OpenMage magento-lts before 20.19.0 Block Whitelist Bypass via {{layout}} Directive

OpenMage magento-lts before 20.19.0 and 21.0.0-beta1 before 21.0.0-beta3 contains an incorrect authorization vulnerability in the template filter's {{layout}} directive, which renders arbitrary layout handles without enforcing the admin/block whitelist that the {{block}} directive applies. Administrators with rights to edit email, CMS, widget or newsletter content can embed {{layout}} with arbitrary handles to render disallowed blocks and disclose limited information.