[5.0.x] Fixed CVE-2024-39330 -- Added extra file name validation in Storage's save method.

Thanks to Josh Schneier for the report, and to Carlton Gibson and Sarah
Boyce for the reviews.

django/core/files/storage/base.py

@@ -34,7 +34,18 @@ class Storage:
         if not hasattr(content, "chunks"):
             content = File(content, name)
 
+        # Ensure that the name is valid, before and after having the storage
+        # system potentially modifying the name. This duplicates the check made
+        # inside `get_available_name` but it's necessary for those cases where
+        # `get_available_name` is overriden and validation is lost.
+        validate_file_name(name, allow_relative_path=True)
+
+        # Potentially find a different name depending on storage constraints.
         name = self.get_available_name(name, max_length=max_length)
+        # Validate the (potentially) new name.
+        validate_file_name(name, allow_relative_path=True)
+
+        # The save operation should return the actual name of the file saved.
         name = self._save(name, content)
         # Ensure that the name returned from the storage system is still valid.
         validate_file_name(name, allow_relative_path=True)

django/core/files/utils.py

@@ -10,10 +10,9 @@ def validate_file_name(name, allow_relative_path=False):
         raise SuspiciousFileOperation("Could not derive file name from '%s'" % name)
 
     if allow_relative_path:
-        # Use PurePosixPath() because this branch is checked only in
-        # FileField.generate_filename() where all file paths are expected to be
-        # Unix style (with forward slashes).
-        path = pathlib.PurePosixPath(name)
+        # Ensure that name can be treated as a pure posix path, i.e. Unix
+        # style (with forward slashes).
+        path = pathlib.PurePosixPath(str(name).replace("\\", "/"))
         if path.is_absolute() or ".." in path.parts:
             raise SuspiciousFileOperation(
                 "Detected path traversal attempt in '%s'" % name