Fixed CVE-2023-31047, Fixed #31710 -- Prevented potential bypass of validation when uploading multiple files using one form field.

Thanks Moataz Al-Sharida and nawaik for reports.

Co-authored-by: Shai Berger <shai@platonix.com>
Co-authored-by: nessita <124304+nessita@users.noreply.github.com>

django/forms/widgets.py

@@ -406,17 +406,41 @@ class MultipleHiddenInput(HiddenInput):
 
 
 class FileInput(Input):
+    allow_multiple_selected = False
     input_type = "file"
     needs_multipart_form = True
     template_name = "django/forms/widgets/file.html"
 
+    def __init__(self, attrs=None):
+        if (
+            attrs is not None
+            and not self.allow_multiple_selected
+            and attrs.get("multiple", False)
+        ):
+            raise ValueError(
+                "%s doesn't support uploading multiple files."
+                % self.__class__.__qualname__
+            )
+        if self.allow_multiple_selected:
+            if attrs is None:
+                attrs = {"multiple": True}
+            else:
+                attrs.setdefault("multiple", True)
+        super().__init__(attrs)
+
     def format_value(self, value):
         """File input never renders a value."""
         return
 
     def value_from_datadict(self, data, files, name):
         "File widgets take data from FILES, not POST"
-        return files.get(name)
+        getter = files.get
+        if self.allow_multiple_selected:
+            try:
+                getter = files.getlist
+            except AttributeError:
+                pass
+        return getter(name)
 
     def value_omitted_from_data(self, data, files, name):
         return name not in files

docs/releases/3.2.19.txt

@@ -6,4 +6,18 @@ Django 3.2.19 release notes
 
 Django 3.2.19 fixes a security issue with severity "low" in 3.2.18.
 
-...
+CVE-2023-31047: Potential bypass of validation when uploading multiple files using one form field
+=================================================================================================
+
+Uploading multiple files using one form field has never been supported by
+:class:`.forms.FileField` or :class:`.forms.ImageField` as only the last
+uploaded file was validated. Unfortunately, :ref:`uploading_multiple_files`
+topic suggested otherwise.
+
+In order to avoid the vulnerability, :class:`~django.forms.ClearableFileInput`
+and :class:`~django.forms.FileInput` form widgets now raise ``ValueError`` when
+the ``multiple`` HTML attribute is set on them. To prevent the exception and
+keep the old behavior, set ``allow_multiple_selected`` to ``True``.
+
+For more details on using the new attribute and handling of multiple files
+through a single field, see :ref:`uploading_multiple_files`.

docs/releases/4.1.9.txt

@@ -6,4 +6,18 @@ Django 4.1.9 release notes
 
 Django 4.1.9 fixes a security issue with severity "low" in 4.1.8.
 
-...
+CVE-2023-31047: Potential bypass of validation when uploading multiple files using one form field
+=================================================================================================
+
+Uploading multiple files using one form field has never been supported by
+:class:`.forms.FileField` or :class:`.forms.ImageField` as only the last
+uploaded file was validated. Unfortunately, :ref:`uploading_multiple_files`
+topic suggested otherwise.
+
+In order to avoid the vulnerability, :class:`~django.forms.ClearableFileInput`
+and :class:`~django.forms.FileInput` form widgets now raise ``ValueError`` when
+the ``multiple`` HTML attribute is set on them. To prevent the exception and
+keep the old behavior, set ``allow_multiple_selected`` to ``True``.
+
+For more details on using the new attribute and handling of multiple files
+through a single field, see :ref:`uploading_multiple_files`.

docs/releases/4.2.1.txt

@@ -7,6 +7,22 @@ Django 4.2.1 release notes
 Django 4.2.1 fixes a security issue with severity "low" and several bugs in
 4.2.
 
+CVE-2023-31047: Potential bypass of validation when uploading multiple files using one form field
+=================================================================================================
+
+Uploading multiple files using one form field has never been supported by
+:class:`.forms.FileField` or :class:`.forms.ImageField` as only the last
+uploaded file was validated. Unfortunately, :ref:`uploading_multiple_files`
+topic suggested otherwise.
+
+In order to avoid the vulnerability, :class:`~django.forms.ClearableFileInput`
+and :class:`~django.forms.FileInput` form widgets now raise ``ValueError`` when
+the ``multiple`` HTML attribute is set on them. To prevent the exception and
+keep the old behavior, set ``allow_multiple_selected`` to ``True``.
+
+For more details on using the new attribute and handling of multiple files
+through a single field, see :ref:`uploading_multiple_files`.
+
 Bugfixes
 ========
 

docs/topics/http/file-uploads.txt

@@ -144,11 +144,27 @@ a :class:`~django.core.files.File` like object to the
             instance = ModelWithFileField(file_field=content_file)
             instance.save()
 
+.. _uploading_multiple_files:
+
 Uploading multiple files
 ------------------------
 
-If you want to upload multiple files using one form field, set the ``multiple``
-HTML attribute of field's widget:
+..
+    Tests in tests.forms_tests.field_tests.test_filefield.MultipleFileFieldTest
+    should be updated after any changes in the following snippets.
+
+If you want to upload multiple files using one form field, create a subclass
+of the field's widget and set the ``allow_multiple_selected`` attribute on it
+to ``True``.
+
+In order for such files to be all validated by your form (and have the value of
+the field include them all), you will also have to subclass ``FileField``. See
+below for an example.
+
+.. admonition:: Multiple file field
+
+    Django is likely to have a proper multiple file field support at some point
+    in the future.
 
 .. code-block:: python
     :caption: ``forms.py``
@@ -156,10 +172,26 @@ HTML attribute of field's widget:
     from django import forms
 
 
+    class MultipleFileInput(forms.ClearableFileInput):
+        allow_multiple_selected = True
+
+
+    class MultipleFileField(forms.FileField):
+        def __init__(self, *args, **kwargs):
+            kwargs.setdefault("widget", MultipleFileInput())
+            super().__init__(*args, **kwargs)
+
+        def clean(self, data, initial=None):
+            single_file_clean = super().clean
+            if isinstance(data, (list, tuple)):
+                result = [single_file_clean(d, initial) for d in data]
+            else:
+                result = single_file_clean(data, initial)
+            return result
+
+
     class FileFieldForm(forms.Form):
-        file_field = forms.FileField(
-            widget=forms.ClearableFileInput(attrs={"multiple": True})
-        )
+        file_field = MultipleFileField()
 
 Then override the ``post`` method of your
 :class:`~django.views.generic.edit.FormView` subclass to handle multiple file
@@ -180,14 +212,32 @@ uploads:
         def post(self, request, *args, **kwargs):
             form_class = self.get_form_class()
             form = self.get_form(form_class)
-            files = request.FILES.getlist("file_field")
             if form.is_valid():
-                for f in files:
-                    ...  # Do something with each file.
                 return self.form_valid(form)
             else:
                 return self.form_invalid(form)
 
+        def form_valid(self, form):
+            files = form.cleaned_data["file_field"]
+            for f in files:
+                ...  # Do something with each file.
+            return super().form_valid()
+
+.. warning::
+
+   This will allow you to handle multiple files at the form level only. Be
+   aware that you cannot use it to put multiple files on a single model
+   instance (in a single field), for example, even if the custom widget is used
+   with a form field related to a model ``FileField``.
+
+.. versionchanged:: 3.2.19
+
+   In previous versions, there was no support for the ``allow_multiple_selected``
+   class attribute, and users were advised to create the widget with the HTML
+   attribute ``multiple`` set through the ``attrs`` argument. However, this
+   caused validation of the form field to be applied only to the last file
+   submitted, which could have adverse security implications.
+
 Upload Handlers
 ===============
 

tests/forms_tests/field_tests/test_filefield.py

@@ -2,7 +2,8 @@ import pickle
 
 from django.core.exceptions import ValidationError
 from django.core.files.uploadedfile import SimpleUploadedFile
-from django.forms import FileField
+from django.core.validators import validate_image_file_extension
+from django.forms import FileField, FileInput
 from django.test import SimpleTestCase
 
 
@@ -109,3 +110,68 @@ class FileFieldTest(SimpleTestCase):
 
     def test_file_picklable(self):
         self.assertIsInstance(pickle.loads(pickle.dumps(FileField())), FileField)
+
+
+class MultipleFileInput(FileInput):
+    allow_multiple_selected = True
+
+
+class MultipleFileField(FileField):
+    def __init__(self, *args, **kwargs):
+        kwargs.setdefault("widget", MultipleFileInput())
+        super().__init__(*args, **kwargs)
+
+    def clean(self, data, initial=None):
+        single_file_clean = super().clean
+        if isinstance(data, (list, tuple)):
+            result = [single_file_clean(d, initial) for d in data]
+        else:
+            result = single_file_clean(data, initial)
+        return result
+
+
+class MultipleFileFieldTest(SimpleTestCase):
+    def test_file_multiple(self):
+        f = MultipleFileField()
+        files = [
+            SimpleUploadedFile("name1", b"Content 1"),
+            SimpleUploadedFile("name2", b"Content 2"),
+        ]
+        self.assertEqual(f.clean(files), files)
+
+    def test_file_multiple_empty(self):
+        f = MultipleFileField()
+        files = [
+            SimpleUploadedFile("empty", b""),
+            SimpleUploadedFile("nonempty", b"Some Content"),
+        ]
+        msg = "'The submitted file is empty.'"
+        with self.assertRaisesMessage(ValidationError, msg):
+            f.clean(files)
+        with self.assertRaisesMessage(ValidationError, msg):
+            f.clean(files[::-1])
+
+    def test_file_multiple_validation(self):
+        f = MultipleFileField(validators=[validate_image_file_extension])
+
+        good_files = [
+            SimpleUploadedFile("image1.jpg", b"fake JPEG"),
+            SimpleUploadedFile("image2.png", b"faux image"),
+            SimpleUploadedFile("image3.bmp", b"fraudulent bitmap"),
+        ]
+        self.assertEqual(f.clean(good_files), good_files)
+
+        evil_files = [
+            SimpleUploadedFile("image1.sh", b"#!/bin/bash -c 'echo pwned!'\n"),
+            SimpleUploadedFile("image2.png", b"faux image"),
+            SimpleUploadedFile("image3.jpg", b"fake JPEG"),
+        ]
+
+        evil_rotations = (
+            evil_files[i:] + evil_files[:i]  # Rotate by i.
+            for i in range(len(evil_files))
+        )
+        msg = "File extension “sh” is not allowed. Allowed extensions are: "
+        for rotated_evil_files in evil_rotations:
+            with self.assertRaisesMessage(ValidationError, msg):
+                f.clean(rotated_evil_files)

tests/forms_tests/widget_tests/test_clearablefileinput.py

@@ -245,3 +245,8 @@ class ClearableFileInputTest(WidgetTest):
             '<input type="file" name="clearable_file" id="id_clearable_file"></div>',
             form.render(),
         )
+
+    def test_multiple_error(self):
+        msg = "ClearableFileInput doesn't support uploading multiple files."
+        with self.assertRaisesMessage(ValueError, msg):
+            ClearableFileInput(attrs={"multiple": True})

tests/forms_tests/widget_tests/test_fileinput.py

@@ -1,4 +1,6 @@
+from django.core.files.uploadedfile import SimpleUploadedFile
 from django.forms import FileField, FileInput, Form
+from django.utils.datastructures import MultiValueDict
 
 from .base import WidgetTest
 
@@ -48,3 +50,45 @@ class FileInputTest(WidgetTest):
             'name="field" required type="file"></div>',
             form.render(),
         )
+
+    def test_multiple_error(self):
+        msg = "FileInput doesn't support uploading multiple files."
+        with self.assertRaisesMessage(ValueError, msg):
+            FileInput(attrs={"multiple": True})
+
+    def test_value_from_datadict_multiple(self):
+        class MultipleFileInput(FileInput):
+            allow_multiple_selected = True
+
+        file_1 = SimpleUploadedFile("something1.txt", b"content 1")
+        file_2 = SimpleUploadedFile("something2.txt", b"content 2")
+        # Uploading multiple files is allowed.
+        widget = MultipleFileInput(attrs={"multiple": True})
+        value = widget.value_from_datadict(
+            data={"name": "Test name"},
+            files=MultiValueDict({"myfile": [file_1, file_2]}),
+            name="myfile",
+        )
+        self.assertEqual(value, [file_1, file_2])
+        # Uploading multiple files is not allowed.
+        widget = FileInput()
+        value = widget.value_from_datadict(
+            data={"name": "Test name"},
+            files=MultiValueDict({"myfile": [file_1, file_2]}),
+            name="myfile",
+        )
+        self.assertEqual(value, file_2)
+
+    def test_multiple_default(self):
+        class MultipleFileInput(FileInput):
+            allow_multiple_selected = True
+
+        tests = [
+            (None, True),
+            ({"class": "myclass"}, True),
+            ({"multiple": False}, False),
+        ]
+        for attrs, expected in tests:
+            with self.subTest(attrs=attrs):
+                widget = MultipleFileInput(attrs=attrs)
+                self.assertIs(widget.attrs["multiple"], expected)