Dancer2 versions from 2.1.0 before 2.2.0 for Perl serve files from outside public_dir via relative path segments in the File route handler.
The handler joins the request path onto public_dir without collapsing relative segments, and checks only that the result is a readable regular file. A request for /../outside.txt escapes public_dir, and percent-encoding the dots reaches the same file.
The handler is off by default. An application is affected once it names File in route_handlers and sets static_handler to 0, which otherwise refuses a dot segment before the route runs.
Any file the worker process can read is served to an unauthenticated request, including the application's config.yml above public_dir.
Upgrade to Dancer2 2.2.0 or later.