Jupyter Server CVE-2026-35397

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

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.