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"""
Cross Site Request Forgery Middleware.
This module provides a middleware that implements protection
against request forgeries from other sites.
"""
from __future__ import unicode_literals
import logging
import re
import string
from django.conf import settings
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.encoding import force_text
from django.utils.http import is_same_domain
from django.utils.six.moves import zip
from django.utils.six.moves.urllib.parse import urlparse
logger = logging.getLogger('django.request')
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_SECRET_LENGTH = 32
CSRF_TOKEN_LENGTH = 2 * CSRF_SECRET_LENGTH
CSRF_ALLOWED_CHARS = string.ascii_letters + string.digits
def _get_failure_view():
"""
Returns 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()
chars = CSRF_ALLOWED_CHARS
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:]
chars = CSRF_ALLOWED_CHARS
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):
"""
Returns 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)
else:
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):
"""
Changes the CSRF token in use for a request - should be done on login
for security purposes.
"""
request.META.update({
"CSRF_COOKIE_USED": True,
"CSRF_COOKIE": _get_new_csrf_token(),
})
request.csrf_cookie_needs_reset = True
def _sanitize_token(token):
# Allow only ASCII alphanumerics
if re.search('[^a-zA-Z0-9]', force_text(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
# length CSRF_TOKEN_LENGTH, all CSRF_ALLOWED_CHARS.
return constant_time_compare(
_unsalt_cipher_token(request_csrf_token),
_unsalt_cipher_token(csrf_token),
)
class CsrfViewMiddleware(MiddlewareMixin):
"""
Middleware that requires a present and correct csrfmiddlewaretoken
for POST requests that have a CSRF cookie, and sets 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):
logger.warning(
'Forbidden (%s): %s', reason, request.path,
extra={
'status_code': 403,
'request': request,
}
)
return _get_failure_view()(request, reason=reason)
def process_view(self, request, callback, callback_args, callback_kwargs):
if getattr(request, 'csrf_processing_done', False):
return None
try:
cookie_token = request.COOKIES[settings.CSRF_COOKIE_NAME]
except KeyError:
csrf_token = None
else:
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
# Use same token next time.
request.META['CSRF_COOKIE'] = csrf_token
# 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 http://example.com/
# An active network attacker (man-in-the-middle, MITM) sends a
# POST form that targets https://example.com/detonate-bomb/ 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
# https://example.com/ we need additional protection that treats
# http://example.com/ 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 = force_text(
request.META.get('HTTP_REFERER'),
strings_only=True,
errors='replace'
)
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, assume we need an exact
# match on host:port. If not, obey the cookie rules.
if settings.CSRF_COOKIE_DOMAIN is None:
# request.get_host() includes the port.
good_referer = request.get_host()
else:
good_referer = settings.CSRF_COOKIE_DOMAIN
server_port = request.get_port()
if server_port not in ('443', '80'):
good_referer = '%s:%s' % (good_referer, server_port)
# Here we generate a list of all acceptable HTTP referers,
# including the current host since that has been validated
# upstream.
good_hosts = list(settings.CSRF_TRUSTED_ORIGINS)
good_hosts.append(good_referer)
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)
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
# CSRF.
return self._reject(request, REASON_NO_CSRF_COOKIE)
# Check non-cookie token for match.
request_csrf_token = ""
if request.method == "POST":
try:
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).
pass
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.
response.set_cookie(settings.CSRF_COOKIE_NAME,
request.META["CSRF_COOKIE"],
max_age=settings.CSRF_COOKIE_AGE,
domain=settings.CSRF_COOKIE_DOMAIN,
path=settings.CSRF_COOKIE_PATH,
secure=settings.CSRF_COOKIE_SECURE,
httponly=settings.CSRF_COOKIE_HTTPONLY
)
# Content varies with the CSRF cookie, so set the Vary header.
patch_vary_headers(response, ('Cookie',))
response.csrf_cookie_set = True
return response
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