Security: prevent path traversal in storage paths

src/documents/templating/filepath.py

@@ -262,6 +262,17 @@ def get_custom_fields_context(
     return field_data
 
 
+def _is_safe_relative_path(value: str) -> bool:
+    if value == "":
+        return True
+
+    path = PurePath(value)
+    if path.is_absolute() or path.drive:
+        return False
+
+    return ".." not in path.parts
+
+
 def validate_filepath_template_and_render(
     template_string: str,
     document: Document | None = None,
@@ -309,6 +320,12 @@ def validate_filepath_template_and_render(
         )
         rendered_template = template.render(context)
 
+        if not _is_safe_relative_path(rendered_template):
+            logger.warning(
+                "Template rendered an unsafe path (absolute or containing traversal).",
+            )
+            return None
+
         # We're good!
         return rendered_template
     except UndefinedError: