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 <https://developer.mozilla.org/en-US/docs/Web/Security/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 <https://developer.mozilla.org/en-US/docs/Web/HTTP/X-Frame-Options>_
to SAMEORIGIN to avoid
clickjacking <https://en.wikipedia.org/wiki/Clickjacking>_.
Sets X-Content-Type-Options <https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Content-Type-Options>_
to prevent content type sniffing.
Sets a strict Content Security Policy <https://developer.mozilla.org/en-US/docs/Web/Security/CSP/Introducing_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 <https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/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 <https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Feature-Policy>_
like Drupal <https://www.drupal.org/project/drupal/issues/3209628>_ 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 <https://flask-seasurf.readthedocs.org/en/latest/>_,
which is based on Django's excellent library.

Installation & Basic Usage

Install via pip <https://pypi.python.org/pypi/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 <https://github.com/wntrblm/flask-talisman/blob/master/example_app>_.

Options

force_https, default True, forces all non-debug connects to
https (about HTTPS <https://developer.mozilla.org/en-US/docs/Glossary/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 <https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-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 <https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Strict-Transport-Security>_).
strict_transport_security_preload, default False, enables HSTS
preloading. If you register your application with
Google's HSTS preload list <https://hstspreload.appspot.com/>_,
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 <https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/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 <https://developer.mozilla.org/en-US/docs/Web/Security/CSP/Using_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 <https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referrer-Policy>_).
feature_policy, default {}, see the Feature Policy_ section (about Feature Policy <https://developer.mozilla.org/en-US/docs/Web/HTTP/Feature_Policy>_).
permissions_policy, default {'browsing-topics': '()'}, see the Permissions Policy_ section (about Permissions Policy <https://developer.mozilla.org/en-US/docs/Web/HTTP/Feature_Policy>_).
document_policy, default {}, see the Document Policy_ section (about Document Policy <https://wicg.github.io/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 <https://docs.microsoft.com/en-us/previous-versions/windows/internet-explorer/ie-developer/compatibility/jj542450(v=vs.85)?redirectedfrom=MSDN>_
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 <https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Content-Type-Options>_).
x_xss_protection, default False, Protects against cross-site scripting (XSS) attacks (about XSS Protection <https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-XSS-Protection>). This option is disabled by default because no modern browser (supports this header <https://caniuse.com/mdn-http_headers_x-xss-protection>) 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 <https://developer.mozilla.org/en-US/docs/Web/Security/CSP/Using_Content_Security_Policy>`_:

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

    <script nonce="{{ csp_nonce() }}">
        //...
    </script>

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 <https://github.com/w3c/webappsec-feature-policy/issues/359>`_
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
<https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Feature-Policy#Browser_compatibility>`_.

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 <https://caniuse.com/permissions-policy>_,
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 <https://www.w3.org/TR/permissions-policy/>_.

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 <https://wicg.github.io/floc/>_ 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 <https://caniuse.com/document-policy>`_,
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 <https://wicg.github.io/document-policy/>`_.

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

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