Smarty is a template engine for PHP, facilitating the separation of presentation (HTML/CSS) from application logic. In affected versions template authors could inject php code by choosing a malicious file name for an extends-tag. Sites that cannot fully trust template authors should update asap. All users are advised to update. There is no patch for users on the v3 branch. There are no known workarounds for this vulnerability.
The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.