A defense-in-depth check was added to mitigate inadequate session validation handling by 3rd party checkout modules. This impacts Magento 1.x prior to 1.9.4.2, Magento Commerce prior to 1.14.4.2, Magento 2.1 prior to 2.1.18, Magento 2.2 prior to 2.2.9 and Magento 2.3 prior to 2.3.2.
Magento Community Edition and Enterprise Edition before 2.0.10 and 2.1.x before 2.1.2 have XSS via e-mail templates that are mishandled during a preview, aka APPSEC-1503.