Clash Royale CLAN TAG#URR8PPP

Using nonce or hash values in content-security-policy for inline styles

I am adding content-security-policy settings for my web app and would like to be strict about inline scripts and inline styles. I have inline scripts working properly but am having trouble with inline styles. For background, my web app uses Elixir/Phoenix on the back end and includes React components on the front end.

Nonce for Inline Scripts - Works

I have just a smattering of inline scripts to get the React components bootstrapped. Per CSP guidelines, I'm using nonce values to handle these inline scripts. Specifically, I generate a new nonce value server side on each page load and include it in the content-security-policy header and also inject it into a nonce attribute in the script tags using server side rendering. This works fine.

nonce

content-security-policy

nonce

Nonce for Inline Styles - Doesn't Work

Inlines styles are another story. Inlines styles are pretty common in React. My hope was that I could use a similar nonce approach for this, but so far I have not been able to get it to work. Nonce values are listed in the style-src page at https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/style-src so it seems like this should work, though that section of the doc references scripts so maybe not. I have tried injecting a nonce attribute into the generated HTML, but this does not work. I continue to receive the normal error messages.

nonce


Refused to apply inline style because it violates the following Content Security Policy directive: "default-src 'self' 'nonce-my-long-ugly-random-nonce-value'". Either the 'unsafe-inline' keyword, a hash ('sha256-specific-hash'), or a nonce ('nonce-...') is required to enable inline execution. Note also that 'style-src' was not explicitly set, so 'default-src' is used as a fallback.



Refused to apply inline style because it violates the following Content Security Policy directive: "default-src 'self' 'nonce-my-long-ugly-random-nonce-value'". Either the 'unsafe-inline' keyword, a hash ('sha256-specific-hash'), or a nonce ('nonce-...') is required to enable inline execution. Note also that 'style-src' was not explicitly set, so 'default-src' is used as a fallback.


I've been searching for a while and have yet to find any hint that nonce values work for inline styles, other than what I mention above. Can anyone confirm that nonce values work for inline styles?

If they don't work, it seems like using hashes is my next best bet. I can get hashes to work, by looking at each error message like the one above and adding the hash to the content-security-policy header. That is not a very scalable solution though, and if I have to go that route I'd like to know if anyone has come up with a way to automate generating the hashes without having to manually navigate everywhere in the app and look at the error message to get the hash.

content-security-policy

Any help you can provide is appreciated.

Thanks.
Justin

nonce

nonce-source

style-src

1 Answer
1

After looking at Content Security Policy allow inline style without unsafe-inline it appears this isn't supported, perhaps because the spec seems to imply that it is referring only to inline styles in a <style> tag, similar how the spec refers to inline scripts in a <script> tag.

<style>

<script>

Assuming that is correct, it looks like I'll have to go the hash route or add 'unsafe-inline' just for style-src.

'unsafe-inline'

style-src

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