[4.2.x] Fixed CVE-2026-33033 -- Mitigated potential DoS in MultiPartParser.

When a multipart file part used `Content-Transfer-Encoding: base64` and
the non-whitespace base64 bytes did not align to a multiple of 4 within
a chunk, the parser entered a loop calling `field_stream.read(1-3)` once
per whitespace byte. Each such call fetched the entire internal buffer,
sliced off 1-3 bytes, and pushed the remainder back via unget(), doing
an O(n) memory copy per call. A 2.5 MB payload of mostly whitespace
produced CPU amplification relative to a normal upload of the same size.

The alignment loop now reads `self._chunk_size` bytes at a time, and
accumulates stripped parts in a list joined once at the end.

Thanks to Seokchan Yoon for the report and the fixing patch.

Backport of 7e9885f99cee771b51692fadc5592bdbf19641aa from main.

1 files changed, 10 insertions, 0 deletions

diff --git a/docs/releases/4.2.30.txt b/docs/releases/4.2.30.txt
index de19a6f08f..c5058d9b84 100644
--- a/docs/releases/4.2.30.txt
+++ b/docs/releases/4.2.30.txt
@@ -46,3 +46,13 @@ instances to be created via forged ``POST`` data.
 
 This issue has severity "low" according to the :ref:`Django security policy
 <security-disclosure>`.
+
+CVE-2026-33033: Potential denial-of-service vulnerability in ``MultiPartParser`` via base64-encoded file upload
+===============================================================================================================
+
+When using ``django.http.multipartparser.MultiPartParser``, multipart uploads
+with ``Content-Transfer-Encoding: base64`` that include excessive whitespace
+may trigger repeated memory copying, potentially degrading performance.
+
+This issue has severity "moderate" according to the :ref:`Django security
+policy <security-disclosure>`.