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

This is a security fix. Disclosure following shortly.

django/core/files/storage.py

 import os
 import errno
-import itertools
 from datetime import datetime
 
 from django.conf import settings
 from django.core.exceptions import SuspiciousFileOperation
 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.module_loading import import_string
@@ -69,13 +69,12 @@ 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

@@ -90,5 +90,14 @@ 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:: 1.7
+
+    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 1.6.6, 1.5.9, and 1.4.14.

docs/ref/files/storage.txt

@@ -112,6 +112,18 @@ The Storage Class
         available for new content to be written to on the target storage
         system.
 
+        .. versionchanged:: 1.7
+
+        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 1.6.6, 1.5.9, and 1.4.14.
+
     .. method:: get_valid_name(name)
 
         Returns a filename based on the ``name`` parameter that's suitable

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.).

docs/releases/1.6.6.txt

@@ -19,6 +19,26 @@ To remedy this, URL reversing now ensures that no URL starts with two slashes
 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.).
+
 Bugfixes
 ========
 

docs/releases/1.7.txt

@@ -588,6 +588,13 @@ File Uploads
   the client. Partially uploaded files are also closed as long as they are
   named ``file`` in the upload handler.
 
+* :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.) to prevent a denial-of-service attack.
+  This change was also made in the 1.6.6, 1.5.9, and 1.4.14 security releases.
+
 Forms
 ^^^^^
 

tests/file_storage/tests.py

@@ -30,6 +30,9 @@ from django.utils._os import upath
 from .models import Storage, temp_storage, temp_storage_location
 
 
+FILE_SUFFIX_REGEX = '[A-Za-z0-9]{7}'
+
+
 class GetStorageClassTests(SimpleTestCase):
 
     def test_get_filesystem_storage(self):
@@ -465,14 +468,16 @@ class FileFieldStorageTests(unittest.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)
         obj2.normal.close()
 
         # 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)
         obj2.normal.close()
 
     def test_filefield_read(self):
@@ -485,17 +490,18 @@ class FileFieldStorageTests(unittest.TestCase):
         self.assertEqual(list(obj.normal.chunks(chunk_size=2)), [b"co", b"nt", b"en", b"t"])
         obj.normal.close()
 
-    def test_file_numbering(self):
-        # Multiple files with the same name get _N appended to them.
-        objs = [Storage() for i in range(3)]
+    def test_duplicate_filename(self):
+        # Multiple files with the same name get _(7 random chars) appended to them.
+        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"]
-        )
-        for o in objs:
-            o.delete()
+        try:
+            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)
+        finally:
+            for o in objs:
+                o.delete()
 
     def test_filefield_default(self):
         # Default values allow an object to access a single file.
@@ -587,10 +593,9 @@ class FileSaveRaceConditionTest(unittest.TestCase):
         self.thread.start()
         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.")
@@ -651,9 +656,10 @@ 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):
         """
@@ -663,8 +669,10 @@ 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 ContentFileStorageTestCase(unittest.TestCase):