flask_wtf flask 的 CSRF 源代码初研究

因为要搞一个基于flask的前后端分离的个人网站,所以需要研究下flask的csrf防护原理.

用的扩展是flask_wtf,也算是比较官方的扩展库了.

先上相关源代码:

 def validate_csrf(data, secret_key=None, time_limit=None, token_key=None):
     """Check if the given data is a valid CSRF token. This compares the given
     signed token to the one stored in the session.

     :param data: The signed CSRF token to be checked.
     :param secret_key: Used to securely sign the token. Default is
         ``WTF_CSRF_SECRET_KEY`` or ``SECRET_KEY``.
     :param time_limit: Number of seconds that the token is valid. Default is
         ``WTF_CSRF_TIME_LIMIT`` or 3600 seconds (60 minutes).
     :param token_key: Key where token is stored in session for comparision.
         Default is ``WTF_CSRF_FIELD_NAME`` or ``'csrf_token'``.

     :raises ValidationError: Contains the reason that validation failed.

     .. versionchanged:: 0.14
         Raises ``ValidationError`` with a specific error message rather than
         returning ``True`` or ``False``.
     """

     secret_key = _get_config(
         secret_key, 'WTF_CSRF_SECRET_KEY', current_app.secret_key,
         message='A secret key is required to use CSRF.'
     )
     field_name = _get_config(
         token_key, 'WTF_CSRF_FIELD_NAME', 'csrf_token',
         message='A field name is required to use CSRF.'
     )
     time_limit = _get_config(
         time_limit, 'WTF_CSRF_TIME_LIMIT', 3600, required=False
     )

     if not data:
         raise ValidationError('The CSRF token is missing.')

     if field_name not in session:
         raise ValidationError('The CSRF session token is missing.')

     s = URLSafeTimedSerializer(secret_key, salt='wtf-csrf-token')

     try:
         token = s.loads(data, max_age=time_limit)
     except SignatureExpired:
         raise ValidationError('The CSRF token has expired.')
     except BadData:
         raise ValidationError('The CSRF token is invalid.')

     if not safe_str_cmp(session[field_name], token):
         raise ValidationError('The CSRF tokens do not match.')

 class CSRFProtect(object):
     """Enable CSRF protection globally for a Flask app.

     ::

         app = Flask(__name__)
         csrf = CsrfProtect(app)

     Checks the ``csrf_token`` field sent with forms, or the ``X-CSRFToken``
     header sent with JavaScript requests. Render the token in templates using
     ``{{ csrf_token() }}``.

     See the :ref:`csrf` documentation.
     """

     def __init__(self, app=None):
         self._exempt_views = set()
         self._exempt_blueprints = set()

         if app:
             self.init_app(app)

     def init_app(self, app):
         app.extensions['csrf'] = self

         app.config.setdefault('WTF_CSRF_ENABLED', True)
         app.config.setdefault('WTF_CSRF_CHECK_DEFAULT', True)
         app.config['WTF_CSRF_METHODS'] = set(app.config.get(
             'WTF_CSRF_METHODS', ['POST', 'PUT', 'PATCH', 'DELETE']
         ))
         app.config.setdefault('WTF_CSRF_FIELD_NAME', 'csrf_token')
         app.config.setdefault(
             'WTF_CSRF_HEADERS', ['X-CSRFToken', 'X-CSRF-Token']
         )
         app.config.setdefault('WTF_CSRF_TIME_LIMIT', 3600)
         app.config.setdefault('WTF_CSRF_SSL_STRICT', True)

         app.jinja_env.globals['csrf_token'] = generate_csrf　　　　　　　　<><><><><><><><><><><><><><><><><><><>
         app.context_processor(lambda: {'csrf_token': generate_csrf})

         @app.before_request
         def csrf_protect():
             if not app.config['WTF_CSRF_ENABLED']:
                 return

             if not app.config['WTF_CSRF_CHECK_DEFAULT']:
                 return

             if request.method not in app.config['WTF_CSRF_METHODS']:
                 return

             if not request.endpoint:
                 return

             view = app.view_functions.get(request.endpoint)

             if not view:
                 return

             if request.blueprint in self._exempt_blueprints:
                 return

             dest = '%s.%s' % (view.__module__, view.__name__)

             if dest in self._exempt_views:
                 return

             self.protect()

     def _get_csrf_token(self):
         # find the ``csrf_token`` field in the subitted form
         # if the form had a prefix, the name will be
         # ``{prefix}-csrf_token``
         field_name = current_app.config['WTF_CSRF_FIELD_NAME']

         for key in request.form:
             if key.endswith(field_name):
                 csrf_token = request.form[key]

                 if csrf_token:
                     return csrf_token

         for header_name in current_app.config['WTF_CSRF_HEADERS']:
             csrf_token = request.headers.get(header_name)

             if csrf_token:
                 return csrf_token

         return None

     def protect(self):
         if request.method not in current_app.config['WTF_CSRF_METHODS']:
             return

         try:
             validate_csrf(self._get_csrf_token())
         except ValidationError as e:
             logger.info(e.args[0])
             self._error_response(e.args[0])

         if request.is_secure and current_app.config['WTF_CSRF_SSL_STRICT']:
             if not request.referrer:
                 self._error_response('The referrer header is missing.')

             good_referrer = 'https://{0}/'.format(request.host)

             if not same_origin(request.referrer, good_referrer):
                 self._error_response('The referrer does not match the host.')

         g.csrf_valid = True  # mark this request as CSRF valid

先说明下csrftoken的普通机制,上面代码中有一行代码后面被我加了一串<>符号,这行代码表明,默认的jinja2渲染的方式就是通过generate_csrf 方法生成csrftoken字符串,所以前后端分离的话,可以直接通过这个方法获取csrftoken,效果是一样的.

进入generate_csrf函数内部,会发现他做了这么点事:生成token,放在session里,然后返回一个加工过的token.这一块说明每当不同的访问触发该函数,那么服务器session内的csrftoken值就会不一样,所以,你可以这么做,获取一次之后在有效期(一个小时内)可以重复使用,但是不建议这么做.然后如果不是form表单提交的话,该csrf系统不会从json中获取token,而会从请求头获取,所以需要在请求头内添加关键字段:X-CSRFToken,将这个值赋值为获取的token即可.

首先获取csrftoken的方式: _get_csrf_token

会先从表单中查找关键字段,如果获取,那么返回该值,获取不到,从请求头获取,方式和django的基本一致,毕竟也就这两种规范方式.

 91         @app.before_request
 92         def csrf_protect():

这两行代码表明wtf是如何实现校验的,通过flask的钩子函数在每次请求开始时进行校验,这是在初始化wtf init_app(app)的时候就已经添加了该钩子函数.

在django里面,一旦中间件的process_request返回任何值,中间件即开始执行响应回调,视图不在执行,那么上面的两行代码下面好像不停地return了好多次,到底啥意思呢,只好再找源码看看.相关源码在下面:

    @setupmethod
    def before_request(self, f):
        """Registers a function to run before each request.

        For example, this can be used to open a database connection, or to load
        the logged in user from the session.

        The function will be called without any arguments. If it returns a
        non-None value, the value is handled as if it was the return value from
        the view, and further request handling is stopped.
        """
        self.before_request_funcs.setdefault(None, []).append(f)
        return f

可以看到添加钩子函数的装饰器执行了什么操作,他只是把钩子函数放进了一个函数列表里,然后我们看看这个函数列表是什么方式处理的.源码如下:

    def preprocess_request(self):
        """Called before the request is dispatched. Calls
        :attr:`url_value_preprocessors` registered with the app and the
        current blueprint (if any). Then calls :attr:`before_request_funcs`
        registered with the app and the blueprint.

        If any :meth:`before_request` handler returns a non-None value, the
        value is handled as if it was the return value from the view, and
        further request handling is stopped.
        """

        bp = _request_ctx_stack.top.request.blueprint

        funcs = self.url_value_preprocessors.get(None, ())
        if bp is not None and bp in self.url_value_preprocessors:
            funcs = chain(funcs, self.url_value_preprocessors[bp])
        for func in funcs:
            func(request.endpoint, request.view_args)

        funcs = self.before_request_funcs.get(None, ())
        if bp is not None and bp in self.before_request_funcs:
            funcs = chain(funcs, self.before_request_funcs[bp])
        for func in funcs:
            rv = func()
            if rv is not None:
                return rv

该方法的注释说明了,如果钩子函数返回任意不为空的数据,那么等同于视图的响应,所以仅仅return 不会导致钩子函数结束,仍然可以访问视图.

现在可以解释def csrf_protect():函数的内容了,即,请求方式不在保护范围内时,跳过校验,未开启防护时,跳过校验,视图无效时跳过校验.

csrf_protect 中会执行 protect ,protect 会执行 validate_csrf(),validate_csrf()是校验的关键,源代码如下:

def validate_csrf(data, secret_key=None, time_limit=None, token_key=None):
    """Check if the given data is a valid CSRF token. This compares the given
    signed token to the one stored in the session.

    :param data: The signed CSRF token to be checked.
    :param secret_key: Used to securely sign the token. Default is
        ``WTF_CSRF_SECRET_KEY`` or ``SECRET_KEY``.
    :param time_limit: Number of seconds that the token is valid. Default is
        ``WTF_CSRF_TIME_LIMIT`` or 3600 seconds (60 minutes).
    :param token_key: Key where token is stored in session for comparision.
        Default is ``WTF_CSRF_FIELD_NAME`` or ``'csrf_token'``.

    :raises ValidationError: Contains the reason that validation failed.

    .. versionchanged:: 0.14
        Raises ``ValidationError`` with a specific error message rather than
        returning ``True`` or ``False``.
    """

    secret_key = _get_config(
        secret_key, 'WTF_CSRF_SECRET_KEY', current_app.secret_key,
        message='A secret key is required to use CSRF.'
    )
    field_name = _get_config(
        token_key, 'WTF_CSRF_FIELD_NAME', 'csrf_token',
        message='A field name is required to use CSRF.'
    )
    time_limit = _get_config(
        time_limit, 'WTF_CSRF_TIME_LIMIT', 3600, required=False
    )

    if not data:
        raise ValidationError('The CSRF token is missing.')

    if field_name not in session:
        raise ValidationError('The CSRF session token is missing.')

    s = URLSafeTimedSerializer(secret_key, salt='wtf-csrf-token')

    try:
        token = s.loads(data, max_age=time_limit)
    except SignatureExpired:
        raise ValidationError('The CSRF token has expired.')
    except BadData:
        raise ValidationError('The CSRF token is invalid.')

    if not safe_str_cmp(session[field_name], token):
        raise ValidationError('The CSRF tokens do not match.')

该方法前面部分就是在获取相关秘钥和关键字,如果不自己自定义的话,这一块通常不会出问题,后面可以看到,方法会从全局变量session中寻找csrftoken字段名,然后最后一步进行校验,所以,wtf是通过比对session中的CSRFtoken和表单中的csrftoken是否一致.

所以前后端分离方式开发的话,需要将csrftoken通过接口或者cookie的方式传给前端,前端将该部分数据取出保存,提交表单的时候带上.

至于关键字,最上面那段代码写的很清楚,默认的,表单是csrf_token, 请求头是 X-CSRFToken.