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327 lines
13 KiB
Python
327 lines
13 KiB
Python
5 years ago
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"""
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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 re
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import string
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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.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.http import is_same_domain
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from django.utils.log import log_response
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logger = logging.getLogger('django.security.csrf')
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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_BAD_TOKEN = "CSRF token missing or incorrect."
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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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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 _salt_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 salt and using it to encrypt the secret.
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"""
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salt = _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 salt))
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cipher = ''.join(chars[(x + y) % len(chars)] for x, y in pairs)
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return salt + cipher
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def _unsalt_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 salt), use it to decrypt
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the second half to produce the original secret.
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"""
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salt = 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 salt))
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return ''.join(chars[x - y] for x, y in pairs) # Note negative values are ok
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def _get_new_csrf_token():
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return _salt_cipher_secret(_get_new_csrf_string())
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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" not in request.META:
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csrf_secret = _get_new_csrf_string()
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request.META["CSRF_COOKIE"] = _salt_cipher_secret(csrf_secret)
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else:
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csrf_secret = _unsalt_cipher_token(request.META["CSRF_COOKIE"])
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request.META["CSRF_COOKIE_USED"] = True
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return _salt_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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request.META.update({
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"CSRF_COOKIE_USED": True,
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"CSRF_COOKIE": _get_new_csrf_token(),
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})
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request.csrf_cookie_needs_reset = True
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def _sanitize_token(token):
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# Allow only ASCII alphanumerics
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if re.search('[^a-zA-Z0-9]', token):
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return _get_new_csrf_token()
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elif len(token) == CSRF_TOKEN_LENGTH:
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return token
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elif 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 unsalted secrets.
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# It's easier to salt 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 _salt_cipher_secret(token)
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return _get_new_csrf_token()
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def _compare_salted_tokens(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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_unsalt_cipher_token(request_csrf_token),
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_unsalt_cipher_token(csrf_token),
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)
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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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# 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%s.' % ('_CLASSES' if settings.MIDDLEWARE is None else '')
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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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csrf_token = _sanitize_token(cookie_token)
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if csrf_token != cookie_token:
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# Cookie token needed to be replaced;
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# the cookie needs to be reset.
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request.csrf_cookie_needs_reset = True
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return csrf_token
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def _set_token(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 process_request(self, request):
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csrf_token = self._get_token(request)
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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 not in ('GET', 'HEAD', 'OPTIONS', 'TRACE'):
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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.
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# It comes after the creation of CSRF cookies, so that
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# everything else continues to work exactly the same
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# (e.g. cookies are sent, etc.), but before any
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# branches that call reject().
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return self._accept(request)
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if request.is_secure():
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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
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# secret we're using. So the MITM can circumvent the CSRF
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# protection. This is true for any HTTP connection, but anyone
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# using HTTPS expects better! For this reason, for
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# https://example.com/ we need additional protection that treats
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# http://example.com/ as completely untrusted. Under HTTPS,
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# Barth et al. found that the Referer header is missing for
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# same-domain requests in only about 0.2% of cases or less, so
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# we can use strict Referer checking.
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referer = request.META.get('HTTP_REFERER')
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if referer is None:
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return self._reject(request, REASON_NO_REFERER)
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referer = urlparse(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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return self._reject(request, 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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return self._reject(request, REASON_INSECURE_REFERER)
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# If there isn't a CSRF_COOKIE_DOMAIN, require an exact match
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# match on host:port. If not, obey the cookie rules (or those
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# for the session cookie, if CSRF_USE_SESSIONS).
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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 not None:
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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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else:
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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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pass
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# Create a list of all acceptable HTTP referers, including the
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# current host if it's permitted by ALLOWED_HOSTS.
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good_hosts = list(settings.CSRF_TRUSTED_ORIGINS)
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if good_referer is not None:
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good_hosts.append(good_referer)
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if not any(is_same_domain(referer.netloc, host) for host in good_hosts):
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reason = REASON_BAD_REFERER % referer.geturl()
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return self._reject(request, reason)
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csrf_token = request.META.get('CSRF_COOKIE')
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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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return self._reject(request, 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 OSError:
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# Handle a broken connection before we've completed reading
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# the POST data. process_view shouldn't raise any
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# exceptions, so we'll ignore and serve the user a 403
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# (assuming they're still listening, which they probably
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# 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,
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# and possible for PUT/DELETE.
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request_csrf_token = request.META.get(settings.CSRF_HEADER_NAME, '')
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request_csrf_token = _sanitize_token(request_csrf_token)
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if not _compare_salted_tokens(request_csrf_token, csrf_token):
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return self._reject(request, REASON_BAD_TOKEN)
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return self._accept(request)
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def process_response(self, request, response):
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if not getattr(request, 'csrf_cookie_needs_reset', False):
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if getattr(response, 'csrf_cookie_set', False):
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return response
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if not request.META.get("CSRF_COOKIE_USED", False):
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return response
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# Set the CSRF cookie even if it's already set, so we renew
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# the expiry timer.
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self._set_token(request, response)
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response.csrf_cookie_set = True
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return response
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