Django中间件CsrfViewMiddleware源码分析

Django Documentation

csrf保护基于以下:

1, 一个CSRF cookie基于一个随机生成的值,其他网站无法得到,次cookie有CsrfViewMiddleware产生.它与每个调用django.middleware.csrf.get_token()(这是一个用于取回CSRF token的方法)的响应一起发送,如果它尚未在请求上设置的话.

为了放置BREACH攻击,token不仅仅是比吗,随机的salt被置于secret之前并用来加密它,出于安全原因,每次用户登陆都会更改密钥的值.

CsrfViewMiddleware.process_request

class CsrfViewMiddleware(MiddlewareMixin):
    def process_request(self, request):
        csrf_token = self._get_token(request)
        # 第一次访问, csrf_token返回None,

        if csrf_token is not None:
            request.META["CSRF_COOKIE"] = csrf_token
            # request.META是一个python字典,包含了所有本次Http请求的Header信息,比如用户IP地址和用户Agent(通常是浏览器的名称版本号).

settings = Lazysettins()

这是一个懒加载

方法_get_token,从名字上看就是获取token, _get_token在后面多处地方用到

    def _get_token(self, request):
        # CSRF_USE_SESSIONS在django/conf/global_settings.py，默认为False,执行else
        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]
                # CSRF_SESSION_KEY= "csrftoken"
            except KeyError:
                # 第一次访问的时候 request.COOKIES = {},所以直接返回
                return None

            csrf_token = _sanitize_token(cookie_token)
            # csrf 对不上 cookie里 的 token，标记csrf_cookie_needs_reset=True，
            # 在process_response的方法中判定
            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

CSRF_SECRET_LENGTH = 32
CSRF_TOKEN_LENGTH = 2 * CSRF_SECRET_LENGTH

def _sanitize_token(token):
    # Allow only ASCII alphanumerics
    if re.search('[^a-zA-Z0-9]', force_text(token)):
        return _get_new_csrf_token()

先跳转到_get_new_csrf_token(),看他的生成方法

def _get_new_csrf_token():
    return _salt_cipher_secret(_get_new_csrf_string())

CSRF_SECRET_LENGTH = 32
CSRF_TOKEN_LENGTH = 2 * CSRF_SECRET_LENGTH

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.
给定一个secret(假设是一串 CSRF_ALLOWED_CHARS), 通过添加一个随机生成值并使用它来加密secret生成一个token
    """
    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

文件位置:django/utils/crypto.py

def get_random_string(length=12,
                      allowed_chars='abcdefghijklmnopqrstuvwxyz'
                                    'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'):
    """
    Returns a securely generated random string.
　　返回安全生成的随机字符串

    The default length of 12 with the a-z, A-Z, 0-9 character set returns
    a 71-bit value. log_2((26+26+10)^12) =~ 71 bits
    """
    if not using_sysrandom:
        # This is ugly, and a hack, but it makes things better than
        # the alternative of predictability. This re-seeds the PRNG
        # using a value that is hard for an attacker to predict, every
        # time a random string is required. This may change the
        # properties of the chosen random sequence slightly, but this
        # is better than absolute predictability.
        random.seed(
            hashlib.sha256(
                ("%s%s%s" % (
                    random.getstate(),
                    time.time(),
                    settings.SECRET_KEY)).encode('utf-8')
            ).digest())
    return ''.join(random.choice(allowed_chars) for i in range(length))

返回的是一个随机的字符串

# 接上面的def _sanitize_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.

        # 较旧的Django版本将cookie设置为CSRF_SECRET_LENGTH字母数字字符的值.为了向后
        # 兼容,接受诸如无保密秘密之类的值.这里更容易加盐并在以后保持一致,而不是在检查中
        # 添加不同的代码路径, 尽管这可能会更有效
        return _salt_cipher_secret(token)
    return _get_new_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
        # 如果装饰器@csrf_exempt生效,则不处理
        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().

                # 关闭CSRF检查测试套件的机制.在创建CSRF cookie之后,所以
                # 其他所有内容继续完全相同(例如发送cookie等),但在调用
                # reject()的任何分支之前
                return self._accept(request)

    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

接上面的CsrfViewMiddleware.process_view的代码

            # is_secure 如果请求是安全的，返回True，意味着发出的是HTTPS请求。
            if request.is_secure():
                referer = request.META.get('HTTP_REFERER')
                if referer is None:
                    return self._reject(request, REASON_NO_REFERER)
                    # _reject就是csrf验证不通过，因为reffer为空

返回一个Forbidden的代码

    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)

referer = urlparse(referer)
                # referer.scheme: 请求的协议,一般为http或者https
                # referer.netloc: host域名

                # Make sure we have a valid URL for Referer.
                # 确保我们在referer中有一个有效的URL
                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 ('', ''):
                        good_referer = '%s:%s' % (good_referer, server_port)
                else:
                    # request.get_host() includes the port.
                    good_referer = request.get_host()

                # Here we generate a list of all acceptable HTTP referers,
                # including the current host since that has been validated
                # upstream.

                # 在这里,我们生成所有可能接受HTTP引用的列表,包括当前主机,因为
                # 它已经在上游验证.
                good_hosts = list(settings.CSRF_TRUSTED_ORIGINS)
                good_hosts.append(good_referer)

                # 禁止跨域
                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.POST.get()相当于获取request.POST["csrfmiddlewaretoken"]
                    # 如果出错就返回'', 这里的csrfmiddlewaretoken是提交的表单中的值,在模板
                    # 中用{% csrf_token %}生成
                    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).

                    # 在我们完成读取POST数据之前处理断开的连接.
                    # process_view不应该引发任何exception.一次我们将忽略并返回403
                    pass

            if request_csrf_token == "":
                # Fall back to X-CSRFToken, to make things easier for AJAX,
                # and possible for PUT/DELETE.
                # ajax中使用"X-CSRFToken"
                # CERF_HEADER_NAME = "HTTP_X_CSRFTOKEN"
                request_csrf_token = request.META.get(settings.CSRF_HEADER_NAME, '')

            request_csrf_token = _sanitize_token(request_csrf_token)
            # 对比两个csrf_token, 一个是表单里隐藏的csrfmiddlewaretoken
            # 或者ajax的header: X_CSRFTOKEN, 另一个是自带的cookies里面的csrf_token
            if not _compare_salted_tokens(request_csrf_token, csrf_token):
                # 匹配不会就拒绝
                return self._reject(request, REASON_BAD_TOKEN)

        return self._accept(request)

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),
    )

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 _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

get_token(important)

get_token是在外部调用,由Template中的{% csrf_token %}触发,由request的cookie不同做出不同的反应.

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:
    # 如果request中不存在csrf, 先生成一个新的secret, 加密赋值到META["CSRF_COOKIE"]
        # 后面用来放到set_cookie之中
        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)

上面返回的一个加密secret将会被填充进入<input type="hidden" name="csrfmiddlewaretoken" value="{}">, value里面,随着表单一起提交并和cookie之中的csrf_token比较

CsrfViewMiddleware.process_response

    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

    # 设置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']就是在上面赋值的
                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,
            )
            # Set the Vary header since content varies with the CSRF cookie.
            patch_vary_headers(response, ('Cookie',))

总结:

第一个访问页面

　　首先第一次访问页面,Template中的{% csrf_token %}会启动get_token(不是私有方法), 产生一个csrf_secret的值

　　这个值在_salt_cipher_secret中随机产生一个与csrf_secret长度相同的salt,利用salt加密csrf_secret, 两个字符串拼接形成csrf_token, request.META["CSRF_COOKIE"] = csrf_token并设置cookie里面

　　get_token返回的用随机生成的例外一个salt加密csrf_secret,同样拼接返回放入process_view进行解密,比对,如果解密出来的数值不同直接返回_reject()

1. 所有传出POST表单中都有一个名为csrfmiddlewaretoken的隐藏表单字段。此字段的值同样是秘密的值。salt添加到它并用于加扰它。每次调用get_token（）时都会重新生成salt，以便在每个此类响应中更改表单字段值。这部分由template的{% csrf_token %}完成。

2. 对于未使用HTTP GET，HEAD，OPTIONS或TRACE的所有传入请求，必须带有CSRF cookie，并且csrfmiddlewaretoken字段必须存在且正确。如果不是，用户将收到403错误。
   验证csrfmiddlewaretoken字段值时，只将secret而不是整个token与cookie值中的secret进行比较。这允许使用不断变化的token。虽然每个请求都可以使用自己的token，但secret仍然是所有人共同的。
   此检查由CsrfViewMiddleware完成。

3. 此外，对于HTTPS请求，严格的引用检查由CsrfViewMiddleware完成。这意味着即使子域可以在您的域上设置或修改cookie，它也不能强制用户发布到您的应用程序，因为该请求不会来自您自己的确切域。 这也解决了在使用会话独立秘密时在HTTPS下可能发生的中间人攻击，因为即使在HTTPS下与站点通信时，HTTP Set-Cookie标头（不幸）也被客户接受了。 。 （对HTTP请求不进行引用检查，因为在HTTP下，Referer头的存在不够可靠。） 如果设置了CSRF_COOKIE_DOMAIN设置，则会将引用者与其进行比较。此设置支持子域。例如，CSRF_COOKIE_DOMAIN ='.example.com'将允许来自www.example.com和api.example.com的POST请求。如果未设置该设置，则referer必须与HTTP Host标头匹配。 可以使用CSRF_TRUSTED_ORIGINS设置将已接受的引用扩展到当前主机或cookie域之外。