CVE-2026-35397 is a path traversal vulnerability (CWE-22) in jupyter-server's REST API affecting versions prior to 2.18.0. An authenticated user can exploit this flaw by crafting requests to the /api/contents endpoint with encoded path components to escape the configured root_dir and access sibling directories whose names begin with the same prefix as the root_dir. This allows unauthorized file operations including reading, writing, and deletion in those sibling directories. Multi-tenant environments with predictable directory naming are especially vulnerable. Version 2.18.0 contains a fix for this issue.

An attacker with authenticated access can bypass directory restrictions to access and manipulate files in sibling directories sharing a common prefix with the root directory. This can lead to unauthorized data exposure, modification, or deletion within those directories. The vulnerability poses a high risk in multi-tenant deployments where directory names are predictable, potentially allowing access to multiple users' data.

Upgrade to jupyter-server version 2.18.0 or later, which contains a fix for this vulnerability. Until upgrading, avoid using folder names that share common prefixes with sibling directories to reduce the risk of exploitation.