next.js/packages/next/src/server/app-render/get-script-nonce-from-header.tsx

import { ESCAPE_REGEX } from '../htmlescape'

export function getScriptNonceFromHeader(
  cspHeaderValue: string
): string | undefined {
  const directives = cspHeaderValue
    // Directives are split by ';'.
    .split(';')
    .map((directive) => directive.trim())

  // First try to find the directive for the 'script-src', otherwise try to
  // fallback to the 'default-src'.
  const directive =
    directives.find((dir) => dir.startsWith('script-src')) ||
    directives.find((dir) => dir.startsWith('default-src'))

  // If no directive could be found, then we're done.
  if (!directive) {
    return
  }

  // Extract the nonce from the directive
  const nonce = directive
    .split(' ')
    // Remove the 'strict-src'/'default-src' string, this can't be the nonce.
    .slice(1)
    .map((source) => source.trim())
    // Find the first source with the 'nonce-' prefix.
    .find(
      (source) =>
        source.startsWith("'nonce-") &&
        source.length > 8 &&
        source.endsWith("'")
    )
    // Grab the nonce by trimming the 'nonce-' prefix.
    ?.slice(7, -1)

  // If we could't find the nonce, then we're done.
  if (!nonce) {
    return
  }

  // Don't accept the nonce value if it contains HTML escape characters.
  // Technically, the spec requires a base64'd value, but this is just an
  // extra layer.
  if (ESCAPE_REGEX.test(nonce)) {
    throw new Error(
      'Nonce value from Content-Security-Policy contained HTML escape characters.\nLearn more: https://nextjs.org/docs/messages/nonce-contained-invalid-characters'
    )
  }

  return nonce
}