nebula-mesh: Web UI and API responses lack security headers (CSP, X-Frame-Options, HSTS, etc.)

None of the response paths in internal/web/ or internal/api/ set the standard browser-security headers. grep for Content-Security-Policy, X-Frame-Options, Strict-Transport-Security, X-Content-Type-Options, Referrer-Policy returns zero matches across the codebase.

Impact

The admin UI signs CA certificates, mints API keys (returned inline once per page), displays TOTP QR codes, and exposes operator-management forms. Missing X-Frame-Options: DENY / frame-ancestors 'none' is a real clickjacking lever against an admin browsing /ui/operators/* or /ui/cas/*. Missing X-Content-Type-Options: nosniff allows MIME confusion on any user-supplied content surface. Missing HSTS on TLS deployments leaves a downgrade window.

Affected

All released versions up to v0.3.0.

Suggested fix

A single response-header middleware mounted at the chi router root in both /ui/* and /api/* paths:

func securityHeadersMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
        h := rw.Header()
        h.Set("Content-Security-Policy",
            "default-src 'self'; frame-ancestors 'none'; base-uri 'none'; form-action 'self'")
        h.Set("X-Content-Type-Options", "nosniff")
        h.Set("Referrer-Policy", "same-origin")
        h.Set("X-Frame-Options", "DENY")  // belt-and-braces; CSP frame-ancestors is the modern path
        if r.TLS != nil {
            h.Set("Strict-Transport-Security", "max-age=31536000; includeSubDomains")
        }
        next.ServeHTTP(rw, r)
    })
}

The inline <script> in layout.html for CSRF wiring (added in the CSRF advisory) will need either a nonce, a hash in CSP, or external-file extraction. Easiest path: a nonce per request (crypto/rand, base64) injected into both the CSP header and the script's nonce="" attribute.