Jupyter: Jupyter Server: Path Traversal via incorrect startswith() root directory check allows access to…
Jupyter
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- Severity
- High, CVSS 8.8
- CVSS vector
- CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- Record
- CVE-2026-35397
- Affects
- Jupyter
- Known exploited
- Not listed in the CISA Known Exploited Vulnerabilities catalog
- Published
- Record first published by the source
- May 5, 2026
- Source last modified
- August 28, 2026
Description
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.
Description quoted from the NVD record.
Who is affected
Jupyter (also called Jupyter Server) is a tool that lets people write and run code alongside notes in a web browser. This flaw affects versions 2.17.0 and earlier. Someone who has a login can use the part that opens and saves files to reach folders outside their own, as long as those folders sit next to it and have names that start with the same letters. For example, a user with a folder named "user1" could reach folders for user10 through user19. They could read, change and delete files there. Setups shared by many users with predictable folder names are most at risk. This flaw is rated High.
What to do
- The person who installed or runs Jupyter for your organization should check which version is in use.
- If it is version
2.17.0or earlier, update Jupyter Server to version2.18.0, which fixes this flaw. - Until you can update, ensure folder names do not share a common prefix with any sibling directory. In other words, no folder should have a name that begins with the full name of another folder next to it.
- If an IT provider or vendor runs Jupyter for you, send them this advisory and ask when the update will be done.
Analysis written from the source record as of August 28, 2026 and reviewed before publishing.