CVE-2026-35397

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.

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.

### 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 es el backend para las aplicaciones web de Jupyter. En las versiones 2.17.0 y anteriores, una vulnerabilidad de salto de ruta en la API REST permite a un usuario autenticado escapar del 'root_dir' configurado y acceder a directorios hermanos cuyos nombres comienzan con el mismo prefijo que el 'root_dir'. Por ejemplo, con un 'root_dir' llamado test , la API permite el acceso a un directorio hermano llamado testtest a través de una solicitud manipulada al endpoint '/api/contents' utilizando componentes de ruta codificados. Un atacante puede leer, escribir y eliminar archivos en los directorios hermanos afectados. Las implementaciones multi-inquilino que utilizan esquemas de nombres predecibles están particularmente en riesgo, ya que un usuario con un directorio llamado user1 podría acceder a los directorios de user10 a user19 y más allá. Un usuario que puede elegir un nombre de carpeta de un solo carácter podría obtener acceso a un número significativo de directorios hermanos. La versión 2.18.0 contiene una corrección. Como solución alternativa, asegúrese de que los nombres de las carpetas no compartan un prefijo común con ningún directorio hermano.