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https://github.com/wntrblm/flask-talisman

HTTP security headers for Flask
https://github.com/wntrblm/flask-talisman

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HTTP security headers for Flask

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README 

        
Talisman: HTTP security headers for Flask

=========================================



|PyPI Version|



Talisman is a small Flask extension that handles setting HTTP headers

that can help protect against a few common web application security

issues.



The default configuration:



-  Forces all connects to ``https``, unless running with debug enabled.

-  Enables `HTTP Strict Transport

   Security `_.

-  Sets Flask's session cookie to ``secure``, so it will never be set if

   your application is somehow accessed via a non-secure connection.

-  Sets Flask's session cookie to ``httponly``, preventing JavaScript

   from being able to access its content. CSRF via Ajax uses a separate

   cookie and should be unaffected.

-  Sets Flask's session cookie to ``Lax``, preventing the cookie to be leaked

   in CSRF-prone request methods.

-  Sets

   `X-Frame-Options `_

   to ``SAMEORIGIN`` to avoid

   `clickjacking `_.

-  Sets `X-Content-Type-Options

   `_

   to prevent content type sniffing.

-  Sets a strict `Content Security

   Policy `__

   of ``default-src: 'self', 'object-src': 'none'``. This is intended to almost completely

   prevent Cross Site Scripting (XSS) attacks. This is probably the only

   setting that you should reasonably change. See the

   `Content Security Policy`_ section.

-  Sets a strict `Referrer-Policy `_

   of ``strict-origin-when-cross-origin`` that governs which referrer information should be included with

   requests made.

-  Disables ``browsing-topics`` by default in the `Permissions-Policy `_

   like `Drupal `_ to enhance privacy protection.



In addition to Talisman, you **should always use a cross-site request

forgery (CSRF) library**. It's highly recommended to use

`Flask-SeaSurf `_,

which is based on Django's excellent library.



Installation & Basic Usage

--------------------------



Install via `pip `_:



::



    pip install flask-talisman



After installing, wrap your Flask app with a ``Talisman``:



.. code:: python



    from flask import Flask

    from flask_talisman import Talisman



    app = Flask(__name__)

    Talisman(app)



There is also a full `Example App `_.



Options

-------



-  ``force_https``, default ``True``, forces all non-debug connects to

   ``https`` (`about HTTPS `_).

-  ``force_https_permanent``, default ``False``, uses ``301`` instead of

   ``302`` for ``https`` redirects.



-  ``frame_options``, default ``SAMEORIGIN``, can be ``SAMEORIGIN``,

   ``DENY``, or ``ALLOWFROM`` (`about Frame Options `_).

-  ``frame_options_allow_from``, default ``None``, a string indicating

   the domains that are allowed to embed the site via iframe.



-  ``strict_transport_security``, default ``True``, whether to send HSTS

   headers (`about HSTS `_).

-  ``strict_transport_security_preload``, default ``False``, enables HSTS

   preloading. If you register your application with

   `Google's HSTS preload list `_,

   Firefox and Chrome will never load your site over a non-secure

   connection.

-  ``strict_transport_security_max_age``, default ``ONE_YEAR_IN_SECS``,

   length of time the browser will respect the HSTS header.

-  ``strict_transport_security_include_subdomains``, default ``True``,

   whether subdomains should also use HSTS.



-  ``content_security_policy``, default ``default-src: 'self'`, 'object-src': 'none'``, see the

   `Content Security Policy`_ section (`about Content Security Policy `_).

-  ``content_security_policy_nonce_in``, default ``[]``. Adds a per-request nonce

   value to the flask request object and also to the specified CSP header section.

   I.e. ``['script-src', 'style-src']``

-  ``content_security_policy_report_only``, default ``False``, whether to set

   the CSP header as "report-only" (as `Content-Security-Policy-Report-Only`)

   to ease deployment by disabling the policy enforcement by the browser,

   requires passing a value with the ``content_security_policy_report_uri``

   parameter

-  ``content_security_policy_report_uri``, default ``None``, a string

   indicating the report URI used for `CSP violation reports

   `_



-  ``referrer_policy``, default ``strict-origin-when-cross-origin``, a string

   that sets the Referrer Policy header to send a full URL when performing a same-origin

   request, only send the origin of the document to an equally secure destination

   (HTTPS->HTTPS), and send no header to a less secure destination (HTTPS->HTTP) (`about Referrer Policy `_).



-  ``feature_policy``, default ``{}``, see the `Feature Policy`_ section (`about Feature Policy `_).



-  ``permissions_policy``, default ``{'browsing-topics': '()'}``, see the `Permissions Policy`_ section (`about Permissions Policy `_).

-  ``document_policy``, default ``{}``, see the `Document Policy`_ section (`about Document Policy `_).



-  ``session_cookie_secure``, default ``True``, set the session cookie

   to ``secure``, preventing it from being sent over plain ``http`` (`about cookies (https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie)_`).

-  ``session_cookie_http_only``, default ``True``, set the session

   cookie to ``httponly``, preventing it from being read by JavaScript.

-  ``session_cookie_samesite``, default ``Lax``, set this to ``Strict`` to prevent the cookie from being sent by the browser to the target site in all cross-site browsing context, even when following a regular link.



-  ``force_file_save``, default ``False``, whether to set the

   `X-Download-Options `_

   header to ``noopen`` to prevent IE >= 8 to from opening file downloads

   directly and only save them instead.



-  ``x_content_type_options``, default ``True``, Protects against MIME sniffing vulnerabilities (`about Content Type Options `_).

-  ``x_xss_protection``, default ``False``, Protects against cross-site scripting (XSS) attacks (`about XSS Protection `_). This option is disabled by default because no modern browser (`supports this header `_) anymore.



For a full list of (security) headers, check out: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers.



Per-view options

~~~~~~~~~~~~~~~~



Sometimes you want to change the policy for a specific view. The

``force_https``, ``frame_options``, ``frame_options_allow_from``,

`content_security_policy``, ``feature_policy``, ``permissions_policy``

and ``document_policy`` options can be changed on a per-view basis.



.. code:: python



    from flask import Flask

    from flask_talisman import Talisman, ALLOW_FROM



    app = Flask(__name__)

    talisman = Talisman(app)



    @app.route('/normal')

    def normal():

        return 'Normal'



    @app.route('/embeddable')

    @talisman(frame_options=ALLOW_FROM, frame_options_allow_from='*')

    def embeddable():

        return 'Embeddable'



Content Security Policy

-----------------------



The default content security policy is extremely strict and will

prevent loading any resources that are not in the same domain as the

application. Most web applications will need to change this policy.

If you're not ready to deploy Content Security Policy, you can set

`content_security_policy` to `False` to disable sending this header

entirely.



A slightly more permissive policy is available at

``flask_talisman.GOOGLE_CSP_POLICY``, which allows loading Google-hosted JS

libraries, fonts, and embeding media from YouTube and Maps.



You can and should create your own policy to suit your site's needs.

Here's a few examples adapted from

`MDN `_:



Example 1

~~~~~~~~~



This is the default policy. A web site administrator wants all content

to come from the site's own origin (this excludes subdomains) and disallow

legacy HTML elements.



.. code:: python



    csp = {

        'default-src': '\'self\'',

        'object-src': '\'none\'',

    }

    talisman = Talisman(app, content_security_policy=csp)



Example 2

~~~~~~~~~



A web site administrator wants to allow content from a trusted domain

and all its subdomains (it doesn't have to be the same domain that the

CSP is set on.)



.. code:: python



    csp = {

        'default-src': [

            '\'self\'',

            '*.trusted.com'

        ]

    }



Example 3

~~~~~~~~~



A web site administrator wants to allow users of a web application to

include images from any origin in their own content, but to restrict

audio or video media to trusted providers, and all scripts only to a

specific server that hosts trusted code.



.. code:: python



    csp = {

        'default-src': '\'self\'',

        'img-src': '*',

        'media-src': [

            'media1.com',

            'media2.com',

        ],

        'script-src': 'userscripts.example.com'

    }



In this example content is only permitted from the document's origin

with the following exceptions:



-  Images may loaded from anywhere (note the ``*`` wildcard).

-  Media is only allowed from media1.com and media2.com (and not from

   subdomains of those sites).

-  Executable script is only allowed from userscripts.example.com.



Example 4

~~~~~~~~~



A web site administrator for an online banking site wants to ensure that

all its content is loaded using SSL, in order to prevent attackers from

eavesdropping on requests.



.. code:: python



    csp = {

        'default-src': 'https://onlinebanking.jumbobank.com'

    }



The server only permits access to documents being loaded specifically

over HTTPS through the single origin onlinebanking.jumbobank.com.



Example 5

~~~~~~~~~



A web site administrator of a web mail site wants to allow HTML in

email, as well as images loaded from anywhere, but not JavaScript or

other potentially dangerous content.



.. code:: python



    csp = {

        'default-src': [

            '\'self\'',

            '*.mailsite.com',

        ],

        'img-src': '*'

    }



Note that this example doesn't specify a ``script-src``; with the

example CSP, this site uses the setting specified by the ``default-src``

directive, which means that scripts can be loaded only from the

originating server.



Example 6

~~~~~~~~~



A web site administrator wants to allow embedded scripts (which might

be generated dynamicially).



.. code:: python



    csp = {

        'default-src': '\'self\'',

        'script-src': '\'self\'',

    }

    talisman = Talisman(

        app,

        content_security_policy=csp,

        content_security_policy_nonce_in=['script-src']

    )



The nonce needs to be added to the script tag in the template:



.. code:: html



    

        //...

    



Note that the CSP directive (`script-src` in the example) to which the `nonce-...`

source should be added needs to be defined explicitly.



Example 7

~~~~~~~~~



A web site adminstrator wants to override the CSP directives via an

environment variable which doesn't support specifying the policy as

a Python dictionary, e.g.:



.. code:: bash



    export CSP_DIRECTIVES="default-src 'self'; image-src *"

    python app.py



Then in the app code you can read the CSP directives from the environment:



.. code:: python



    import os

    from flask_talisman import Talisman, DEFAULT_CSP_POLICY



    talisman = Talisman(

        app,

        content_security_policy=os.environ.get("CSP_DIRECTIVES", DEFAULT_CSP_POLICY),

    )



As you can see above the policy can be defined simply just like the official

specification requires the HTTP header to be set: As a semicolon separated

list of individual CSP directives.



Feature Policy

--------------



**Note:** Feature Policy has largely been `renamed Permissions Policy `_

in the latest draft and some features are likely to move to Document Policy.

At this writing, most browsers support the ``Feature-Policy`` HTTP Header name.

See the `Permissions Policy`_ and `Document Policy`_ sections below should you wish

to set these.



Also note that the Feature Policy specification did not progress beyond the `draft https://wicg.github.io/feature-policy/`

stage before being renamed, but is `supported in some form in most browsers

`_.



The default feature policy is empty, as this is the default expected behaviour.



Geolocation Example

~~~~~~~~~~~~~~~~~~~



Disable access to Geolocation interface.



.. code:: python



    feature_policy = {

        'geolocation': '\'none\''

    }

    talisman = Talisman(app, feature_policy=feature_policy)



Permissions Policy

------------------



Feature Policy has been split into Permissions Policy and Document Policy but

at this writing `browser support of Permissions Policy is very limited `_,

and it is recommended to still set the ``Feature-Policy`` HTTP Header.

Permission Policy support is included in Talisman for when this becomes more

widely supported.



Note that the `Permission Policy is still an Working Draft `_.



When the same feature or permission is set in both Feature Policy and Permission Policy,

the Permission Policy setting will take precedence in browsers that support both.



It should be noted that the syntax differs between Feature Policy and Permission Policy

as can be seen from the ``geolocation`` examples provided.



The default Permissions Policy is ``browsing-topics=()``, which opts sites out of

`Federated Learning of Cohorts `_ an interest-based advertising initiative

called Topics API.



Permission Policy can be set either using a dictionary, or using a string.



Geolocation and Microphone Example

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~



Disable access to Geolocation interface and Microphone using dictionary syntax



.. code:: python



    permissions_policy = {

        'geolocation': '()',

        'microphone': '()'

    }

    talisman = Talisman(app, permissions_policy=permissions_policy)



Disable access to Geolocation interface and Microphone using string syntax



.. code:: python



    permissions_policy = 'geolocation=(), microphone=()'

    talisman = Talisman(app, permissions_policy=permissions_policy)



Document Policy

---------------



Feature Policy has been split into Permissions Policy and Document Policy but

at this writing `browser support of Document Policy is very limited `_,

and it is recommended to still set the ``Feature-Policy`` HTTP Header.

Document Policy support is included in Talisman for when this becomes more

widely supported.



Note that the `Document Policy is still an Unofficial Draft `_.



The default Document Policy is empty, as this is the default expected behaviour.



Document Policy can be set either using a dictionary, or using a string.



Oversized-Images Example

~~~~~~~~~~~~~~~~~~~~~~~~



Forbid oversized-images using dictionary syntax:



.. code:: python



    document_policy = {

        'oversized-images': '?0'

    }

    talisman = Talisman(app, document_policy=document_policy)



Forbid oversized-images using string syntax:



.. code:: python



    document_policy = 'oversized-images=?0'

    talisman = Talisman(app, document_policy=document_policy)



Disclaimer

----------



This code originated at Google, but is not an official Google product,

experimental or otherwise. It was forked on June 6th, 2021 from the

unmaintained GoogleCloudPlatform/flask-talisman.



There is no silver bullet for web application security. Talisman can

help, but security is more than just setting a few headers. Any

public-facing web application should have a comprehensive approach to

security.



Contributing changes

--------------------



-  See `CONTRIBUTING.md`_



Licensing

---------



- Apache 2.0 - See `LICENSE`_



.. _LICENSE: https://github.com/wntrblm/flask-talisman/blob/master/LICENSE

.. _CONTRIBUTING.md: https://github.com/wntrblm/flask-talisman/blob/master/CONTRIBUTING.md

.. |PyPI Version| image:: https://img.shields.io/pypi/v/flask-talisman.svg

   :target: https://pypi.python.org/pypi/flask-talisman