Affected: Encode Starlette

Directory traversal vulnerability in Starlette versions 0.13.5 and later and prior to 0.27.0 allows a remote unauthenticated attacker to view files in a web service which was built using Starlette.

Affected: PyPI: starlette

### 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…

Affected: PyPI: starlette

Directory traversal vulnerability in Starlette versions 0.13.5 and later and prior to 0.27.0 allows a remote unauthenticated attacker to view files in a web service which was built using Starlette.