Harden the default Content-Security-Policy: use a per-request nonce instead of 'unsafe-inline' / 'unsafe-eval'

Summary

pgAdmin 4's default CONTENT_SECURITY_POLICY allowed 'unsafe-inline' and 'unsafe-eval' in script-src, which leaves the policy unable to stop an injected inline script — the main protection a CSP is meant to provide.

Inline scripts now run under a per-request nonce instead:

• A per-request nonce is generated with secrets.token_urlsafe and cached on flask.g, so the same value is emitted in the templates and in the CSP response header.
• A {nonce} placeholder in CONTENT_SECURITY_POLICY is substituted at runtime.
• Inline <script> and <style> tags carry the nonce.
• The new default is script-src 'self' 'nonce-{nonce}', with neither 'unsafe-inline' nor 'unsafe-eval'.

Two deliberate limits:

• style-src keeps 'unsafe-inline'. MUI and React inject runtime styles and inline style="" attributes that cannot carry a nonce.
• Development bundles are built with webpack's eval devtool, which a strict nonce policy blocks. get_content_security_policy() appends 'unsafe-eval' to script-src automatically when config.DEBUG is set and the policy uses a nonce, so developers no longer have to add it by hand in config_local.py. Production is unaffected, and a custom non-nonce policy passes through untouched.

Fix

Fixed in pgAdmin 4 9.18 by commit fb0ca5c4c.

Unit tests cover nonce generation, caching and per-request freshness, {nonce} substitution, pass-through of custom/None/empty policies, header emission, and the development-mode behaviour.

Not a vulnerability report

This is a hardening change to pgAdmin's own default policy rather than a fix for a reported flaw, so no CVE has been requested.

The requested hardening is already recorded as fixed by commit fb0ca5c4c in pgAdmin 4 9.18. Review that commit and its unit tests, which cover nonce generation and caching, policy substitution, header emission, custom policies, and development-mode behavior; no newcomer implementation work remains on this issue.