clean_string(self, field: str, data: str) -> str:
             cleaned, remove_newline=remove_newline
         )
 
+        if is_markdown:
+            cleaned = clean_markdown(cleaned)
+
         return cleaned
 
     def clean_data(self, data: dict) -> dict:

src/backend/InvenTree/InvenTree/settings.py

@@ -1231,23 +1231,29 @@
             'abbr',
             'b',
             'blockquote',
+            'code',
             'em',
             'h1',
             'h2',
             'h3',
+            'h4',
+            'h5',
+            'hr',
             'i',
             'img',
             'li',
             'ol',
             'p',
+            'pre',
+            's',
             'strong',
-            'ul',
             'table',
             'thead',
             'tbody',
             'th',
             'tr',
             'td',
+            'ul',
         ],
     }
 }

src/backend/InvenTree/company/test_api.py

@@ -156,6 +156,47 @@ def test_company_active(self):
             len(self.get(url, data={'active': False}, expected_code=200).data), 1
         )
 
+    def test_company_notes(self):
+        """Test the markdown 'notes' field for the Company model."""
+        pk = Company.objects.first().pk
+
+        # Attempt to inject malicious markdown into the "notes" field
+        xss = [
+            '[Click me](javascript:alert(123))',
+            '![x](javascript:alert(123))',
+            '![Uh oh...]("onerror="alert(\'XSS\'))',
+        ]
+
+        for note in xss:
+            response = self.patch(
+                reverse('api-company-detail', kwargs={'pk': pk}),
+                {'notes': note},
+                expected_code=400,
+            )
+
+            self.assertIn(
+                'Data contains prohibited markdown content', str(response.data)
+            )
+
+        # The following markdown is safe, and should be accepted
+        good = [
+            'This is a **bold** statement',
+            'This is a *italic* statement',
+            'This is a [link](https://www.google.com)',
+            'This is an ![image](https://www.google.com/test.jpg)',
+            'This is a `code` block',
+            'This text has ~~strikethrough~~ formatting',
+        ]
+
+        for note in good:
+            response = self.patch(
+                reverse('api-company-detail', kwargs={'pk': pk}),
+                {'notes': note},
+                expected_code=200,
+            )
+
+            self.assertEqual(response.data['notes'], note)
+
 
 class ContactTest(InvenTreeAPITestCase):
     """Tests for the Contact models."""