Fixed CVE-2021-28658 -- Fixed potential directory-traversal via uploaded files.

Thanks Claude Paroz for the initial patch.
Thanks Dennis Brinkrolf for the report.

tests/file_uploads/tests.py

@@ -23,6 +23,22 @@ UNICODE_FILENAME = 'test-0123456789_中文_Orléans.jpg'
 MEDIA_ROOT = sys_tempfile.mkdtemp()
 UPLOAD_TO = os.path.join(MEDIA_ROOT, 'test_upload')
 
+CANDIDATE_TRAVERSAL_FILE_NAMES = [
+    '/tmp/hax0rd.txt',          # Absolute path, *nix-style.
+    'C:\\Windows\\hax0rd.txt',  # Absolute path, win-style.
+    'C:/Windows/hax0rd.txt',    # Absolute path, broken-style.
+    '\\tmp\\hax0rd.txt',        # Absolute path, broken in a different way.
+    '/tmp\\hax0rd.txt',         # Absolute path, broken by mixing.
+    'subdir/hax0rd.txt',        # Descendant path, *nix-style.
+    'subdir\\hax0rd.txt',       # Descendant path, win-style.
+    'sub/dir\\hax0rd.txt',      # Descendant path, mixed.
+    '../../hax0rd.txt',         # Relative path, *nix-style.
+    '..\\..\\hax0rd.txt',       # Relative path, win-style.
+    '../..\\hax0rd.txt',        # Relative path, mixed.
+    '../hax0rd.txt',       # HTML entities.
+    '../hax0rd.txt',        # HTML entities.
+]
+
 
 @override_settings(MEDIA_ROOT=MEDIA_ROOT, ROOT_URLCONF='file_uploads.urls', MIDDLEWARE=[])
 class FileUploadTests(TestCase):
@@ -251,22 +267,8 @@ class FileUploadTests(TestCase):
         # a malicious payload with an invalid file name (containing os.sep or
         # os.pardir). This similar to what an attacker would need to do when
         # trying such an attack.
-        scary_file_names = [
-            "/tmp/hax0rd.txt",          # Absolute path, *nix-style.
-            "C:\\Windows\\hax0rd.txt",  # Absolute path, win-style.
-            "C:/Windows/hax0rd.txt",    # Absolute path, broken-style.
-            "\\tmp\\hax0rd.txt",        # Absolute path, broken in a different way.
-            "/tmp\\hax0rd.txt",         # Absolute path, broken by mixing.
-            "subdir/hax0rd.txt",        # Descendant path, *nix-style.
-            "subdir\\hax0rd.txt",       # Descendant path, win-style.
-            "sub/dir\\hax0rd.txt",      # Descendant path, mixed.
-            "../../hax0rd.txt",         # Relative path, *nix-style.
-            "..\\..\\hax0rd.txt",       # Relative path, win-style.
-            "../..\\hax0rd.txt"         # Relative path, mixed.
-        ]
-
         payload = client.FakePayload()
-        for i, name in enumerate(scary_file_names):
+        for i, name in enumerate(CANDIDATE_TRAVERSAL_FILE_NAMES):
             payload.write('\r\n'.join([
                 '--' + client.BOUNDARY,
                 'Content-Disposition: form-data; name="file%s"; filename="%s"' % (i, name),
@@ -286,7 +288,7 @@ class FileUploadTests(TestCase):
         response = self.client.request(**r)
         # The filenames should have been sanitized by the time it got to the view.
         received = response.json()
-        for i, name in enumerate(scary_file_names):
+        for i, name in enumerate(CANDIDATE_TRAVERSAL_FILE_NAMES):
             got = received["file%s" % i]
             self.assertEqual(got, "hax0rd.txt")
 
@@ -597,6 +599,47 @@ class FileUploadTests(TestCase):
         # shouldn't differ.
         self.assertEqual(os.path.basename(obj.testfile.path), 'MiXeD_cAsE.txt')
 
+    def test_filename_traversal_upload(self):
+        os.makedirs(UPLOAD_TO, exist_ok=True)
+        self.addCleanup(shutil.rmtree, MEDIA_ROOT)
+        tests = [
+            '../test.txt',
+            '../test.txt',
+        ]
+        for file_name in tests:
+            with self.subTest(file_name=file_name):
+                payload = client.FakePayload()
+                payload.write(
+                    '\r\n'.join([
+                        '--' + client.BOUNDARY,
+                        'Content-Disposition: form-data; name="my_file"; '
+                        'filename="%s";' % file_name,
+                        'Content-Type: text/plain',
+                        '',
+                        'file contents.\r\n',
+                        '\r\n--' + client.BOUNDARY + '--\r\n',
+                    ]),
+                )
+                r = {
+                    'CONTENT_LENGTH': len(payload),
+                    'CONTENT_TYPE': client.MULTIPART_CONTENT,
+                    'PATH_INFO': '/upload_traversal/',
+                    'REQUEST_METHOD': 'POST',
+                    'wsgi.input': payload,
+                }
+                response = self.client.request(**r)
+                result = response.json()
+                self.assertEqual(response.status_code, 200)
+                self.assertEqual(result['file_name'], 'test.txt')
+                self.assertIs(
+                    os.path.exists(os.path.join(MEDIA_ROOT, 'test.txt')),
+                    False,
+                )
+                self.assertIs(
+                    os.path.exists(os.path.join(UPLOAD_TO, 'test.txt')),
+                    True,
+                )
+
 
 @override_settings(MEDIA_ROOT=MEDIA_ROOT)
 class DirectoryCreationTests(SimpleTestCase):
@@ -666,6 +709,15 @@ class MultiParserTests(SimpleTestCase):
         }, StringIO('x'), [], 'utf-8')
         self.assertEqual(multipart_parser._content_length, 0)
 
+    def test_sanitize_file_name(self):
+        parser = MultiPartParser({
+            'CONTENT_TYPE': 'multipart/form-data; boundary=_foo',
+            'CONTENT_LENGTH': '1'
+        }, StringIO('x'), [], 'utf-8')
+        for file_name in CANDIDATE_TRAVERSAL_FILE_NAMES:
+            with self.subTest(file_name=file_name):
+                self.assertEqual(parser.sanitize_file_name(file_name), 'hax0rd.txt')
+
     def test_rfc2231_parsing(self):
         test_data = (
             (b"Content-Type: application/x-stuff; title*=us-ascii'en-us'This%20is%20%2A%2A%2Afun%2A%2A%2A",