8000 [AssetMapper] Document how to make it work with a Content Security Policy by nicwortel · Pull Request #19865 · symfony/symfony-docs · GitHub
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[AssetMapper] Document how to make it work with a Content Security Policy #19865

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May 7, 2024
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[AssetMapper] Document how to make it work with a Content Security Policy #19865

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23 changes: 23 additions & 0 deletions frontend/asset_mapper.rst
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Expand Up @@ -1061,6 +1061,27 @@ have *one* importmap, so ``importmap()`` must be called exactly once.
If, for some reason, you want to execute *only* ``checkout.js``
and *not* ``app.js``, pass only ``checkout`` to ``importmap()``.

Using a Content Security Policy (CSP)
-------------------------------------

If you're using a `Content Security Policy`_ (CSP) to prevent cross-site
scripting attacks, the inline ``<script>`` tags rendered by the ``importmap()``
function will likely violate that policy and will not be executed by the browser.

To allow these scripts to run without disabling the security provided by
the CSP, you can generate a secure random string for every request (called
a *nonce*) and include it in the CSP header and in a ``nonce`` attribute on
the ``<script>`` tags.
The ``importmap()`` function accepts an optional second argument that can be
used to pass attributes to the rendered ``<script>`` tags.
You can use the `NelmioSecurityBundle`_ to generate the nonce and include
it in the CSP header, and then pass the same nonce to the Twig function:

.. code-block:: twig

{# the csp_nonce() function is defined by the NelmioSecurityBundle #}
{{ importmap('app', {'nonce': csp_nonce('script')}) }}
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@nicwortel nicwortel May 6, 2024 •
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The csp_nonce function is unknown because it is part of the NelmioSecurityBundle.

Should I remove it from the example and instead just refer to the documentation of the NelmioSecurityBundle?

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I think it's OK because you linked to the bundle in the previous paragraph ... but I'd add a comment in this code to explain that this function comes from that bundle:

.. code-block:: twig

    {# the csp_nonce() function is defined by the NelmioSecurityBundle #}
    {{ importmap('app', {'nonce': csp_nonce('script')}) }}

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I added the comment 👍 however, that doesn't solve the error reported by the "Code Blocks" workflow, so the build still fails.


The AssetMapper Component Caching System in dev
-----------------------------------------------

Expand Down Expand Up @@ -1143,3 +1164,5 @@ command as part of your CI to be warned anytime a new vulnerability is found.
.. _`dist/css/bootstrap.min.css file`: https://www.jsdelivr.com/package/npm/bootstrap?tab=files&path=dist%2Fcss#tabRouteFiles
.. _`dynamic import`: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import
.. _`package.json configuration file`: https://docs.npmjs.com/creating-a-package-json-file
.. _Content Security Policy: https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP
.. _NelmioSecurityBundle: https://symfony.com/bundles/NelmioSecurityBundle/current/index.html#nonce-for-inline-script-handling
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