CVE-2026-23990 Flux Operator Web UI Impersonation Bypass via Empty OIDC Claims
Attackers can bypass RBAC impersonation in Flux Operator Web UI to execute API requests with operator privileges, leading to privilege escalation and data exposure.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-23990?
CVE-2026-23990 is a medium-severity vulnerability in controlplaneio-fluxcd flux-operator, published on 21 Jan 2026. Any signed-in user can exploit it over the network, with no action from the victim.
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.
Which versions are affected by CVE-2026-23990?
3 open source packages ship an affected version, including github.com/controlplaneio-fluxcd/flux-operator. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
github.com/controlplaneio-fluxcd/flux-operator Go | From 0.36.0 | 0.40.0 |
github.com/controlplaneio-fluxcd/flux-operator Go | From 0.36.0 | 0.40.0 |
github.com/controlplaneio-fluxcd/flux-operator Go | From 0.36.0 | 0.40.0 |
Products include Control-Plane Flux Operator.
How do I fix CVE-2026-23990?
Upgrade github.com/controlplaneio-fluxcd/flux-operator to 0.40.0 or later.
Is CVE-2026-23990 being exploited?
There is no known exploitation of CVE-2026-23990 in the wild and no public exploit code.
Sources
Also known as GHSA-4xh5-jcj2-ch8q, GO-2026-4351.