[2.2.x] Fixed CVE-2021-31542 -- Tightened path & file name sanitation in file uploads.

Author: apollo13

django/core/files/storage.py

@@ -1,11 +1,13 @@
 import os
+import pathlib
 from datetime import datetime
 from urllib.parse import urljoin
 
 from django.conf import settings
 from django.core.exceptions import SuspiciousFileOperation
 from django.core.files import File, locks
 from django.core.files.move import file_move_safe
+from django.core.files.utils import validate_file_name
 from django.core.signals import setting_changed
 from django.utils import timezone
 from django.utils._os import safe_join
@@ -66,6 +68,9 @@ def get_available_name(self, name, max_length=None):
         available for new content to be written to.
         """
         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)
+        validate_file_name(file_name)
         file_root, file_ext = os.path.splitext(file_name)
         # If the filename already exists, add an underscore and a random 7
         # character alphanumeric string (before the file extension, if one
@@ -98,6 +103,8 @@ def generate_filename(self, filename):
         """
         # `filename` may include a path as returned by FileField.upload_to.
         dirname, filename = os.path.split(filename)
+        if '..' in pathlib.PurePath(dirname).parts:
+            raise SuspiciousFileOperation("Detected path traversal attempt in '%s'" % dirname)
         return os.path.normpath(os.path.join(dirname, self.get_valid_name(filename)))
 
     def path(self, name):

django/core/files/uploadedfile.py

@@ -8,6 +8,7 @@
 from django.conf import settings
 from django.core.files import temp as tempfile
 from django.core.files.base import File
+from django.core.files.utils import validate_file_name
 
 __all__ = ('UploadedFile', 'TemporaryUploadedFile', 'InMemoryUploadedFile',
            'SimpleUploadedFile')
@@ -47,6 +48,8 @@ def _set_name(self, name):
                 ext = ext[:255]
                 name = name[:255 - len(ext)] + ext
 
+            name = validate_file_name(name)
+
         self._name = name
 
     name = property(_get_name, _set_name)

django/core/files/utils.py

@@ -1,3 +1,19 @@
+import os
+
+from django.core.exceptions import SuspiciousFileOperation
+
+
+def validate_file_name(name):
+    if name != os.path.basename(name):
+        raise SuspiciousFileOperation("File name '%s' includes path elements" % name)
+
+    # Remove potentially dangerous names
+    if name in {'', '.', '..'}:
+        raise SuspiciousFileOperation("Could not derive file name from '%s'" % name)
+
+    return name
+
+
 class FileProxyMixin:
     """
     A mixin class used to forward file methods to an underlaying file

django/db/models/fields/files.py

@@ -6,6 +6,7 @@
 from django.core.files.base import File
 from django.core.files.images import ImageFile
 from django.core.files.storage import default_storage
+from django.core.files.utils import validate_file_name
 from django.db.models import signals
 from django.db.models.fields import Field
 from django.utils.translation import gettext_lazy as _
@@ -299,6 +300,7 @@ def generate_filename(self, instance, filename):
         Until the storage layer, all file paths are expected to be Unix style
         (with forward slashes).
         """
+        filename = validate_file_name(filename)
         if callable(self.upload_to):
             filename = self.upload_to(instance, filename)
         else:

django/http/multipartparser.py

@@ -7,7 +7,7 @@
 import base64
 import binascii
 import cgi
-import os
+import html
 from urllib.parse import unquote
 
 from django.conf import settings
@@ -19,7 +19,6 @@
 )
 from django.utils.datastructures import MultiValueDict
 from django.utils.encoding import force_text
-from django.utils.text import unescape_entities
 
 __all__ = ('MultiPartParser', 'MultiPartParserError', 'InputStreamExhausted')
 
@@ -295,10 +294,25 @@ def handle_file_complete(self, old_field_name, counters):
                 break
 
     def sanitize_file_name(self, file_name):
-        file_name = unescape_entities(file_name)
-        # Cleanup Windows-style path separators.
-        file_name = file_name[file_name.rfind('\\') + 1:].strip()
-        return os.path.basename(file_name)
+        """
+        Sanitize the filename of an upload.
+
+        Remove all possible path separators, even though that might remove more
+        than actually required by the target system. Filenames that could
+        potentially cause problems (current/parent dir) are also discarded.
+
+        It should be noted that this function could still return a "filepath"
+        like "C:some_file.txt" which is handled later on by the storage layer.
+        So while this function does sanitize filenames to some extent, the
+        resulting filename should still be considered as untrusted user input.
+        """
+        file_name = html.unescape(file_name)
+        file_name = file_name.rsplit('/')[-1]
+        file_name = file_name.rsplit('\\')[-1]
+
+        if file_name in {'', '.', '..'}:
+            return None
+        return file_name
 
     IE_sanitize = sanitize_file_name
 

django/utils/text.py

@@ -4,6 +4,7 @@
 from gzip import GzipFile
 from io import BytesIO
 
+from django.core.exceptions import SuspiciousFileOperation
 from django.utils.functional import SimpleLazyObject, keep_lazy_text, lazy
 from django.utils.translation import gettext as _, gettext_lazy, pgettext
 
@@ -216,7 +217,7 @@ def _truncate_html(self, length, truncate, text, truncate_len, words):
 
 
 @keep_lazy_text
-def get_valid_filename(s):
+def get_valid_filename(name):
     """
     Return the given string converted to a string that can be used for a clean
     filename. Remove leading and trailing spaces; convert other spaces to
@@ -225,8 +226,11 @@ def get_valid_filename(s):
     >>> get_valid_filename("john's portrait in 2004.jpg")
     'johns_portrait_in_2004.jpg'
     """
-    s = str(s).strip().replace(' ', '_')
-    return re.sub(r'(?u)[^-\w.]', '', s)
+    s = str(name).strip().replace(' ', '_')
+    s = re.sub(r'(?u)[^-\w.]', '', s)
+    if s in {'', '.', '..'}:
+        raise SuspiciousFileOperation("Could not derive file name from '%s'" % name)
+    return s
 
 
 @keep_lazy_text

docs/releases/2.2.21.txt

@@ -0,0 +1,17 @@
+===========================
+Django 2.2.21 release notes
+===========================
+
+*May 4, 2021*
+
+Django 2.2.21 fixes a security issue in 2.2.20.
+
+CVE-2021-31542: Potential directory-traversal via uploaded files
+================================================================
+
+``MultiPartParser``, ``UploadedFile``, and ``FieldFile`` allowed
+directory-traversal via uploaded files with suitably crafted file names.
+
+In order to mitigate this risk, stricter basename and path sanitation is now
+applied. Specifically, empty file names and paths with dot segments will be
+rejected.

docs/releases/index.txt

@@ -25,6 +25,7 @@ versions of the documentation contain the release notes for any later releases.
 .. toctree::
    :maxdepth: 1
 
+   2.2.21
    2.2.20
    2.2.19
    2.2.18

tests/file_storage/test_generate_filename.py

@@ -1,7 +1,8 @@
 import os
 
+from django.core.exceptions import SuspiciousFileOperation
 from django.core.files.base import ContentFile
-from django.core.files.storage import Storage
+from django.core.files.storage import FileSystemStorage, Storage
 from django.db.models import FileField
 from django.test import SimpleTestCase
 
@@ -36,6 +37,44 @@ def generate_filename(self, filename):
 
 
 class GenerateFilenameStorageTests(SimpleTestCase):
+    def test_storage_dangerous_paths(self):
+        candidates = [
+            ('/tmp/..', '..'),
+            ('/tmp/.', '.'),
+            ('', ''),
+        ]
+        s = FileSystemStorage()
+        msg = "Could not derive file name from '%s'"
+        for file_name, base_name in candidates:
+            with self.subTest(file_name=file_name):
+                with self.assertRaisesMessage(SuspiciousFileOperation, msg % base_name):
+                    s.get_available_name(file_name)
+                with self.assertRaisesMessage(SuspiciousFileOperation, msg % base_name):
+                    s.generate_filename(file_name)
+
+    def test_storage_dangerous_paths_dir_name(self):
+        file_name = '/tmp/../path'
+        s = FileSystemStorage()
+        msg = "Detected path traversal attempt in '/tmp/..'"
+        with self.assertRaisesMessage(SuspiciousFileOperation, msg):
+            s.get_available_name(file_name)
+        with self.assertRaisesMessage(SuspiciousFileOperation, msg):
+            s.generate_filename(file_name)
+
+    def test_filefield_dangerous_filename(self):
+        candidates = ['..', '.', '', '???', '$.$.$']
+        f = FileField(upload_to='some/folder/')
+        msg = "Could not derive file name from '%s'"
+        for file_name in candidates:
+            with self.subTest(file_name=file_name):
+                with self.assertRaisesMessage(SuspiciousFileOperation, msg % file_name):
+                    f.generate_filename(None, file_name)
+
+    def test_filefield_dangerous_filename_dir(self):
+        f = FileField(upload_to='some/folder/')
+        msg = "File name '/tmp/path' includes path elements"
+        with self.assertRaisesMessage(SuspiciousFileOperation, msg):
+            f.generate_filename(None, '/tmp/path')
 
     def test_filefield_generate_filename(self):
         f = FileField(upload_to='some/folder/')

tests/file_uploads/tests.py

@@ -8,8 +8,9 @@
 from io import BytesIO, StringIO
 from urllib.parse import quote
 
+from django.core.exceptions import SuspiciousFileOperation
 from django.core.files import temp as tempfile
-from django.core.files.uploadedfile import SimpleUploadedFile
+from django.core.files.uploadedfile import SimpleUploadedFile, UploadedFile
 from django.http.multipartparser import (
     MultiPartParser, MultiPartParserError, parse_header,
 )
@@ -37,6 +38,16 @@
     '../hax0rd.txt',       # HTML entities.
 ]
 
+CANDIDATE_INVALID_FILE_NAMES = [
+    '/tmp/',        # Directory, *nix-style.
+    'c:\\tmp\\',    # Directory, win-style.
+    '/tmp/.',       # Directory dot, *nix-style.
+    'c:\\tmp\\.',   # Directory dot, *nix-style.
+    '/tmp/..',      # Parent directory, *nix-style.
+    'c:\\tmp\\..',  # Parent directory, win-style.
+    '',             # Empty filename.
+]
+
 
 @override_settings(MEDIA_ROOT=MEDIA_ROOT, ROOT_URLCONF='file_uploads.urls', MIDDLEWARE=[])
 class FileUploadTests(TestCase):
@@ -52,6 +63,22 @@ def tearDownClass(cls):
         shutil.rmtree(MEDIA_ROOT)
         super().tearDownClass()
 
+    def test_upload_name_is_validated(self):
+        candidates = [
+            '/tmp/',
+            '/tmp/..',
+            '/tmp/.',
+        ]
+        if sys.platform == 'win32':
+            candidates.extend([
+                'c:\\tmp\\',
+                'c:\\tmp\\..',
+                'c:\\tmp\\.',
+            ])
+        for file_name in candidates:
+            with self.subTest(file_name=file_name):
+                self.assertRaises(SuspiciousFileOperation, UploadedFile, name=file_name)
+
     def test_simple_upload(self):
         with open(__file__, 'rb') as fp:
             post_data = {
@@ -631,6 +658,15 @@ def test_sanitize_file_name(self):
             with self.subTest(file_name=file_name):
                 self.assertEqual(parser.sanitize_file_name(file_name), 'hax0rd.txt')
 
+    def test_sanitize_invalid_file_name(self):
+        parser = MultiPartParser({
+            'CONTENT_TYPE': 'multipart/form-data; boundary=_foo',
+            'CONTENT_LENGTH': '1',
+        }, StringIO('x'), [], 'utf-8')
+        for file_name in CANDIDATE_INVALID_FILE_NAMES:
+            with self.subTest(file_name=file_name):
+                self.assertIsNone(parser.sanitize_file_name(file_name))
+
     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",