CMB2 <= 2.13.1 - Unauthenticated Stored Cross-Site Scripting via 'textarea_code' Field

The CMB2 plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the '<textarea_code field id> (e.g. kl_code, kl_post_code)' parameter in all versions up to, and including, 2.13.1 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. The front-end save path requires only a CMB2 box nonce, which is emitted to all visitors including unauthenticated guests via a simple GET request, making the attack trivially reachable without any credentials on sites that expose a public CMB2 form writing a textarea_code field.