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How CVE-2026-54338 Works
JupyterHub is a multi-user server for Jupyter notebooks. Prior to version 5.5.0, the software contained a vulnerability in its form-based login authenticators—such as the default PAM Authenticator. This vulnerability, identified as CVE-2026-54338, allows an unauthenticated attacker to cause a denial of service.
The core of the issue lies in how JupyterHub handles failed login attempts. When a login fails, the username provided in the request is logged. However, versions before 5.5.0 do not properly validate or limit the size of this username input. An attacker can exploit this by sending a large number of login requests, each with an extremely long, attacker-controlled string as the username.
Because there is no bound on the size of the username that can be logged, each failed attempt writes a massive amount of data to the logging system. Over time, this unbounded logging output consumes significant disk space and can overwhelm the system’s logging and storage resources. This resource exhaustion can lead to the entire JupyterHub service becoming unstable or completely unavailable, effectively denying service to legitimate users.
It is important to note that this vulnerability only affects form-based authenticators. The more widely used OAuthenticator, which does not use a login form, is not affected. The issue is fixed in JupyterHub version 5.5.0.
DailyCVE Form
Platform: JupyterHub
Version: < 5.5.0
Vulnerability: Unauthenticated DoS
Severity: Medium (CVSS 5.3)
Date: 2026-08-07
Prediction: 2026-06-10 (already patched)
What Undercode Say:
Analytics
- Attack Vector: Network
- Attack Complexity: Low
- Privileges Required: None
- User Interaction: None
- Scope: Unchanged
- Confidentiality Impact: None
- Integrity Impact: None
- Availability Impact: Low
- CVSS Base Score: 5.3
Exploit: (Educational Purposes!)
An attacker can exploit this vulnerability by sending a barrage of POST requests to the login endpoint with an oversized username parameter. Below is a conceptual example using curl:
for i in {1..1000}; do
curl -X POST https://jupyterhub.example.com/hub/login \
-d "username=$(python -c "print('A'1000000)")" \
-d "password=anything"
done
This script will repeatedly attempt to log in with a one-million-character username. Each failed attempt will log this massive string, quickly filling up the server’s disk and causing a denial of service.
Protection
- Upgrade: The primary and most effective mitigation is to upgrade JupyterHub to version 5.5.0 or later.
- Workaround: If an immediate upgrade is not possible, switch to an authenticator that does not use a login form, such as OAuthenticator.
- Input Validation: As a general security practice, implement strict input validation and length limits on all user-supplied data, especially usernames.
Impact
- Resource Exhaustion: Unauthenticated attackers can cause a denial of service by exhausting the system’s logging and storage resources.
- Service Disruption: The resource exhaustion can lead to the JupyterHub service becoming unstable or completely unavailable, impacting all users.
- No Data Breach: The vulnerability does not allow for data theft or modification; it is purely a denial-of-service issue.
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Sources:
Reported By: github.com
Extra Source Hub:
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