django/django/middleware/csrf.py

"""
Cross Site Request Forgery Middleware.

This module provides a middleware that implements protection
against request forgeries from other sites.
"""
import logging
import string
from collections import defaultdict
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.functional import cached_property
from django.utils.http import is_same_domain
from django.utils.log import log_response
from django.utils.regex_helper import _lazy_re_compile

logger = logging.getLogger('django.security.csrf')
# This matches if any character is not in CSRF_ALLOWED_CHARS.
invalid_token_chars_re = _lazy_re_compile('[^a-zA-Z0-9]')

REASON_BAD_ORIGIN = "Origin checking failed - %s does not match any trusted origins."
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_CSRF_TOKEN_INCORRECT = 'CSRF token incorrect.'
REASON_CSRF_TOKEN_MISSING = 'CSRF token missing.'
REASON_MALFORMED_REFERER = "Referer checking failed - Referer is malformed."
REASON_INSECURE_REFERER = "Referer checking failed - Referer is insecure while host is secure."
# The reason strings below are for passing to InvalidTokenFormat. They are
# phrases without a subject because they can be in reference to either the CSRF
# cookie or non-cookie token.
REASON_INCORRECT_LENGTH = 'has incorrect length'
REASON_INVALID_CHARACTERS = 'has invalid characters'

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 _mask_cipher_secret(secret):
    """
    Given a secret (assumed to be a string of CSRF_ALLOWED_CHARS), generate a
    token by adding a mask and applying it to the secret.
    """
    mask = _get_new_csrf_string()
    chars = CSRF_ALLOWED_CHARS
    pairs = zip((chars.index(x) for x in secret), (chars.index(x) for x in mask))
    cipher = ''.join(chars[(x + y) % len(chars)] for x, y in pairs)
    return mask + cipher


def _unmask_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 mask), use it to decrypt
    the second half to produce the original secret.
    """
    mask = 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 mask))
    return ''.join(chars[x - y] for x, y in pairs)  # Note negative values are ok


def _get_new_csrf_token():
    return _mask_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"] = _mask_cipher_secret(csrf_secret)
    else:
        csrf_secret = _unmask_cipher_token(request.META["CSRF_COOKIE"])
    request.META["CSRF_COOKIE_USED"] = True
    return _mask_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


class InvalidTokenFormat(Exception):
    def __init__(self, reason):
        self.reason = reason


def _sanitize_token(token):
    if len(token) not in (CSRF_TOKEN_LENGTH, CSRF_SECRET_LENGTH):
        raise InvalidTokenFormat(REASON_INCORRECT_LENGTH)
    # Make sure all characters are in CSRF_ALLOWED_CHARS.
    if invalid_token_chars_re.search(token):
        raise InvalidTokenFormat(REASON_INVALID_CHARACTERS)
    if 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 unmasked secrets.
        # It's easier to mask here and be consistent later, rather than add
        # different code paths in the checks, although that might be a tad more
        # efficient.
        return _mask_cipher_secret(token)
    return token


def _compare_masked_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(
        _unmask_cipher_token(request_csrf_token),
        _unmask_cipher_token(csrf_token),
    )


class RejectRequest(Exception):
    def __init__(self, reason):
        self.reason = reason


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.
    """
    @cached_property
    def csrf_trusted_origins_hosts(self):
        return [
            urlparse(origin).netloc.lstrip('*')
            for origin in settings.CSRF_TRUSTED_ORIGINS
        ]

    @cached_property
    def allowed_origins_exact(self):
        return {
            origin for origin in settings.CSRF_TRUSTED_ORIGINS
            if '*' not in origin
        }

    @cached_property
    def allowed_origin_subdomains(self):
        """
        A mapping of allowed schemes to list of allowed netlocs, where all
        subdomains of the netloc are allowed.
        """
        allowed_origin_subdomains = defaultdict(list)
        for parsed in (urlparse(origin) for origin in settings.CSRF_TRUSTED_ORIGINS if '*' in origin):
            allowed_origin_subdomains[parsed.scheme].append(parsed.netloc.lstrip('*'))
        return allowed_origin_subdomains

    # 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.'
                )
        else:
            try:
                cookie_token = request.COOKIES[settings.CSRF_COOKIE_NAME]
            except KeyError:
                return None

            # This can raise InvalidTokenFormat.
            csrf_token = _sanitize_token(cookie_token)

            if csrf_token != cookie_token:
                # Then the cookie token had length CSRF_SECRET_LENGTH, so flag
                # to replace it with the masked version.
                request.csrf_cookie_needs_reset = True
            return csrf_token

    def _set_token(self, request, response):
        if settings.CSRF_USE_SESSIONS:
            if request.session.get(CSRF_SESSION_KEY) != request.META['CSRF_COOKIE']:
                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 _origin_verified(self, request):
        request_origin = request.META['HTTP_ORIGIN']
        try:
            good_host = request.get_host()
        except DisallowedHost:
            pass
        else:
            good_origin = '%s://%s' % (
                'https' if request.is_secure() else 'http',
                good_host,
            )
            if request_origin == good_origin:
                return True
        if request_origin in self.allowed_origins_exact:
            return True
        try:
            parsed_origin = urlparse(request_origin)
        except ValueError:
            return False
        request_scheme = parsed_origin.scheme
        request_netloc = parsed_origin.netloc
        return any(
            is_same_domain(request_netloc, host)
            for host in self.allowed_origin_subdomains.get(request_scheme, ())
        )

    def _check_referer(self, request):
        referer = request.META.get('HTTP_REFERER')
        if referer is None:
            raise RejectRequest(REASON_NO_REFERER)

        try:
            referer = urlparse(referer)
        except ValueError:
            raise RejectRequest(REASON_MALFORMED_REFERER)

        # Make sure we have a valid URL for Referer.
        if '' in (referer.scheme, referer.netloc):
            raise RejectRequest(REASON_MALFORMED_REFERER)

        # Ensure that our Referer is also secure.
        if referer.scheme != 'https':
            raise RejectRequest(REASON_INSECURE_REFERER)

        if any(
            is_same_domain(referer.netloc, host)
            for host in self.csrf_trusted_origins_hosts
        ):
            return
        # Allow matching the configured cookie domain.
        good_referer = (
            settings.SESSION_COOKIE_DOMAIN
            if settings.CSRF_USE_SESSIONS
            else settings.CSRF_COOKIE_DOMAIN
        )
        if good_referer is None:
            # If no cookie domain is configured, allow matching the current
            # host:port exactly if it's permitted by ALLOWED_HOSTS.
            try:
                # request.get_host() includes the port.
                good_referer = request.get_host()
            except DisallowedHost:
                raise RejectRequest(REASON_BAD_REFERER % referer.geturl())
        else:
            server_port = request.get_port()
            if server_port not in ('443', '80'):
                good_referer = '%s:%s' % (good_referer, server_port)

        if not is_same_domain(referer.netloc, good_referer):
            raise RejectRequest(REASON_BAD_REFERER % referer.geturl())

    def process_request(self, request):
        try:
            csrf_token = self._get_token(request)
        except InvalidTokenFormat:
            csrf_token = _get_new_csrf_token()
            request.csrf_cookie_needs_reset = True

        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 in ('GET', 'HEAD', 'OPTIONS', 'TRACE'):
            return self._accept(request)

        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)

        # Reject the request if the Origin header doesn't match an allowed
        # value.
        if 'HTTP_ORIGIN' in request.META:
            if not self._origin_verified(request):
                return self._reject(request, REASON_BAD_ORIGIN % request.META['HTTP_ORIGIN'])
        elif request.is_secure():
            # If the Origin header wasn't provided, reject HTTPS requests if
            # the Referer header doesn't match an allowed value.
            #
            # 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.
            try:
                self._check_referer(request)
            except RejectRequest as exc:
                return self._reject(request, exc.reason)

        # Access csrf_token via self._get_token() as rotate_token() may have
        # been called by an authentication middleware during the
        # process_request() phase.
        try:
            csrf_token = self._get_token(request)
        except InvalidTokenFormat as exc:
            return self._reject(request, f'CSRF cookie {exc.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 OSError:
                # 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.
            try:
                request_csrf_token = request.META[settings.CSRF_HEADER_NAME]
            except KeyError:
                return self._reject(request, REASON_CSRF_TOKEN_MISSING)

        try:
            request_csrf_token = _sanitize_token(request_csrf_token)
        except InvalidTokenFormat as exc:
            return self._reject(request, f'CSRF token {exc.reason}.')

        if not _compare_masked_tokens(request_csrf_token, csrf_token):
            return self._reject(request, REASON_CSRF_TOKEN_INCORRECT)

        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