Twig is a template language for PHP. When using the ?? operator, output escaping was missing for the expression on the left side of the operator. This vulnerability is fixed in 3.19.0.
https://github.com/twigphp/Twig/security/advisories/GHSA-3xg3-cgvq-2xwr
https://github.com/twigphp/Twig/commit/38576b12f05df3cc871bf68f39ccb46b418334a3