flask_wtf flask 的 CSRF 源代码初研究

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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