When your Content Security Policy disallows eval(), Unpoly cannot directly run JavaScript code in HTML attributes. This affects [up-on-...] attributes like [up-on-loaded] or [up-on-accepted].
For example, the following callback would crash the fragment update with an error like Uncaught EvalError: call to Function() blocked by CSP:
<a href="/path" up-follow up-on-loaded="alert()">Click me</a>
This page outlines several solutions for this.
Solution 1: Move the callback into your JavaScript
One solution is to move the handler from the HTML to the JavaScript file that we loaded via an allowed <script src>.
In the example above we could invent a new .alert-on-loaded class that we assign to the link:
<a href="/path" up-follow class="alert-on-loaded">Click me</a>
Then this JavaScript listener intercepts clicks on elements with that class:
up.on('up:link:follow', '.alert-on-loaded', (event) => {
event.renderOptions.onLoaded = () => alert()
})
Solution 2: Prefix a CSP nonce
Unpoly lets you work around this by prefixing your callback with a CSP nonce:
<a href="/path" up-follow up-on-loaded="nonce-kO52Iphm8B alert()">Click me</a>
The nonce- prefix is static, followed by your random nonce in Base64, followed by a space and your original JavaScript code.
The embedded nonce must match a script-src nonce of the response that you're currently rendering:
Content-Type: text/html
Content-Security-Policy: script-src 'self' 'nonce-kO52Iphm8B'
...
<a href="/path" up-follow up-on-loaded="nonce-kO52Iphm8B alert()">Click me</a>
For this to work you must also include the <meta name="csp-nonce"> tag in the <head> of the initial page that booted Unpoly:
<head>
<meta name="csp-nonce" content="nonce-kO52Iphm8B">
...
</head>
To provide the nonce through another method, configure up.protocol.config.cspNonce.
When responding to a fragment update, you may use a CSP nonce unique to that latest response.
You do not need to reuse the nonce of the initial page that booted Unpoly.
Neither to you need to update the <meta> tag with the latest nonce.
Tip
If you're using the unpoly-rails gem you can prefix a nonce
using the up.safe_callback() helper.
Solution 3: Relax your CSP
Depending on your requirements, you may decide to prioritize developer convenience over a strict CSP.
You can use all [up-on-] attributes without restriction by passing the 'unsafe-eval' directive:
Content-Security-Policy: script-src 'self' 'unsafe-eval'
Note that this allows the use of eval() from all allowed scripts on your page, not just from Unpoly. However, it does not allow the creation of inline <script> tags.
