456 lines
18 KiB
Python
456 lines
18 KiB
Python
"""
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Cross Site Request Forgery Middleware.
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This module provides a middleware that implements protection
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against request forgeries from other sites.
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"""
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import logging
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import string
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from collections import defaultdict
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from urllib.parse import urlparse
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from django.conf import settings
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from django.core.exceptions import DisallowedHost, ImproperlyConfigured
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from django.http import UnreadablePostError
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from django.http.request import HttpHeaders
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from django.urls import get_callable
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from django.utils.cache import patch_vary_headers
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from django.utils.crypto import constant_time_compare, get_random_string
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from django.utils.deprecation import MiddlewareMixin
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from django.utils.functional import cached_property
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from django.utils.http import is_same_domain
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from django.utils.log import log_response
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from django.utils.regex_helper import _lazy_re_compile
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logger = logging.getLogger('django.security.csrf')
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# This matches if any character is not in CSRF_ALLOWED_CHARS.
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invalid_token_chars_re = _lazy_re_compile('[^a-zA-Z0-9]')
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REASON_BAD_ORIGIN = "Origin checking failed - %s does not match any trusted origins."
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REASON_NO_REFERER = "Referer checking failed - no Referer."
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REASON_BAD_REFERER = "Referer checking failed - %s does not match any trusted origins."
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REASON_NO_CSRF_COOKIE = "CSRF cookie not set."
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REASON_CSRF_TOKEN_MISSING = 'CSRF token missing.'
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REASON_MALFORMED_REFERER = "Referer checking failed - Referer is malformed."
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REASON_INSECURE_REFERER = "Referer checking failed - Referer is insecure while host is secure."
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# The reason strings below are for passing to InvalidTokenFormat. They are
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# phrases without a subject because they can be in reference to either the CSRF
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# cookie or non-cookie token.
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REASON_INCORRECT_LENGTH = 'has incorrect length'
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REASON_INVALID_CHARACTERS = 'has invalid characters'
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CSRF_SECRET_LENGTH = 32
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CSRF_TOKEN_LENGTH = 2 * CSRF_SECRET_LENGTH
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CSRF_ALLOWED_CHARS = string.ascii_letters + string.digits
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CSRF_SESSION_KEY = '_csrftoken'
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def _get_failure_view():
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"""Return the view to be used for CSRF rejections."""
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return get_callable(settings.CSRF_FAILURE_VIEW)
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def _get_new_csrf_string():
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return get_random_string(CSRF_SECRET_LENGTH, allowed_chars=CSRF_ALLOWED_CHARS)
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def _mask_cipher_secret(secret):
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"""
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Given a secret (assumed to be a string of CSRF_ALLOWED_CHARS), generate a
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token by adding a mask and applying it to the secret.
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"""
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mask = _get_new_csrf_string()
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chars = CSRF_ALLOWED_CHARS
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pairs = zip((chars.index(x) for x in secret), (chars.index(x) for x in mask))
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cipher = ''.join(chars[(x + y) % len(chars)] for x, y in pairs)
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return mask + cipher
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def _unmask_cipher_token(token):
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"""
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Given a token (assumed to be a string of CSRF_ALLOWED_CHARS, of length
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CSRF_TOKEN_LENGTH, and that its first half is a mask), use it to decrypt
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the second half to produce the original secret.
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"""
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mask = token[:CSRF_SECRET_LENGTH]
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token = token[CSRF_SECRET_LENGTH:]
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chars = CSRF_ALLOWED_CHARS
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pairs = zip((chars.index(x) for x in token), (chars.index(x) for x in mask))
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return ''.join(chars[x - y] for x, y in pairs) # Note negative values are ok
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def _add_new_csrf_cookie(request):
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"""Generate a new random CSRF_COOKIE value, and add it to request.META."""
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csrf_secret = _get_new_csrf_string()
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request.META.update({
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'CSRF_COOKIE': _mask_cipher_secret(csrf_secret),
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'CSRF_COOKIE_NEEDS_UPDATE': True,
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})
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return csrf_secret
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def get_token(request):
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"""
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Return the CSRF token required for a POST form. The token is an
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alphanumeric value. A new token is created if one is not already set.
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A side effect of calling this function is to make the csrf_protect
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decorator and the CsrfViewMiddleware add a CSRF cookie and a 'Vary: Cookie'
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header to the outgoing response. For this reason, you may need to use this
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function lazily, as is done by the csrf context processor.
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"""
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if 'CSRF_COOKIE' in request.META:
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csrf_secret = _unmask_cipher_token(request.META["CSRF_COOKIE"])
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# Since the cookie is being used, flag to send the cookie in
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# process_response() (even if the client already has it) in order to
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# renew the expiry timer.
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request.META['CSRF_COOKIE_NEEDS_UPDATE'] = True
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else:
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csrf_secret = _add_new_csrf_cookie(request)
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return _mask_cipher_secret(csrf_secret)
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def rotate_token(request):
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"""
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Change the CSRF token in use for a request - should be done on login
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for security purposes.
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"""
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_add_new_csrf_cookie(request)
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class InvalidTokenFormat(Exception):
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def __init__(self, reason):
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self.reason = reason
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def _sanitize_token(token):
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if len(token) not in (CSRF_TOKEN_LENGTH, CSRF_SECRET_LENGTH):
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raise InvalidTokenFormat(REASON_INCORRECT_LENGTH)
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# Make sure all characters are in CSRF_ALLOWED_CHARS.
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if invalid_token_chars_re.search(token):
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raise InvalidTokenFormat(REASON_INVALID_CHARACTERS)
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if len(token) == CSRF_SECRET_LENGTH:
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# Older Django versions set cookies to values of CSRF_SECRET_LENGTH
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# alphanumeric characters. For backwards compatibility, accept
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# such values as unmasked secrets.
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# It's easier to mask here and be consistent later, rather than add
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# different code paths in the checks, although that might be a tad more
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# efficient.
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return _mask_cipher_secret(token)
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return token
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def _does_token_match(request_csrf_token, csrf_token):
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# Assume both arguments are sanitized -- that is, strings of
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# length CSRF_TOKEN_LENGTH, all CSRF_ALLOWED_CHARS.
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return constant_time_compare(
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_unmask_cipher_token(request_csrf_token),
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_unmask_cipher_token(csrf_token),
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)
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class RejectRequest(Exception):
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def __init__(self, reason):
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self.reason = reason
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class CsrfViewMiddleware(MiddlewareMixin):
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"""
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Require a present and correct csrfmiddlewaretoken for POST requests that
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have a CSRF cookie, and set an outgoing CSRF cookie.
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This middleware should be used in conjunction with the {% csrf_token %}
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template tag.
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"""
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@cached_property
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def csrf_trusted_origins_hosts(self):
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return [
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urlparse(origin).netloc.lstrip('*')
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for origin in settings.CSRF_TRUSTED_ORIGINS
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]
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@cached_property
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def allowed_origins_exact(self):
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return {
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origin for origin in settings.CSRF_TRUSTED_ORIGINS
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if '*' not in origin
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}
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@cached_property
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def allowed_origin_subdomains(self):
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"""
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A mapping of allowed schemes to list of allowed netlocs, where all
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subdomains of the netloc are allowed.
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"""
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allowed_origin_subdomains = defaultdict(list)
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for parsed in (urlparse(origin) for origin in settings.CSRF_TRUSTED_ORIGINS if '*' in origin):
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allowed_origin_subdomains[parsed.scheme].append(parsed.netloc.lstrip('*'))
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return allowed_origin_subdomains
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# The _accept and _reject methods currently only exist for the sake of the
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# requires_csrf_token decorator.
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def _accept(self, request):
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# Avoid checking the request twice by adding a custom attribute to
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# request. This will be relevant when both decorator and middleware
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# are used.
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request.csrf_processing_done = True
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return None
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def _reject(self, request, reason):
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response = _get_failure_view()(request, reason=reason)
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log_response(
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'Forbidden (%s): %s', reason, request.path,
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response=response,
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request=request,
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logger=logger,
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)
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return response
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def _get_token(self, request):
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if settings.CSRF_USE_SESSIONS:
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try:
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return request.session.get(CSRF_SESSION_KEY)
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except AttributeError:
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raise ImproperlyConfigured(
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'CSRF_USE_SESSIONS is enabled, but request.session is not '
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'set. SessionMiddleware must appear before CsrfViewMiddleware '
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'in MIDDLEWARE.'
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)
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else:
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try:
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cookie_token = request.COOKIES[settings.CSRF_COOKIE_NAME]
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except KeyError:
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return None
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# This can raise InvalidTokenFormat.
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csrf_token = _sanitize_token(cookie_token)
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if csrf_token != cookie_token:
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# Then the cookie token had length CSRF_SECRET_LENGTH, so flag
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# to replace it with the masked version.
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request.META['CSRF_COOKIE_NEEDS_UPDATE'] = True
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return csrf_token
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def _set_csrf_cookie(self, request, response):
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if settings.CSRF_USE_SESSIONS:
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if request.session.get(CSRF_SESSION_KEY) != request.META['CSRF_COOKIE']:
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request.session[CSRF_SESSION_KEY] = request.META['CSRF_COOKIE']
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else:
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response.set_cookie(
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settings.CSRF_COOKIE_NAME,
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request.META['CSRF_COOKIE'],
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max_age=settings.CSRF_COOKIE_AGE,
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domain=settings.CSRF_COOKIE_DOMAIN,
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path=settings.CSRF_COOKIE_PATH,
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secure=settings.CSRF_COOKIE_SECURE,
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httponly=settings.CSRF_COOKIE_HTTPONLY,
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samesite=settings.CSRF_COOKIE_SAMESITE,
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)
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# Set the Vary header since content varies with the CSRF cookie.
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patch_vary_headers(response, ('Cookie',))
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def _origin_verified(self, request):
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request_origin = request.META['HTTP_ORIGIN']
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try:
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good_host = request.get_host()
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except DisallowedHost:
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pass
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else:
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good_origin = '%s://%s' % (
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'https' if request.is_secure() else 'http',
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good_host,
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)
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if request_origin == good_origin:
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return True
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if request_origin in self.allowed_origins_exact:
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return True
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try:
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parsed_origin = urlparse(request_origin)
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except ValueError:
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return False
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request_scheme = parsed_origin.scheme
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request_netloc = parsed_origin.netloc
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return any(
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is_same_domain(request_netloc, host)
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for host in self.allowed_origin_subdomains.get(request_scheme, ())
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)
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def _check_referer(self, request):
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referer = request.META.get('HTTP_REFERER')
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if referer is None:
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raise RejectRequest(REASON_NO_REFERER)
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try:
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referer = urlparse(referer)
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except ValueError:
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raise RejectRequest(REASON_MALFORMED_REFERER)
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# Make sure we have a valid URL for Referer.
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if '' in (referer.scheme, referer.netloc):
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raise RejectRequest(REASON_MALFORMED_REFERER)
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# Ensure that our Referer is also secure.
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if referer.scheme != 'https':
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raise RejectRequest(REASON_INSECURE_REFERER)
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if any(
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is_same_domain(referer.netloc, host)
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for host in self.csrf_trusted_origins_hosts
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):
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return
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# Allow matching the configured cookie domain.
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good_referer = (
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settings.SESSION_COOKIE_DOMAIN
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if settings.CSRF_USE_SESSIONS
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else settings.CSRF_COOKIE_DOMAIN
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)
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if good_referer is None:
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# If no cookie domain is configured, allow matching the current
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# host:port exactly if it's permitted by ALLOWED_HOSTS.
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try:
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# request.get_host() includes the port.
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good_referer = request.get_host()
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except DisallowedHost:
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raise RejectRequest(REASON_BAD_REFERER % referer.geturl())
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else:
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server_port = request.get_port()
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if server_port not in ('443', '80'):
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good_referer = '%s:%s' % (good_referer, server_port)
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if not is_same_domain(referer.netloc, good_referer):
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raise RejectRequest(REASON_BAD_REFERER % referer.geturl())
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def _bad_token_message(self, reason, token_source):
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if token_source != 'POST':
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# Assume it is a settings.CSRF_HEADER_NAME value.
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header_name = HttpHeaders.parse_header_name(token_source)
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token_source = f'the {header_name!r} HTTP header'
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return f'CSRF token from {token_source} {reason}.'
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def _check_token(self, request):
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# Access csrf_token via self._get_token() as rotate_token() may have
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# been called by an authentication middleware during the
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# process_request() phase.
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try:
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csrf_token = self._get_token(request)
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except InvalidTokenFormat as exc:
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raise RejectRequest(f'CSRF cookie {exc.reason}.')
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if csrf_token is None:
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# No CSRF cookie. For POST requests, we insist on a CSRF cookie,
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# and in this way we can avoid all CSRF attacks, including login
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# CSRF.
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raise RejectRequest(REASON_NO_CSRF_COOKIE)
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# Check non-cookie token for match.
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request_csrf_token = ''
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if request.method == 'POST':
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try:
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request_csrf_token = request.POST.get('csrfmiddlewaretoken', '')
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except UnreadablePostError:
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# Handle a broken connection before we've completed reading the
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# POST data. process_view shouldn't raise any exceptions, so
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# we'll ignore and serve the user a 403 (assuming they're still
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# listening, which they probably aren't because of the error).
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pass
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if request_csrf_token == '':
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# Fall back to X-CSRFToken, to make things easier for AJAX, and
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# possible for PUT/DELETE.
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try:
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request_csrf_token = request.META[settings.CSRF_HEADER_NAME]
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except KeyError:
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raise RejectRequest(REASON_CSRF_TOKEN_MISSING)
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token_source = settings.CSRF_HEADER_NAME
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else:
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token_source = 'POST'
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try:
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request_csrf_token = _sanitize_token(request_csrf_token)
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except InvalidTokenFormat as exc:
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reason = self._bad_token_message(exc.reason, token_source)
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raise RejectRequest(reason)
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if not _does_token_match(request_csrf_token, csrf_token):
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reason = self._bad_token_message('incorrect', token_source)
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raise RejectRequest(reason)
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def process_request(self, request):
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try:
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csrf_token = self._get_token(request)
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except InvalidTokenFormat:
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_add_new_csrf_cookie(request)
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else:
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if csrf_token is not None:
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# Use same token next time.
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request.META['CSRF_COOKIE'] = csrf_token
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def process_view(self, request, callback, callback_args, callback_kwargs):
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if getattr(request, 'csrf_processing_done', False):
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return None
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# Wait until request.META["CSRF_COOKIE"] has been manipulated before
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# bailing out, so that get_token still works
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if getattr(callback, 'csrf_exempt', False):
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return None
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# Assume that anything not defined as 'safe' by RFC7231 needs protection
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if request.method in ('GET', 'HEAD', 'OPTIONS', 'TRACE'):
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return self._accept(request)
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if getattr(request, '_dont_enforce_csrf_checks', False):
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# Mechanism to turn off CSRF checks for test suite. It comes after
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# the creation of CSRF cookies, so that everything else continues
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# to work exactly the same (e.g. cookies are sent, etc.), but
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# before any branches that call the _reject method.
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return self._accept(request)
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# Reject the request if the Origin header doesn't match an allowed
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# value.
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if 'HTTP_ORIGIN' in request.META:
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if not self._origin_verified(request):
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return self._reject(request, REASON_BAD_ORIGIN % request.META['HTTP_ORIGIN'])
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elif request.is_secure():
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# If the Origin header wasn't provided, reject HTTPS requests if
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# the Referer header doesn't match an allowed value.
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#
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# Suppose user visits http://example.com/
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# An active network attacker (man-in-the-middle, MITM) sends a
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# POST form that targets https://example.com/detonate-bomb/ and
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# submits it via JavaScript.
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#
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# The attacker will need to provide a CSRF cookie and token, but
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# that's no problem for a MITM and the session-independent secret
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# we're using. So the MITM can circumvent the CSRF protection. This
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# is true for any HTTP connection, but anyone using HTTPS expects
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# better! For this reason, for https://example.com/ we need
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# additional protection that treats http://example.com/ as
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# completely untrusted. Under HTTPS, Barth et al. found that the
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# Referer header is missing for same-domain requests in only about
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# 0.2% of cases or less, so we can use strict Referer checking.
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try:
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self._check_referer(request)
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except RejectRequest as exc:
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return self._reject(request, exc.reason)
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try:
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self._check_token(request)
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except RejectRequest as exc:
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return self._reject(request, exc.reason)
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return self._accept(request)
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def process_response(self, request, response):
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if request.META.get('CSRF_COOKIE_NEEDS_UPDATE'):
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self._set_csrf_cookie(request, response)
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# Unset the flag to prevent _set_csrf_cookie() from being
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# unnecessarily called again in process_response() by other
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# instances of CsrfViewMiddleware. This can happen e.g. when both a
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# decorator and middleware are used. However,
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# CSRF_COOKIE_NEEDS_UPDATE is still respected in subsequent calls
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# e.g. in case rotate_token() is called in process_response() later
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# by custom middleware but before those subsequent calls.
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request.META['CSRF_COOKIE_NEEDS_UPDATE'] = False
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return response
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