Meta Box < 5.9.4 - Contributor+ Arbitrary Posts' Custom Field Disclosure

Proof of Concept

1. ADMIN: Install Meta Box
2. ADMIN: Add Meta Box fields through code or the premium add-on (https://gist.github.com/sc0ttkclark/f4f1b94d3a8bc7f00614acf5d80dbd2e)
3. CONTRIBUTOR: Add shortcode to any post and specify/guess any post ID + field key and save
4. CONTRIBUTOR: Preview the post and see that custom field is output without any further checks for access

Example shortcode: `[rwmb_meta object_id="ANY_POST_ID" id="ANY_META_BOX_FIELD_KEY"]`

Example shortcode for my Gist: `[rwmb_meta object_id="1234" id="test_field"]`