CVE-2020-26250

Workarounds

Replacing c.Authenticator.whitelist = ... with c.Authenticator.allowed_users = ... avoids the issue.

In the jupyterhub helm chart prior to 0.10.6, this can be done via hub.extraConfig:

auth:
  allowedUsers:
  - user1
  - user2

hub:
  extraConfig:
    allowedUsers: |
        # set new field not exposed in helm chart < 0.10.6
        set_config_if_not_none(c.Authenticator, "allowed_users", "auth.allowedUsers")

For more information

If you have any questions or comments about this advisory:

• Open a thread on the Jupyter forum
• Email us at [email protected]

What goes wrong?

The deprecated (in jupyterhub 1.2) configuration Authenticator.whitelist, which should be transparently mapped to Authenticator.allowed_users with a warning, is instead ignored by OAuthenticator classes, resulting in the same behavior as if this configuration has not been set. If this is the only mechanism of authorization restriction (i.e. no group or team restrictions in configuration) then all authenticated users will be allowed. Provider-based restrictions, including deprecated values such as GitHubOAuthenticator.org_whitelist are not affected.

Who is impacted?

All users of OAuthenticator 0.12.0 and 0.12.1 with JupyterHub 1.2 (JupyterHub Helm chart 0.10.0-0.10.5) who use the admin.whitelist.users configuration in the jupyterhub helm chart or the c.Authenticator.whitelist configuration directly. Users of other deprecated configuration, e.g. c.GitHubOAuthenticator.team_whitelist are not affected.

If you see a log line like this and expect a specific list of allowed usernames:

[I 2020-11-27 16:51:54.528 JupyterHub app:1717] Not using allowed_users. Any authenticated user will be allowed.

you are likely affected.

The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions. Typical impact: unauthorized data access or execution of privileged operations.

CVE-2020-26250 has a CVSS score of 6.3 (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 (0.12.2); upgrading removes the vulnerable code path.

Kodem intelligence

• Replacing deprecated c.Authenticator.whitelist = ... with c.Authenticator.allowed_users = ... avoids the issue.
• Update oauthenticator to 0.12.2
• Update jupyterhub helm chart to 0.10.6

If any users have been authorized during this time who should not have been, they must be deleted via the API or admin interface, per the documentation.

1. What is CVE-2020-26250? CVE-2020-26250 is a high-severity incorrect authorization vulnerability in oauthenticator (pip), affecting versions >= 0.12.0, < 0.12.2. It is fixed in 0.12.2. The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions.
2. How severe is CVE-2020-26250? CVE-2020-26250 has a CVSS score of 6.3 (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 oauthenticator are affected by CVE-2020-26250? oauthenticator (pip) versions >= 0.12.0, < 0.12.2 is affected.
4. Is there a fix for CVE-2020-26250? Yes. CVE-2020-26250 is fixed in 0.12.2. Upgrade to this version or later.
5. Is CVE-2020-26250 exploitable, and should I be worried? Whether CVE-2020-26250 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-2020-26250 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-2020-26250? Upgrade oauthenticator to 0.12.2 or later.

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