CVE-2026-66787 is a high severity (CVSS 8.7) flaw in the lighthouse component of Red Hat Advanced Cluster Management for Kubernetes. A compromised spoke cluster can create EndpointSlices with attacker controlled IP addresses and redirect service traffic between clusters. Red Hat fixed it in RHSA-2026:63016, which brings Submariner v0.24 for ACM 2.17.
What's affected
The lighthouse component of Red Hat Advanced Cluster Management for Kubernetes. Red Hat says it stems from insufficient validation of advertised IP addresses within EndpointSlice objects. A compromised spoke cluster can create EndpointSlices with attacker controlled IP addresses, and the lighthouse DNS on other clusters then points legitimate service names at malicious endpoints.
Red Hat rates the issue Important. Its CVE page shows a CVSS v3 base score of 8.7 with the vector CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:N, which matches the record's 8.7. The weakness is CWE-345, insufficient verification of data authenticity. The record has no KEV listing for it, and Red Hat's page does not mention exploitation.
The fix ships in Red Hat's errata RHSA-2026:63016, issued on 3 September 2026. Its synopsis is "Important: Submariner v0.24 security fixes and container updates", and it applies to Red Hat Advanced Cluster Management for Kubernetes 2.17. The errata page lists 13 CVEs fixed in the same release, including this one and its sibling entry.
Red Hat's CVE page says mitigation for this issue is not available, or the available options do not meet its criteria for ease of use and wide applicability. That makes the update the main control.
How to check if you're exposed
Find the clusters where Submariner or Red Hat Advanced Cluster Management multi-cluster networking is installed, and read the installed version. These commands help on a hub or spoke cluster:
kubectl get csv -A | grep -i submariner
subctl show versions
Compare what you find with Submariner v0.24 from the errata. The CVE page lists the affected product as Red Hat Advanced Cluster Management for Kubernetes. I did not find a per version range on the pages I fetched, so use the errata and your Red Hat subscription portal to confirm which builds you run.
The precondition is a compromised or malicious spoke cluster that is connected to your other clusters. That is the scenario to model: ask which of your connected clusters are less trusted than the others.
What to do now
- List every cluster that joins a Submariner or ACM multi-cluster setup.
- Apply RHSA-2026:63016, which brings Submariner v0.24 for ACM 2.17.
- Identify the spoke clusters with the weakest controls and consider whether they should be joined to the same fabric as your most sensitive ones.
- Monitor for unexpected network ranges or endpoints published by spoke clusters.
- Rotate credentials for any spoke cluster you suspect is compromised.
Why it matters
Lighthouse provides cross-cluster service discovery. A service in one cluster can be found by name from another, and the DNS answer comes from EndpointSlice data that the spoke publishes. If the receiving side accepts any IP address in that data, the answer can point to a machine the attacker controls.
Red Hat calls the outcome a transparent man in the middle attack on cross-cluster service communication, without altering the target service, which could lead to disclosure and data manipulation. The service itself is never changed. Only the path to it is.
The CVSS vector uses a changed scope and high privileges required. The privilege is control of one spoke, and the changed scope shows that the harm reaches other clusters. A flaw like this erodes the isolation between clusters, which is the reason people split workloads across them.
Quick check
What does the attacker control?
A spoke cluster connected to yours. Red Hat's wording is a compromised spoke cluster.
Is the real service modified?
Red Hat says the attack works without altering the target service. The DNS answer is what changes.
Which release fixes it?
RHSA-2026:63016, Submariner v0.24 for ACM 2.17.
Sources
Last reviewed: 18 October 2026.
