Twig is a template language for PHP. Versions 1.x prior to 1.44.7, 2.x prior to 2.15.3, and 3.x prior to 3.4.3 encounter an issue when the filesystem loader loads templates for which the name is a user input. It is possible to use the `source` or `include` statement to read arbitrary files from outside the templates' directory when using a namespace like `@somewhere/../some.file`. In such a case, validation is bypassed. Versions 1.44.7, 2.15.3, and 3.4.3 contain a fix for validation of such template names. There are no known workarounds aside from upgrading.
The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.