[4.2.x] Fixed CVE-2026-33034 -- Enforced DATA_UPLOAD_MAX_MEMORY_SIZE on body size in ASGI requests.

The `body` property in `HttpRequest` checks DATA_UPLOAD_MAX_MEMORY_SIZE
against the declared `Content-Length` header before reading. On the ASGI
path, chunked requests carry no `Content-Length`, so the check evaluated
to 0 and always passed regardless of the actual body size.

This work adds a new check on the actual number of bytes consumed.

Thanks to Superior for the report, and to Jake Howard and Jacob Walls
for reviews.

Backport of 953c238058c0ce387a1a41cb491bfc1875d73ad0 from main.

3 files changed, 204 insertions, 11 deletions

diff --git a/django/http/request.py b/django/http/request.py
index 4d1d077ec1..0b86444c4b 100644
--- a/django/http/request.py
+++ b/django/http/request.py
@@ -1,5 +1,6 @@
 import codecs
 import copy
+import os
 from io import BytesIO
 from itertools import chain
 from urllib.parse import parse_qsl, quote, urlencode, urljoin, urlsplit
@@ -328,25 +329,49 @@ class HttpRequest:
                     "You cannot access body after reading from request's data stream"
                 )
 
-            # Limit the maximum request data size that will be handled in-memory.
-            if (
-                settings.DATA_UPLOAD_MAX_MEMORY_SIZE is not None
-                and int(self.META.get("CONTENT_LENGTH") or 0)
-                > settings.DATA_UPLOAD_MAX_MEMORY_SIZE
-            ):
-                raise RequestDataTooBig(
-                    "Request body exceeded settings.DATA_UPLOAD_MAX_MEMORY_SIZE."
-                )
+            # Limit the maximum request data size that will be handled
+            # in-memory. Reject early when Content-Length is present and
+            # already exceeds the limit, avoiding reading the body at all.
+            self._check_data_too_big(int(self.META.get("CONTENT_LENGTH") or 0))
+
+            # Content-Length can be absent or understated (e.g.
+            # `Transfer-Encoding: chunked` on ASGI), so for seekable
+            # streams (e.g. SpooledTemporaryFile on ASGI), check the actual
+            # buffered size before reading it all into memory.
+            if hasattr(self._stream, "seekable") and self._stream.seekable():
+                stream_size = self._stream.seek(0, os.SEEK_END)
+                self._check_data_too_big(stream_size)
+                self._stream.seek(0)
+                did_check = True
+            else:
+                did_check = False
 
             try:
-                self._body = self.read()
+                if settings.DATA_UPLOAD_MAX_MEMORY_SIZE is not None and not did_check:
+                    # Read one byte past the limit to detect an oversize body
+                    # without loading it all into memory first.
+                    self._body = self.read(settings.DATA_UPLOAD_MAX_MEMORY_SIZE + 1)
+                else:
+                    self._body = self.read()
             except OSError as e:
                 raise UnreadablePostError(*e.args) from e
             finally:
                 self._stream.close()
             self._stream = BytesIO(self._body)
+            if not did_check:
+                stream_size = self._stream.seek(0, os.SEEK_END)
+                self._check_data_too_big(stream_size)
+                self._stream.seek(0)
         return self._body
 
+    def _check_data_too_big(self, length):
+        if (
+            settings.DATA_UPLOAD_MAX_MEMORY_SIZE is not None
+            and length > settings.DATA_UPLOAD_MAX_MEMORY_SIZE
+        ):
+            msg = "Request body exceeded settings.DATA_UPLOAD_MAX_MEMORY_SIZE."
+            raise RequestDataTooBig(msg)
+
     def _mark_post_parse_error(self):
         self._post = QueryDict()
         self._files = MultiValueDict()
diff --git a/docs/releases/4.2.30.txt b/docs/releases/4.2.30.txt
index c5058d9b84..8382907068 100644
--- a/docs/releases/4.2.30.txt
+++ b/docs/releases/4.2.30.txt
@@ -56,3 +56,14 @@ may trigger repeated memory copying, potentially degrading performance.
 
 This issue has severity "moderate" according to the :ref:`Django security
 policy <security-disclosure>`.
+
+CVE-2026-33034: Potential denial-of-service vulnerability in ASGI requests via memory upload limit bypass
+=========================================================================================================
+
+ASGI requests with a missing or understated ``Content-Length`` header could
+bypass the :setting:`DATA_UPLOAD_MAX_MEMORY_SIZE` limit when reading
+``HttpRequest.body``, potentially loading an unbounded request body into
+memory and causing service degradation.
+
+This issue has severity "low" according to the :ref:`Django security policy
+<security-disclosure>`.
diff --git a/tests/asgi/tests.py b/tests/asgi/tests.py
index bf623c2083..08715026a9 100644
--- a/tests/asgi/tests.py
+++ b/tests/asgi/tests.py
@@ -1,12 +1,14 @@
 import asyncio
 import sys
 import threading
+from io import BytesIO
 from pathlib import Path
 
 from asgiref.testing import ApplicationCommunicator
 
 from django.contrib.staticfiles.handlers import ASGIStaticFilesHandler
-from django.core.asgi import get_asgi_application
+from django.core.asgi import ASGIHandler, get_asgi_application
+from django.core.exceptions import RequestDataTooBig
 from django.core.handlers.asgi import ASGIRequest
 from django.core.signals import request_finished, request_started
 from django.db import close_old_connections
@@ -22,6 +24,7 @@ from django.utils.http import http_date
 from .urls import sync_waiter, test_filename
 
 TEST_STATIC_ROOT = Path(__file__).parent / "project" / "static"
+TOO_MUCH_DATA_MSG = "Request body exceeded settings.DATA_UPLOAD_MAX_MEMORY_SIZE."
 
 
 @override_settings(ROOT_URLCONF="asgi.urls")
@@ -356,3 +359,157 @@ class ASGITest(SimpleTestCase):
         self.assertEqual(len(sync_waiter.active_threads), 2)
 
         sync_waiter.active_threads.clear()
+
+
+class DataUploadMaxMemorySizeASGITests(SimpleTestCase):
+    def make_request(
+        self,
+        body,
+        content_type=b"application/octet-stream",
+        content_length=None,
+        stream=None,
+    ):
+        scope = AsyncRequestFactory()._base_scope(method="POST", path="/")
+        scope["headers"] = [(b"content-type", content_type)]
+        if content_length is not None:
+            scope["headers"].append((b"content-length", str(content_length).encode()))
+        return ASGIRequest(scope, stream if stream is not None else BytesIO(body))
+
+    def test_body_size_not_exceeded_without_content_length(self):
+        body = b"x" * 5
+        with self.settings(DATA_UPLOAD_MAX_MEMORY_SIZE=5):
+            self.assertEqual(self.make_request(body).body, body)
+
+    def test_body_size_exceeded_without_content_length(self):
+        request = self.make_request(b"x" * 10)
+        with self.assertRaisesMessage(RequestDataTooBig, TOO_MUCH_DATA_MSG):
+            with self.settings(DATA_UPLOAD_MAX_MEMORY_SIZE=5):
+                request.body
+
+    def test_body_size_check_fires_before_read(self):
+        # The seekable size check rejects oversized bodies before reading
+        # them into memory (i.e. before calling self.read()).
+        class TrackingBytesIO(BytesIO):
+            calls = []
+
+            def read(self, *args, **kwargs):
+                self.calls.append((args, kwargs))
+                return super().read(*args, **kwargs)
+
+        stream = TrackingBytesIO(b"x" * 10)
+        request = self.make_request(b"x" * 10, stream=stream)
+        with self.assertRaisesMessage(RequestDataTooBig, TOO_MUCH_DATA_MSG):
+            with self.settings(DATA_UPLOAD_MAX_MEMORY_SIZE=5):
+                request.body
+
+        self.assertEqual(stream.calls, [])
+
+    def test_post_size_exceeded_without_content_length(self):
+        request = self.make_request(
+            b"a=" + b"x" * 10,
+            content_type=b"application/x-www-form-urlencoded",
+        )
+        with self.assertRaisesMessage(RequestDataTooBig, TOO_MUCH_DATA_MSG):
+            with self.settings(DATA_UPLOAD_MAX_MEMORY_SIZE=5):
+                request.POST
+
+    def test_no_limit(self):
+        body = b"x" * 100
+        with self.settings(DATA_UPLOAD_MAX_MEMORY_SIZE=None):
+            self.assertEqual(self.make_request(body).body, body)
+
+    async def test_read_body_no_limit(self):
+        chunks = [
+            {"type": "http.request", "body": b"x" * 100, "more_body": True},
+            {"type": "http.request", "body": b"x" * 100, "more_body": False},
+        ]
+
+        async def receive():
+            return chunks.pop(0)
+
+        handler = ASGIHandler()
+        with self.settings(DATA_UPLOAD_MAX_MEMORY_SIZE=None):
+            body_file = await handler.read_body(receive)
+            self.addCleanup(body_file.close)
+
+        body_file.seek(0)
+        self.assertEqual(body_file.read(), b"x" * 200)
+
+    def test_non_multipart_body_size_enforced(self):
+        # DATA_UPLOAD_MAX_MEMORY_SIZE is enforced on non-multipart bodies.
+        request = self.make_request(b"x" * 100)
+        with self.assertRaisesMessage(RequestDataTooBig, TOO_MUCH_DATA_MSG):
+            with self.settings(DATA_UPLOAD_MAX_MEMORY_SIZE=10):
+                request.body
+
+    def test_multipart_file_upload_not_limited_by_data_upload_max(self):
+        # DATA_UPLOAD_MAX_MEMORY_SIZE applies to non-file fields only; a file
+        # upload whose total body exceeds the limit must still succeed.
+        boundary = "testboundary"
+        file_content = b"x" * 100
+        body = (
+            (
+                f"--{boundary}\r\n"
+                f'Content-Disposition: form-data; name="file"; filename="test.txt"\r\n'
+                f"Content-Type: application/octet-stream\r\n"
+                f"\r\n"
+            ).encode()
+            + file_content
+            + f"\r\n--{boundary}--\r\n".encode()
+        )
+        request = self.make_request(
+            body,
+            content_type=f"multipart/form-data; boundary={boundary}".encode(),
+            content_length=len(body),
+        )
+        with self.settings(
+            DATA_UPLOAD_MAX_MEMORY_SIZE=10, FILE_UPLOAD_MAX_MEMORY_SIZE=10
+        ):
+            files = request.FILES
+        self.assertEqual(len(files), 1)
+        uploaded = files["file"]
+        self.addCleanup(uploaded.close)
+        self.assertEqual(uploaded.read(), file_content)
+
+    async def test_read_body_buffers_all_chunks(self):
+        # read_body() consumes all chunks regardless of
+        # DATA_UPLOAD_MAX_MEMORY_SIZE; the limit is enforced later when
+        # HttpRequest.body is accessed.
+        chunks = [
+            {"type": "http.request", "body": b"x" * 10, "more_body": True},
+            {"type": "http.request", "body": b"y" * 10, "more_body": True},
+            {"type": "http.request", "body": b"z" * 10, "more_body": False},
+        ]
+
+        async def receive():
+            return chunks.pop(0)
+
+        handler = ASGIHandler()
+        with self.settings(DATA_UPLOAD_MAX_MEMORY_SIZE=15):
+            body_file = await handler.read_body(receive)
+            self.addCleanup(body_file.close)
+
+        self.assertEqual(len(chunks), 0)  # All chunks were consumed.
+        body_file.seek(0)
+        self.assertEqual(body_file.read(), b"x" * 10 + b"y" * 10 + b"z" * 10)
+
+    async def test_read_body_multipart_not_limited(self):
+        # All chunks are consumed regardless of DATA_UPLOAD_MAX_MEMORY_SIZE;
+        # multipart size enforcement happens inside MultiPartParser, not here.
+        chunks = [
+            {"type": "http.request", "body": b"x" * 10, "more_body": True},
+            {"type": "http.request", "body": b"y" * 10, "more_body": True},
+            {"type": "http.request", "body": b"z" * 10, "more_body": False},
+        ]
+
+        async def receive():
+            return chunks.pop(0)
+
+        handler = ASGIHandler()
+        with self.settings(DATA_UPLOAD_MAX_MEMORY_SIZE=15):
+            body_file = await handler.read_body(receive)
+            self.addCleanup(body_file.close)
+
+        self.assertEqual(len(chunks), 0)  # All chunks were consumed.
+        body_file.seek(0)
+        self.assertEqual(body_file.read(), b"x" * 10 + b"y" * 10 + b"z" * 10)