[AssetMapper] Document usage of strict-dynamic in a CSP
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AssetMapper will include special importmap entries for CSS files, which get resolved to `data:application/javascript`. See https://symfony.com/doc/current/frontend/asset_mapper.html#handling-css. Browsers will report those as CSP violations, as `data:` scripts can also be used for XSS attacks. For the same reason, allowing `data:` in the CSP is not a safe solution. symfony/symfony#58416 (comment) provides a solution: using `strict-dynamic` in the `script-src` directive will allow the importmap to include other resources. This commit adds that solution to the documentation.
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nicwortel
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Oct 30, 2024
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Nicely explained 👏 Thanks Nic! |
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AssetMapper will include special importmap entries for CSS files, which get resolved to
data:application/javascript.See https://symfony.com/doc/current/frontend/asset_mapper.html#handling-css.
Browsers will report those as CSP violations, as
data:scripts can also be used for XSS attacks. For the same reason, allowingdata:in the CSP is not a safe solution.symfony/symfony#58416 (comment) provides a solution: using
strict-dynamicin thescript-srcdirective will allow the importmap to include other resources.This PR adds that solution to the documentation.