[1.5.x] Fixed #23157 -- Removed O(n) algorithm when uploading duplicate file names.

This is a security fix. Disclosure following shortly.

django/core/files/storage.py

@@ -4,13 +4,13 @@ try:
     from urllib.parse import urljoin
 except ImportError:     # Python 2
     from urlparse import urljoin
-import itertools
 from datetime import datetime
 
 from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured, SuspiciousOperation
 from django.core.files import locks, File
 from django.core.files.move import file_move_safe
+from django.utils.crypto import get_random_string
 from django.utils.encoding import force_text, filepath_to_uri
 from django.utils.functional import LazyObject
 from django.utils.importlib import import_module
@@ -66,13 +66,12 @@ class Storage(object):
         """
         dir_name, file_name = os.path.split(name)
         file_root, file_ext = os.path.splitext(file_name)
-        # If the filename already exists, add an underscore and a number (before
-        # the file extension, if one exists) to the filename until the generated
-        # filename doesn't exist.
-        count = itertools.count(1)
+        # If the filename already exists, add an underscore and a random 7
+        # character alphanumeric string (before the file extension, if one
+        # exists) to the filename until the generated filename doesn't exist.
         while self.exists(name):
             # file_ext includes the dot.
-            name = os.path.join(dir_name, "%s_%s%s" % (file_root, next(count), file_ext))
+            name = os.path.join(dir_name, "%s_%s%s" % (file_root, get_random_string(7), file_ext))
 
         return name
 

docs/howto/custom-file-storage.txt

@@ -83,5 +83,14 @@ the provided filename into account. The ``name`` argument passed to this method
 will have already cleaned to a filename valid for the storage system, according
 to the ``get_valid_name()`` method described above.
 
-The code provided on ``Storage`` simply appends ``"_1"``, ``"_2"``, etc. to the
-filename until it finds one that's available in the destination directory.
+.. versionchanged:: 1.5.9
+
+    If a file with ``name`` already exists, an underscore plus a random 7
+    character alphanumeric string is appended to the filename before the
+    extension.
+
+    Previously, an underscore followed by a number (e.g. ``"_1"``, ``"_2"``,
+    etc.) was appended to the filename until an avaible name in the destination
+    directory was found. A malicious user could exploit this deterministic
+    algorithm to create a denial-of-service attack. This change was also made
+    in Django 1.4.14.

docs/ref/files/storage.txt

@@ -81,6 +81,17 @@ The Storage Class
         available for new content to be written to on the target storage
         system.
 
+        .. versionchanged:: 1.5.9
+
+        If a file with ``name`` already exists, an underscore plus a random 7
+        character alphanumeric string is appended to the filename before the
+        extension.
+
+        Previously, an underscore followed by a number (e.g. ``"_1"``, ``"_2"``,
+        etc.) was appended to the filename until an avaible name in the
+        destination directory was found. A malicious user could exploit this
+        deterministic algorithm to create a denial-of-service attack. This
+        change was also made in Django 1.4.14.
 
     .. method:: get_valid_name(name)
 

docs/releases/1.4.14.txt

@@ -18,3 +18,23 @@ To remedy this, URL reversing now ensures that no URL starts with two slashes
 (//), replacing the second slash with its URL encoded counterpart (%2F). This
 approach ensures that semantics stay the same, while making the URL relative to
 the domain and not to the scheme.
+
+File upload denial-of-service
+=============================
+
+Before this release, Django's file upload handing in its default configuration
+may degrade to producing a huge number of ``os.stat()`` system calls when a
+duplicate filename is uploaded. Since ``stat()`` may invoke IO, this may produce
+a huge data-dependent slowdown that slowly worsens over time. The net result is
+that given enough time, a user with the ability to upload files can cause poor
+performance in the upload handler, eventually causing it to become very slow
+simply by uploading 0-byte files. At this point, even a slow network connection
+and few HTTP requests would be all that is necessary to make a site unavailable.
+
+We've remedied the issue by changing the algorithm for generating file names
+if a file with the uploaded name already exists.
+:meth:`Storage.get_available_name()
+<django.core.files.storage.Storage.get_available_name>` now appends an
+underscore plus a random 7 character alphanumeric string (e.g. ``"_x3a1gho"``),
+rather than iterating through an underscore followed by a number (e.g. ``"_1"``,
+``"_2"``, etc.).

docs/releases/1.5.9.txt

@@ -18,3 +18,23 @@ To remedy this, URL reversing now ensures that no URL starts with two slashes
 (//), replacing the second slash with its URL encoded counterpart (%2F). This
 approach ensures that semantics stay the same, while making the URL relative to
 the domain and not to the scheme.
+
+File upload denial-of-service
+=============================
+
+Before this release, Django's file upload handing in its default configuration
+may degrade to producing a huge number of ``os.stat()`` system calls when a
+duplicate filename is uploaded. Since ``stat()`` may invoke IO, this may produce
+a huge data-dependent slowdown that slowly worsens over time. The net result is
+that given enough time, a user with the ability to upload files can cause poor
+performance in the upload handler, eventually causing it to become very slow
+simply by uploading 0-byte files. At this point, even a slow network connection
+and few HTTP requests would be all that is necessary to make a site unavailable.
+
+We've remedied the issue by changing the algorithm for generating file names
+if a file with the uploaded name already exists.
+:meth:`Storage.get_available_name()
+<django.core.files.storage.Storage.get_available_name>` now appends an
+underscore plus a random 7 character alphanumeric string (e.g. ``"_x3a1gho"``),
+rather than iterating through an underscore followed by a number (e.g. ``"_1"``,
+``"_2"``, etc.).

tests/modeltests/files/tests.py

@@ -9,11 +9,14 @@ from django.core.files import File
 from django.core.files.base import ContentFile
 from django.core.files.uploadedfile import SimpleUploadedFile
 from django.test import TestCase
-from django.utils import unittest
+from django.utils import six, unittest
 
 from .models import Storage, temp_storage, temp_storage_location
 
 
+FILE_SUFFIX_REGEX = '[A-Za-z0-9]{7}'
+
+
 class FileStorageTests(TestCase):
     def tearDown(self):
         shutil.rmtree(temp_storage_location)
@@ -59,27 +62,28 @@ class FileStorageTests(TestCase):
         # Save another file with the same name.
         obj2 = Storage()
         obj2.normal.save("django_test.txt", ContentFile("more content"))
-        self.assertEqual(obj2.normal.name, "tests/django_test_1.txt")
+        obj2_name = obj2.normal.name
+        six.assertRegex(self, obj2_name, "tests/django_test_%s.txt" % FILE_SUFFIX_REGEX)
         self.assertEqual(obj2.normal.size, 12)
 
         # Push the objects into the cache to make sure they pickle properly
         cache.set("obj1", obj1)
         cache.set("obj2", obj2)
-        self.assertEqual(cache.get("obj2").normal.name, "tests/django_test_1.txt")
+        six.assertRegex(self, cache.get("obj2").normal.name, "tests/django_test_%s.txt" % FILE_SUFFIX_REGEX)
 
         # Deleting an object does not delete the file it uses.
         obj2.delete()
         obj2.normal.save("django_test.txt", ContentFile("more content"))
-        self.assertEqual(obj2.normal.name, "tests/django_test_2.txt")
+        self.assertNotEqual(obj2_name, obj2.normal.name)
+        six.assertRegex(self, obj2.normal.name, "tests/django_test_%s.txt" % FILE_SUFFIX_REGEX)
 
         # Multiple files with the same name get _N appended to them.
-        objs = [Storage() for i in range(3)]
+        objs = [Storage() for i in range(2)]
         for o in objs:
             o.normal.save("multiple_files.txt", ContentFile("Same Content"))
-        self.assertEqual(
-            [o.normal.name for o in objs],
-            ["tests/multiple_files.txt", "tests/multiple_files_1.txt", "tests/multiple_files_2.txt"]
-        )
+        names = [o.normal.name for o in objs]
+        self.assertEqual(names[0], "tests/multiple_files.txt")
+        six.assertRegex(self, names[1], "tests/multiple_files_%s.txt" % FILE_SUFFIX_REGEX)
         for o in objs:
             o.delete()
 

tests/regressiontests/file_storage/tests.py

@@ -40,6 +40,9 @@ except ImportError:
         Image = None
 
 
+FILE_SUFFIX_REGEX = '[A-Za-z0-9]{7}'
+
+
 class GetStorageClassTests(SimpleTestCase):
 
     def test_get_filesystem_storage(self):
@@ -431,10 +434,9 @@ class FileSaveRaceConditionTest(unittest.TestCase):
         self.thread.start()
         name = self.save_file('conflict')
         self.thread.join()
-        self.assertTrue(self.storage.exists('conflict'))
-        self.assertTrue(self.storage.exists('conflict_1'))
-        self.storage.delete('conflict')
-        self.storage.delete('conflict_1')
+        files = sorted(os.listdir(self.storage_dir))
+        self.assertEqual(files[0], 'conflict')
+        six.assertRegex(self, files[1], 'conflict_%s' % FILE_SUFFIX_REGEX)
 
 @unittest.skipIf(sys.platform.startswith('win'), "Windows only partially supports umasks and chmod.")
 class FileStoragePermissions(unittest.TestCase):
@@ -478,9 +480,10 @@ class FileStoragePathParsing(unittest.TestCase):
         self.storage.save('dotted.path/test', ContentFile("1"))
         self.storage.save('dotted.path/test', ContentFile("2"))
 
+        files = sorted(os.listdir(os.path.join(self.storage_dir, 'dotted.path')))
         self.assertFalse(os.path.exists(os.path.join(self.storage_dir, 'dotted_.path')))
-        self.assertTrue(os.path.exists(os.path.join(self.storage_dir, 'dotted.path/test')))
-        self.assertTrue(os.path.exists(os.path.join(self.storage_dir, 'dotted.path/test_1')))
+        self.assertEqual(files[0], 'test')
+        six.assertRegex(self, files[1], 'test_%s' % FILE_SUFFIX_REGEX)
 
     def test_first_character_dot(self):
         """
@@ -490,8 +493,10 @@ class FileStoragePathParsing(unittest.TestCase):
         self.storage.save('dotted.path/.test', ContentFile("1"))
         self.storage.save('dotted.path/.test', ContentFile("2"))
 
-        self.assertTrue(os.path.exists(os.path.join(self.storage_dir, 'dotted.path/.test')))
-        self.assertTrue(os.path.exists(os.path.join(self.storage_dir, 'dotted.path/.test_1')))
+        files = sorted(os.listdir(os.path.join(self.storage_dir, 'dotted.path')))
+        self.assertFalse(os.path.exists(os.path.join(self.storage_dir, 'dotted_.path')))
+        self.assertEqual(files[0], '.test')
+        six.assertRegex(self, files[1], '.test_%s' % FILE_SUFFIX_REGEX)
 
 class DimensionClosingBug(unittest.TestCase):
     """