CVE-2026-23990 is a medium-severity (CVSS 5.3) Improper Privilege Management vulnerability in github.com/controlplaneio-fluxcd/flux-operator. A fix is available for github.com/controlplaneio-fluxcd/flux-operator — see the affected versions and patch details below.
Flux Operator Web UI Impersonation Bypass via Empty OIDC Claims
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-23990.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-23990 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
github.com/controlplaneio-fluxcd/flux-operatorReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
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.
Impact
- 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
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.
- Cluster admin configures OIDC authentication with a provider that does not include
emailorgroupsclaims in tokens - User authenticates with a valid token from that provider
- The default CEL expressions evaluate to empty values:
- Username:
has(claims.email) ? claims.email : ''→"" - Groups:
has(claims.groups) ? claims.groups : []→[]
- Username:
- Authentication succeeds (token signature is valid)
- A userClient is created with empty impersonation config
- All subsequent API requests bypass impersonation and execute as the flux-operator service account
- User gains operator-level read access across all namespaces
Patches
This vulnerability was fixed in Flux Operator v0.40.0.
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/controlplaneio-fluxcd/flux-operator | ≥ 0.36.0&&< 0.40.0 | 0.40.0go get github.com/controlplaneio-fluxcd/flux-operator@v0.40.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/controlplaneio-fluxcd/flux-operator, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/controlplaneio-fluxcd/flux-operator to 0.40.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-23990 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-23990 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-23990. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-23990 in your dependencies?
O3 Security finds CVE-2026-23990 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.