CVE-2026-35397

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

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

Workarounds

Use folder names that do not overlap.

Acknowledgments

Thank you to @stef41 for providing a useful PoC.

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.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2026-35397 has a CVSS score of 7.1 (High). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.18.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-35397? CVE-2026-35397 is a high-severity path traversal vulnerability in jupyter-server (pip), affecting versions <= 2.17.0. It is fixed in 2.18.0. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2026-35397? CVE-2026-35397 has a CVSS score of 7.1 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of jupyter-server are affected by CVE-2026-35397? jupyter-server (pip) versions <= 2.17.0 is affected.
4. Is there a fix for CVE-2026-35397? Yes. CVE-2026-35397 is fixed in 2.18.0. Upgrade to this version or later.
5. Is CVE-2026-35397 exploitable, and should I be worried? Whether CVE-2026-35397 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-35397 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-35397? Upgrade jupyter-server to 2.18.0 or later.