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Cross Site Request Forgery Middleware.
This module provides a middleware that implements protection
against request forgeries from other sites.
import logging
import re
import string
from urllib.parse import urlparse
from django.conf import settings
from django.core.exceptions import DisallowedHost, ImproperlyConfigured
from django.urls import get_callable
from django.utils.cache import patch_vary_headers
from django.utils.crypto import constant_time_compare, get_random_string
from django.utils.deprecation import MiddlewareMixin
from django.utils.http import is_same_domain
from django.utils.log import log_response
logger = logging.getLogger('')
REASON_NO_REFERER = "Referer checking failed - no Referer."
REASON_BAD_REFERER = "Referer checking failed - %s does not match any trusted origins."
REASON_NO_CSRF_COOKIE = "CSRF cookie not set."
REASON_BAD_TOKEN = "CSRF token missing or incorrect."
REASON_MALFORMED_REFERER = "Referer checking failed - Referer is malformed."
REASON_INSECURE_REFERER = "Referer checking failed - Referer is insecure while host is secure."
CSRF_ALLOWED_CHARS = string.ascii_letters + string.digits
CSRF_SESSION_KEY = '_csrftoken'
def _get_failure_view():
"""Return the view to be used for CSRF rejections."""
return get_callable(settings.CSRF_FAILURE_VIEW)
def _get_new_csrf_string():
return get_random_string(CSRF_SECRET_LENGTH, allowed_chars=CSRF_ALLOWED_CHARS)
def _salt_cipher_secret(secret):
Given a secret (assumed to be a string of CSRF_ALLOWED_CHARS), generate a
token by adding a salt and using it to encrypt the secret.
salt = _get_new_csrf_string()
pairs = zip((chars.index(x) for x in secret), (chars.index(x) for x in salt))
cipher = ''.join(chars[(x + y) % len(chars)] for x, y in pairs)
return salt + cipher
def _unsalt_cipher_token(token):
Given a token (assumed to be a string of CSRF_ALLOWED_CHARS, of length
CSRF_TOKEN_LENGTH, and that its first half is a salt), use it to decrypt
the second half to produce the original secret.
salt = token[:CSRF_SECRET_LENGTH]
token = token[CSRF_SECRET_LENGTH:]
pairs = zip((chars.index(x) for x in token), (chars.index(x) for x in salt))
secret = ''.join(chars[x - y] for x, y in pairs) # Note negative values are ok
return secret
def _get_new_csrf_token():
return _salt_cipher_secret(_get_new_csrf_string())
def get_token(request):
Return the CSRF token required for a POST form. The token is an
alphanumeric value. A new token is created if one is not already set.
A side effect of calling this function is to make the csrf_protect
decorator and the CsrfViewMiddleware add a CSRF cookie and a 'Vary: Cookie'
header to the outgoing response. For this reason, you may need to use this
function lazily, as is done by the csrf context processor.
if "CSRF_COOKIE" not in request.META:
csrf_secret = _get_new_csrf_string()
request.META["CSRF_COOKIE"] = _salt_cipher_secret(csrf_secret)
csrf_secret = _unsalt_cipher_token(request.META["CSRF_COOKIE"])
request.META["CSRF_COOKIE_USED"] = True
return _salt_cipher_secret(csrf_secret)
def rotate_token(request):
Change the CSRF token in use for a request - should be done on login
for security purposes.
"CSRF_COOKIE": _get_new_csrf_token(),
request.csrf_cookie_needs_reset = True
def _sanitize_token(token):
# Allow only ASCII alphanumerics
if'[^a-zA-Z0-9]', token):
return _get_new_csrf_token()
elif len(token) == CSRF_TOKEN_LENGTH:
return token
elif len(token) == CSRF_SECRET_LENGTH:
# Older Django versions set cookies to values of CSRF_SECRET_LENGTH
# alphanumeric characters. For backwards compatibility, accept
# such values as unsalted secrets.
# It's easier to salt here and be consistent later, rather than add
# different code paths in the checks, although that might be a tad more
# efficient.
return _salt_cipher_secret(token)
return _get_new_csrf_token()
def _compare_salted_tokens(request_csrf_token, csrf_token):
# Assume both arguments are sanitized -- that is, strings of
return constant_time_compare(
class CsrfViewMiddleware(MiddlewareMixin):
Require a present and correct csrfmiddlewaretoken for POST requests that
have a CSRF cookie, and set an outgoing CSRF cookie.
This middleware should be used in conjunction with the {% csrf_token %}
template tag.
# The _accept and _reject methods currently only exist for the sake of the
# requires_csrf_token decorator.
def _accept(self, request):
# Avoid checking the request twice by adding a custom attribute to
# request. This will be relevant when both decorator and middleware
# are used.
request.csrf_processing_done = True
return None
def _reject(self, request, reason):
response = _get_failure_view()(request, reason=reason)
'Forbidden (%s): %s', reason, request.path,
return response
def _get_token(self, request):
if settings.CSRF_USE_SESSIONS:
return request.session.get(CSRF_SESSION_KEY)
except AttributeError:
raise ImproperlyConfigured(
'CSRF_USE_SESSIONS is enabled, but request.session is not '
'set. SessionMiddleware must appear before CsrfViewMiddleware '
'in MIDDLEWARE%s.' % ('_CLASSES' if settings.MIDDLEWARE is None else '')
cookie_token = request.COOKIES[settings.CSRF_COOKIE_NAME]
except KeyError:
return None
csrf_token = _sanitize_token(cookie_token)
if csrf_token != cookie_token:
# Cookie token needed to be replaced;
# the cookie needs to be reset.
request.csrf_cookie_needs_reset = True
return csrf_token
def _set_token(self, request, response):
if settings.CSRF_USE_SESSIONS:
request.session[CSRF_SESSION_KEY] = request.META['CSRF_COOKIE']
# Set the Vary header since content varies with the CSRF cookie.
patch_vary_headers(response, ('Cookie',))
def process_request(self, request):
csrf_token = self._get_token(request)
if csrf_token is not None:
# Use same token next time.
request.META['CSRF_COOKIE'] = csrf_token
def process_view(self, request, callback, callback_args, callback_kwargs):
if getattr(request, 'csrf_processing_done', False):
return None
# Wait until request.META["CSRF_COOKIE"] has been manipulated before
# bailing out, so that get_token still works
if getattr(callback, 'csrf_exempt', False):
return None
# Assume that anything not defined as 'safe' by RFC7231 needs protection
if request.method not in ('GET', 'HEAD', 'OPTIONS', 'TRACE'):
if getattr(request, '_dont_enforce_csrf_checks', False):
# Mechanism to turn off CSRF checks for test suite.
# It comes after the creation of CSRF cookies, so that
# everything else continues to work exactly the same
# (e.g. cookies are sent, etc.), but before any
# branches that call reject().
return self._accept(request)
if request.is_secure():
# Suppose user visits
# An active network attacker (man-in-the-middle, MITM) sends a
# POST form that targets and
# submits it via JavaScript.
# The attacker will need to provide a CSRF cookie and token, but
# that's no problem for a MITM and the session-independent
# secret we're using. So the MITM can circumvent the CSRF
# protection. This is true for any HTTP connection, but anyone
# using HTTPS expects better! For this reason, for
# we need additional protection that treats
# as completely untrusted. Under HTTPS,
# Barth et al. found that the Referer header is missing for
# same-domain requests in only about 0.2% of cases or less, so
# we can use strict Referer checking.
referer = request.META.get('HTTP_REFERER')
if referer is None:
return self._reject(request, REASON_NO_REFERER)
referer = urlparse(referer)
# Make sure we have a valid URL for Referer.
if '' in (referer.scheme, referer.netloc):
return self._reject(request, REASON_MALFORMED_REFERER)
# Ensure that our Referer is also secure.
if referer.scheme != 'https':
return self._reject(request, REASON_INSECURE_REFERER)
# If there isn't a CSRF_COOKIE_DOMAIN, require an exact match
# match on host:port. If not, obey the cookie rules (or those
# for the session cookie, if CSRF_USE_SESSIONS).
good_referer = (
else settings.CSRF_COOKIE_DOMAIN
if good_referer is not None:
server_port = request.get_port()
if server_port not in ('443', '80'):
good_referer = '%s:%s' % (good_referer, server_port)
# request.get_host() includes the port.
good_referer = request.get_host()
except DisallowedHost:
# Create a list of all acceptable HTTP referers, including the
# current host if it's permitted by ALLOWED_HOSTS.
good_hosts = list(settings.CSRF_TRUSTED_ORIGINS)
if good_referer is not None:
if not any(is_same_domain(referer.netloc, host) for host in good_hosts):
reason = REASON_BAD_REFERER % referer.geturl()
return self._reject(request, reason)
csrf_token = request.META.get('CSRF_COOKIE')
if csrf_token is None:
# No CSRF cookie. For POST requests, we insist on a CSRF cookie,
# and in this way we can avoid all CSRF attacks, including login
return self._reject(request, REASON_NO_CSRF_COOKIE)
# Check non-cookie token for match.
request_csrf_token = ""
if request.method == "POST":
request_csrf_token = request.POST.get('csrfmiddlewaretoken', '')
except IOError:
# Handle a broken connection before we've completed reading
# the POST data. process_view shouldn't raise any
# exceptions, so we'll ignore and serve the user a 403
# (assuming they're still listening, which they probably
# aren't because of the error).
if request_csrf_token == "":
# Fall back to X-CSRFToken, to make things easier for AJAX,
# and possible for PUT/DELETE.
request_csrf_token = request.META.get(settings.CSRF_HEADER_NAME, '')
request_csrf_token = _sanitize_token(request_csrf_token)
if not _compare_salted_tokens(request_csrf_token, csrf_token):
return self._reject(request, REASON_BAD_TOKEN)
return self._accept(request)
def process_response(self, request, response):
if not getattr(request, 'csrf_cookie_needs_reset', False):
if getattr(response, 'csrf_cookie_set', False):
return response
if not request.META.get("CSRF_COOKIE_USED", False):
return response
# Set the CSRF cookie even if it's already set, so we renew
# the expiry timer.
self._set_token(request, response)
response.csrf_cookie_set = True
return response