CVE-2023-29159: Path Traversal

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([fullpath, 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 ├── staticdisallow │ ├── 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/../staticdisallow/' curl --path-as-is 'localhost:8000/static/../static1.txt' The static1.txt and the directory staticdisallow 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.