Resolved - Drupal.org has released Drupal version 8.6.10. This release is classified as a security release for all previous versions of Drupal 8.[1] Pantheon has made the update available for one-click update in site dashboards. We urge all users to update all environments as soon as possible. Instructions for applying core updates can be found in our Support Center.[2] This release addresses SA-CORE-2019-003[3]; all users are urged to upgrade their sites as soon as possible.

[1] - https://www.drupal.org/project/drupal/releases/8.6.10
[2] - https://pantheon.io/docs/upstream-updates/
[3] - https://www.drupal.org/sa-core-2019-003
Feb 20, 12:30 PST