CVE-2026-35397

### 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.

Jupyter Server is the backend for Jupyter web applications. In versions 2.17.0 and earlier, a path traversal vulnerability in the REST API allows an authenticated user to escape the configured root_dir and access sibling directories whose names begin with the same prefix as the root_dir. For example, with a root_dir named "test", the API permits access to a sibling directory named "testtest" through a crafted request to the /api/contents endpoint using encoded path components. An attacker can read, write, and delete files in affected sibling directories. Multi-tenant deployments using predictable naming schemes are particularly at risk, as a user with a directory named "user1" could access directories for user10 through user19 and beyond. A user who can choose a single-character folder name could gain access to a significant number of sibling directories. Version 2.18.0 contains a fix. As a workaround, ensure folder names do not share a common prefix with any sibling directory.