CSP nonce cached on pages when disabling unsafe-inline

[Dnd-Coch]

Preconditions

• Magento 2.4.7
• Custom configuration as following in etc/config.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
    <default>
       <csp>
           <policies>
               <storefront>
                   <scripts>
                       <inline>0</inline>
                   </scripts>
               </storefront>
           </policies>
       </csp>
    </default>
</config>

Steps to reproduce

1. Add above content in the etc/config.xml of a custom module
2. Clear the cache
3. Load the site homepage in your browser
4. Inspect the Content-Security-Policy-Report-Only HTTP header of the page

Expected result

• The unsafe-inline expression is not present in the script-src policy
• Hashes of the inline scripts of the page are present in the script-src policy

Actual result

• The unsafe-inline expression is not present in the script-src policy
• A generated nonce is present in the script-src policy instead of hashes
• The nonce is cached by FPC

As it is mentioned in this architecture document, the nonce approach is incompatible with Magento's page cache.

So the fact that nonce takes precedence over hashes in this method \Magento\Csp\Helper\InlineUtil::processTag is a problem for merchants who want to remove the unsafe-inline expression of their policies to improve overall security.

Nonce should be generated only for non-cacheable pages.

Examples

See \Magento\Csp\Helper\InlineUtil::processTag method.

Proposed solution

No response

Release note

No response

Triage and priority

• Severity: S0 - Affects critical data or functionality and leaves users without workaround.
• Severity: S1 - Affects critical data or functionality and forces users to employ a workaround.
• Severity: S2 - Affects non-critical data or functionality and forces users to employ a workaround.
• Severity: S3 - Affects non-critical data or functionality and does not force users to employ a workaround.
• Severity: S4 - Affects aesthetics, professional look and feel, “quality” or “usability”.

[m2-assistant]

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[m2-assistant]

Hi @engcom-November. Thank you for working on this issue.
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[engcom-November]

Hello @Dnd-Coch,

Thank you for the report and collaboration!

This issue is not reproducible in 2.4-develop.
Added the config file with inline as 0, but in the Content-Security-Policy-Report-Only header we were able to see the hash instead of the generated nonce.

Please take a look at the screenshot :

Please let us know if we are missing anything.

Thank you.

[hostep]

@engcom-November: you can't test this yet on 2.4-develop, some code which was released in 2.4.7 isn't merged yet into 2.4-develop, you should either test with 2.4.7, or wait until the merge has happened.
Here's an example of changes that haven't been merged in 2.4-develop that might be relevant to this issue: 25cf7f0

[engcom-November]

Hello @Dnd-Coch,

Verified in 2.4.7, as suggested by @hostep,
We are able to see a nonce in Content-Security-Policy-Report-Only header along with the hashes.
As nonce is incompatible with Magento's page cache, this issue can be confirmed.

Please take a look at the screenshot:

Thank you.