CVE-2026-23990

A privilege escalation vulnerability exists in the Flux Operator Web UI authentication code that allows an attacker to bypass Kubernetes RBAC impersonation and execute API requests with the operator's service account privileges.

After OIDC token claims are processed through CEL expressions, there is no validation that the resulting username and groups values are non-empty. When both values are empty, the Kubernetes client-go library does not add impersonation headers to API requests, causing them to be executed with the flux-operator service account's credentials instead of the authenticated user's limited permissions.

Attack Scenario

Prerequisite: Cluster admins must configure the Flux Operator with an OIDC provider that issues tokens lacking the expected claims (e.g., email, groups), or configure custom CEL expressions that can evaluate to empty values.

1. Cluster admin configures OIDC authentication with a provider that does not include email or groups claims in tokens
2. User authenticates with a valid token from that provider
3. The default CEL expressions evaluate to empty values:
   • Username: has(claims.email) ? claims.email : '' → ""
   • Groups: has(claims.groups) ? claims.groups : [] → []
4. Authentication succeeds (token signature is valid)
5. A userClient is created with empty impersonation config
6. All subsequent API requests bypass impersonation and execute as the flux-operator service account
7. User gains operator-level read access across all namespaces

Workarounds

The workaround is to make the email and groups claims required in the web config impersonation section.

Example config:

apiVersion: web.fluxcd.controlplane.io/v1
kind: Config
spec:
  baseURL: https://flux.example.com
  authentication:
    type: OAuth2
    oauth2:
      provider: OIDC
      clientID: "<redacted>"
      clientSecret: "<redacted>"
      issuerURL: "https://login.microsoftonline.com/<redacted>/v2.0"
      scopes: [openid, profile, email, offline_access]
      impersonation:
        username: claims.email
        groups: claims.groups

References

See the Pull Request fixing this vulnerability https://github.com/controlplaneio-fluxcd/flux-operator/pull/610

Credits

This vulnerability was discovered by the Flux Operator maintainers during a debugging session with end-users.

• Privilege Escalation: Any authenticated user can escalate to operator-level read permissions and perform suspend/resume/reconcile actions
• Data Exposure: Unauthorized read access to Flux resources across all namespaces, bypassing RBAC restrictions
• Information Disclosure: View sensitive GitOps pipeline configurations, source URLs, and deployment status across the entire cluster

The application assigns, modifies, tracks, or checks privileges incorrectly, allowing a user to gain elevated access. Typical impact: privilege escalation beyond the intended level.

CVE-2026-23990 has a CVSS score of 5.3 (Medium). 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 (0.40.0); upgrading removes the vulnerable code path.

Kodem intelligence

This vulnerability was fixed in Flux Operator v0.40.0.

1. What is CVE-2026-23990? CVE-2026-23990 is a medium-severity improper privilege management vulnerability in github.com/controlplaneio-fluxcd/flux-operator (go), affecting versions >= 0.36.0, < 0.40.0. It is fixed in 0.40.0. The application assigns, modifies, tracks, or checks privileges incorrectly, allowing a user to gain elevated access.
2. How severe is CVE-2026-23990? CVE-2026-23990 has a CVSS score of 5.3 (Medium). 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 github.com/controlplaneio-fluxcd/flux-operator are affected by CVE-2026-23990? github.com/controlplaneio-fluxcd/flux-operator (go) versions >= 0.36.0, < 0.40.0 is affected.
4. Is there a fix for CVE-2026-23990? Yes. CVE-2026-23990 is fixed in 0.40.0. Upgrade to this version or later.
5. Is CVE-2026-23990 exploitable, and should I be worried? Whether CVE-2026-23990 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-2026-23990 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-2026-23990? Upgrade github.com/controlplaneio-fluxcd/flux-operator to 0.40.0 or later.