GHSA-v5gw-mw7f-84px Starlette has Path Traversal vulnerability in StaticFiles

• PyPI starlette: >= 0.13.5, < 0.27.0 → fixed in 0.27.0

Summary When 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. Details The root cause of this issue is the usage of `os.path.commonprefix()`: https://github.com/encode/starlette/blob/4bab981d9e870f6cee1bd4cd59b87ddaf355b2dc/starlette/staticfiles.py#L172-L174 As 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. PoC In 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: ```py import uvicorn from starlette.applications import Starlette from starlette.routing import Mount from starlette.staticfiles import StaticFiles routes = [ 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: ```shell curl --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. Impact Confidentiality is breached: An attacker may obtain files that should not be open to the public. Credits Security 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.