[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, 9 insertions, 6 deletions

diff --git a/django/http/multipartparser.py b/django/http/multipartparser.py
index 5ab63455ef..856ab68059 100644
--- a/django/http/multipartparser.py
+++ b/django/http/multipartparser.py
@@ -302,15 +302,18 @@ class MultiPartParser:
                                 # We should always decode base64 chunks by
                                 # multiple of 4, ignoring whitespace.
 
-                                stripped_chunk = b"".join(chunk.split())
+                                stripped_parts = [b"".join(chunk.split())]
+                                stripped_length = len(stripped_parts[0])
 
-                                remaining = len(stripped_chunk) % 4
-                                while remaining != 0:
-                                    over_chunk = field_stream.read(4 - remaining)
+                                while stripped_length % 4 != 0:
+                                    over_chunk = field_stream.read(self._chunk_size)
                                     if not over_chunk:
                                         break
-                                    stripped_chunk += b"".join(over_chunk.split())
-                                    remaining = len(stripped_chunk) % 4
+                                    over_stripped = b"".join(over_chunk.split())
+                                    stripped_parts.append(over_stripped)
+                                    stripped_length += len(over_stripped)
+
+                                stripped_chunk = b"".join(stripped_parts)
 
                                 try:
                                     chunk = base64.b64decode(stripped_chunk)