CVE-2026-40109 is a low-severity (CVSS 3.1) Improper Authentication vulnerability in github.com/fluxcd/notification-controller. A fix is available for github.com/fluxcd/notification-controller — see the affected versions and patch details below.
Flux notification-controller GCR Receiver missing email validation allows unauthorized reconciliation triggering
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-2026-40109.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
github.com/fluxcd/notification-controllerReal-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
Impact
The gcr Receiver type in Flux notification-controller does not validate the email claim of Google OIDC tokens used for Pub/Sub push authentication. This allows any valid Google-issued token, to authenticate against the Receiver webhook endpoint, triggering unauthorized Flux reconciliations.
Exploitation requires the attacker to know the Receiver's webhook URL. The webhook path is generated as /hook/sha256sum(token+name+namespace), where the token is a random string stored in a Kubernetes Secret. There is no API or endpoint that enumerates webhook URLs. An attacker cannot discover the path without either having access to the cluster and permissions to read the Receiver's .status.webhookPath in the target namespace, or obtaining the URL through other means (e.g. leaked secrets or access to Pub/Sub config).
Upon successful authentication, the controller triggers a reconciliation for all resources listed in the Receiver's .spec.resources. However, the practical impact is limited: Flux reconciliation is idempotent, so if the desired state in the configured sources (Git, OCI, Helm) has not changed, the reconciliation results in a no-op with no effect on cluster state. Additionally, Flux controllers deduplicate reconciliation requests, sending many requests in a short period results in only a single reconciliation being processed.
Patches
The fix in notification-controller v1.8.3 refactors the GCR Receiver authentication to allow users to extend the verification to email and audience claims in the JWT. This enables operators to configure their Receiver's secret with the expected GCP Service Account email and audience, which the controller will validate against the token's claims before accepting the request.
Email validation example:
apiVersion: v1
kind: Secret
metadata:
name: gcr-webhook-token
namespace: apps
type: Opaque
stringData:
token: <random token>
email: <service-account>@<project>.iam.gserviceaccount.com
audience: https://<hostname>/hook/<sha256(token+name+namespace)>
For more information, please see the GCR Receiver documentation: https://fluxcd.io/flux/components/notification/receivers/#gcr
Credits
Thanks to Saroj Khadka for reporting this issue to the Flux Security Team.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/fluxcd/notification-controller | all versions | 1.8.3go get github.com/fluxcd/notification-controller@v1.8.3 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/fluxcd/notification-controller, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/fluxcd/notification-controller to 1.8.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40109 is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.
Frequently Asked Questions
Is CVE-2026-40109 in your dependencies?
Find it across Go, including transitive dependencies.