Vulnerability · Published Jan 21, 2026 · Assigned by GitHub_M
CVE-2026-23990Flux Operator Web UI Impersonation Bypass via Empty OIDC Claims
Medium 5.3
In plain English
A medium-severity privilege mismanagement in flux-operator from controlplaneio-fluxcd. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 39% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
The Flux Operator is a Kubernetes CRD controller that manages the lifecycle of CNCF Flux CD and the ControlPlane enterprise distribution. Starting in version 0.36.0 and prior to version 0.40.0, 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. In order to be vulnerable, 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. 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. This can result in privilege escalation, data exposure, and/or information disclosure. Version 0.40.0 patches the issue.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/controlplaneio-fluxcd/flux-operator | 0.40.0 | GHSA-4xh5-jcj2-ch8q |
| Go | github.com/controlplaneio-fluxcd/flux-operator | 0.40.0 | GO-2026-4351 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| controlplaneio-fluxcd | flux-operator | >= 0.36.0, < 0.40.0 |
Weakness: CWE-269, CWE-862 · Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
References
- https://github.com/controlplaneio-fluxcd/flux-operator/security/advisories/GHSA-4xh5-jcj2-ch8q x_refsource_CONFIRM
- https://github.com/controlplaneio-fluxcd/flux-operator/pull/610 x_refsource_MISC
- https://github.com/controlplaneio-fluxcd/flux-operator/commit/084540424f6de8ba5d88fb1fd1e8472ba29afd7e x_refsource_MISC
- https://github.com/controlplaneio-fluxcd/flux-operator/releases/tag/v0.40.0 x_refsource_MISC