Twig is affected by path traversal vulnerability when used with Twig_Loader_Filesystem for loading Twig templates but only if the application is using non-trusted template names (names provided by a end-user for instance).

When affected, it is possible to go up one directory for the paths configured in the application's loader.

For instance, if the filesystem loader is configured with /path/to/templates as a path to look for templates, an attacker can force Twig to include a file stored in /path/to by prepending the path with /../ like in {% include "/../somefile_in_path_to" %}

Note that using anything else (like ../somefile, /../../somefile, or ../../somefile) won’t work and the application will return a proper exception.