CMB2 < 2.13.0 - Subscriber+ Arbitrary Option Corruption via oEmbed Handler

Description

The plugin does not perform any capability check on one of its AJAX actions, allowing users with a role as low as Subscriber to create arbitrary WordPress options and corrupt existing ones, which can break core site settings and take the site offline.
Exploitation requires the site's theme or another plugin to have declared an oEmbed field, as the plugin registers none of its own. The stored value is never attacker-controlled, so the issue does not lead to privilege escalation.