CVE-2026-86049 Jupyter Server: 5xx request logging leaks token-bearing Referer header values
Attackers with log access can recover Jupyter tokens from Referer headers in 5xx logs, enabling account takeover.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-86049?
CVE-2026-86049 is a high-severity vulnerability in jupyter-server jupyter_server, published on 17 Sep 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Official description
Jupyter Server is the backend for Jupyter web applications. Prior to version 2.21.0, the 5xx request logging path in jupyter_server/log.py copies the Referer header into a JSON header block without applying the token scrubbing used for the request URI. A request that returns HTTP 500 while the Referer contains a token-bearing URL can therefore write that token to server logs in plaintext. An attacker who can read those logs can recover the token and use the affected user's Jupyter Server permissions. This issue is fixed in version 2.21.0.
Weakness: CWE-532
Which versions are affected by CVE-2026-86049?
3 open source packages ship an affected version, including jupyter-server and jupyter_server.
| Package | Affected | Fixed in |
|---|---|---|
jupyter-server PyPI | All versions | 2.21.0 |
jupyter-server PyPI | All versions | 2.21.0 |
jupyter_server PyPI | All versions | 2.21.0 |
How do I fix CVE-2026-86049?
Upgrade jupyter-server to 2.21.0 or later.
Is CVE-2026-86049 being exploited?
There is no known exploitation of CVE-2026-86049 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-86049, GHSA-c3mw-737p-c7g2, MINI-c7g2-m6hg-q67w, PYSEC-2026-4054, UBUNTU-CVE-2026-86049.