Fixed CVE-2021-45452 -- Fixed potential path traversal in storage subsystem.

Thanks to Dennis Brinkrolf for the report.

django/core/files/storage.py

@@ -51,7 +51,10 @@ class Storage:
             content = File(content, name)
 
         name = self.get_available_name(name, max_length=max_length)
-        return self._save(name, content)
+        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)
+        return name
 
     # These methods are part of the public API, with default implementations.
 
@@ -75,6 +78,7 @@ class Storage:
         Return a filename that's free on the target storage system and
         available for new content to be written to.
         """
+        name = str(name).replace('\\', '/')
         dir_name, file_name = os.path.split(name)
         if '..' in pathlib.PurePath(dir_name).parts:
             raise SuspiciousFileOperation("Detected path traversal attempt in '%s'" % dir_name)
@@ -108,6 +112,7 @@ class Storage:
         Validate the filename by calling get_valid_name() and return a filename
         to be passed to the save() method.
         """
+        filename = str(filename).replace('\\', '/')
         # `filename` may include a path as returned by FileField.upload_to.
         dirname, filename = os.path.split(filename)
         if '..' in pathlib.PurePath(dirname).parts:
@@ -297,6 +302,8 @@ class FileSystemStorage(Storage):
         if self.file_permissions_mode is not None:
             os.chmod(full_path, self.file_permissions_mode)
 
+        # Ensure the saved path is always relative to the storage root.
+        name = os.path.relpath(full_path, self.location)
         # Store filenames with forward slashes, even on Windows.
         return str(name).replace('\\', '/')
 

docs/releases/2.2.26.txt

@@ -33,6 +33,11 @@ resolution logic, that will not call methods, nor allow indexing on
 dictionaries.
 
 As a reminder, all untrusted user input should be validated before use.
+CVE-2021-45452: Potential directory-traversal via ``Storage.save()``
+====================================================================
+
+``Storage.save()`` allowed directory-traversal if directly passed suitably
+crafted file names.
 
 This issue has severity "low" according to the :ref:`Django security policy
 <security-disclosure>`.

docs/releases/3.2.11.txt

@@ -33,6 +33,11 @@ resolution logic, that will not call methods, nor allow indexing on
 dictionaries.
 
 As a reminder, all untrusted user input should be validated before use.
+CVE-2021-45452: Potential directory-traversal via ``Storage.save()``
+====================================================================
+
+``Storage.save()`` allowed directory-traversal if directly passed suitably
+crafted file names.
 
 This issue has severity "low" according to the :ref:`Django security policy
 <security-disclosure>`.

docs/releases/4.0.1.txt

@@ -33,6 +33,11 @@ resolution logic, that will not call methods, nor allow indexing on
 dictionaries.
 
 As a reminder, all untrusted user input should be validated before use.
+CVE-2021-45452: Potential directory-traversal via ``Storage.save()``
+====================================================================
+
+``Storage.save()`` allowed directory-traversal if directly passed suitably
+crafted file names.
 
 This issue has severity "low" according to the :ref:`Django security policy
 <security-disclosure>`.

tests/file_storage/test_generate_filename.py

@@ -53,9 +53,16 @@ class GenerateFilenameStorageTests(SimpleTestCase):
                     s.generate_filename(file_name)
 
     def test_storage_dangerous_paths_dir_name(self):
-        file_name = '/tmp/../path'
+        candidates = [
+            ('tmp/../path', 'tmp/..'),
+            ('tmp\\..\\path', 'tmp/..'),
+            ('/tmp/../path', '/tmp/..'),
+            ('\\tmp\\..\\path', '/tmp/..'),
+        ]
         s = FileSystemStorage()
-        msg = "Detected path traversal attempt in '/tmp/..'"
+        for file_name, path in candidates:
+            msg = "Detected path traversal attempt in '%s'" % path
+            with self.subTest(file_name=file_name):
                 with self.assertRaisesMessage(SuspiciousFileOperation, msg):
                     s.get_available_name(file_name)
                 with self.assertRaisesMessage(SuspiciousFileOperation, msg):

tests/file_storage/tests.py

@@ -298,6 +298,12 @@ class FileStorageTests(SimpleTestCase):
 
         self.storage.delete('path/to/test.file')
 
+    def test_file_save_abs_path(self):
+        test_name = 'path/to/test.file'
+        f = ContentFile('file saved with path')
+        f_name = self.storage.save(os.path.join(self.temp_dir, test_name), f)
+        self.assertEqual(f_name, test_name)
+
     @unittest.skipUnless(symlinks_supported(), 'Must be able to symlink to run this test.')
     def test_file_save_broken_symlink(self):
         """A new path is created on save when a broken symlink is supplied."""