# Django 2.1.8.dev20190330215650 documentation

# Source code for django.middleware.csrf

    """
    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('django.security.csrf')

    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
    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()
        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):
        """
        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)
        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):
        """
        Change 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]', 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):
        """
        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)
            log_response(
                'Forbidden (%s): %s', reason, request.path,
                response=response,
                request=request,
                logger=logger,
            )
            return response

        def _get_token(self, request):
            if settings.CSRF_USE_SESSIONS:
                try:
                    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 '')
                    )
            else:
                try:
                    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']
            else:
                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,
                    samesite=settings.CSRF_COOKIE_SAMESITE,
                )
                # 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 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 = 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 = (
                        settings.SESSION_COOKIE_DOMAIN
                        if settings.CSRF_USE_SESSIONS
                        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)
                    else:
                        try:
                            # request.get_host() includes the port.
                            good_referer = request.get_host()
                        except DisallowedHost:
                            pass

                    # 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:
                        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)

                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
                    # 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.
            self._set_token(request, response)
            response.csrf_cookie_set = True
            return response