(CSP) Content Security Policy. Way to handle inline styles added from external liblary

Question

Is there way to handle inline script/styles added from external library? In my own styles i just use nonce but i can't add it to external library.

I use tooltip.io and problem appears when liblary try to run:

function() {
                var n = e("./styles/css/styles.scss")
                  , t = document.createElement("style");
                t.innerHTML = n,
                document.head.appendChild(t)
            }(),

CSP shows

[Report Only] Refused to apply inline style because it violates the following Content Security Policy directive: "style-src 'self' 'nonce-b123d558a63bc7e84aa7' ". Either the 'unsafe-inline' keyword, a hash ('sha256-SamqqFx+xVKq8AnoIWgeammNlDl/h4AP94HthfQO43Q='), or a nonce ('nonce-...') is required to enable inline execution.

Is there any way to handle this kind of errors?

Answer 1

Recently I faced the similar issue for Jquery unobtrusive javascript file I used in one of project it was adding inline style="diplay:none" on html element Ul So, I prefered Hash approach to allow inline style in this you need to add style-src 'self' 'unsafe-hashes' 'sha-256 {sha hashed string}'

if you use chrome in chrome developer tools in console where csp violation is shown, in last line of error message it also shows sha-256 string you can add to your content security policy header, so that inline css or js can be allowed by csp, this unsafe-hashes approach is better than unsafe-nonces as nonce needs to be unique per request and hash is irreversible so content matching hash only allowed always

I feel you should try to open your page which gives error in chrome and see if it shows sha-256 hash as last line in console error message, if you want to manually generate hash like sha-512, you can generate hash for the styles.css generated after./styles/css/styles.scss is processed and add this hash in csp attribute like

Content-Security-Policy: default-src 'none'; style-src 'self' 'sha-512{Manually generated sha-512}';

last option you can use is unsafe-inline in style src but you can give try for unsafe-hashes which will be better than unsafe-inline

Answer 2

You must specify what is allowed in the head section of your html..

To allow inline styles:

Content-Security-Policy: style-src 'unsafe-inline';

The above Content Security Policy will allow inline styles like the element, and the style attribute on any element:

<style>
#inline-style { background: red; }
</style>

<div style="display:none">Foo</div>

You can use a nonce-source to only allow specific inline style blocks:

Content-Security-Policy: style-src 'nonce-2726c7f26c'

You will have to set the same nonce on the element:

<style nonce="2726c7f26c">
#inline-style { background: red; }
</style>

or by domain:

Content-Security-Policy: style-src https://example.com/

To allow inline scripts and inline event handlers:

Content-Security-Policy: script-src 'unsafe-inline';

The above Content Security Policy will allow inline elements

<script> 
  var inline = 1; 
</script>

You can use a nonce-source to only allow specific inline script blocks:

Content-Security-Policy: script-src 'nonce-2726c7f26c'

You will have to set the same nonce on the element:

<script nonce="2726c7f26c">
  var inline = 1;
</script>

or by domain:

Content-Security-Policy: script-src https://example.com/

Answer 3

I will answer question that I've given the bounty. I think that the only way to make library work with CSP is to add nonce option to CSP. And add suport for CSP with nonce to the library. To do that the library will need to change all inline style with <style> tag, with that nonce.

jsLibrary({
 nonce: '<XXXX>'
});

<style nonce="<XXXX>">

</style>

if you know any other way to use CSP with 3rd party library with minimal modification that can be done on the server, feel free to add an answer.