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Add support for automatic nonce generation for Rails UJS #32018
Because the UJS library creates a script tag to process responses it normally requires the script-src attribute of the content security policy to include 'unsafe-inline'.
To work around this we generate a per-request nonce value that is embedded in a meta tag in a similar fashion to how CSRF protection embeds its token in a meta tag. The UJS library can then read the nonce value and set it on the dynamically generated script tag to enable it to execute without needing 'unsafe-inline' enabled.
Nonce generation isn't 100% safe - if your script tag is including user generated content in someway then it may be possible to exploit an XSS vulnerability which can take advantage of the nonce. It is however an improvement on a blanket permission for inline scripts.
It is also possible to use the nonce within your own script tags by using
An example of how nonce can be abused is in this blog post: