Silverstripe Advanced Workflow: Remote code execution via advanced workflow email template

Silverstripe Advanced Workflow is a highly configurable step-based workflow module. Prior to 6.4.5, 7.1.3, and 7.2.1, an attacker with permission to author the advanced workflow email template can place a specially crafted server-side template payload in NotifyUsersWorkflowAction.EmailTemplate. When NotifyUsersWorkflowAction renders the field through the Silverstripe template engine SSTemplateParser, the payload can cause PHP evaluation and arbitrary code execution on the server; the regression coverage is in tests/php/WorkflowEngineTest.php. This issue is fixed in versions 6.4.5, 7.1.3, and 7.2.1.