Critical Red Hat ACM Vulnerability Lets Attackers Gain Cluster-Admin Access

Key Takeaways

• A critical privilege escalation flaw (CVE-2026-10090) has been identified in Red Hat Advanced Cluster Management for Kubernetes (ACM).
• The vulnerability allows users with basic namespace “edit” permissions on an ACM hub to gain full cluster-admin access across managed clusters.
• The flaw stems from insufficient authorization checks in the Application Subscription controller, enabling unauthorized deployment of cluster-scoped resources.
• Red Hat has confirmed the vulnerability with a CVSS score of 9.9 but has not yet released an official patch or dedicated mitigation.
• Organizations are advised to audit permissions, monitor resource creation, and implement temporary admission control policies to mitigate risk.

Red Hat has disclosed a severe privilege escalation vulnerability, tracked as CVE-2026-10090, impacting the Application Subscription controller within Red Hat Advanced Cluster Management for Kubernetes (ACM). This critical flaw presents a significant risk, potentially allowing low-privileged users to seize complete control over entire fleets of managed clusters.

The vulnerability carries an “Important” rating with a CVSS score of 9.9, underscoring its extreme severity. It enables a user possessing only namespace-scoped “edit” permissions on an ACM hub to elevate their privileges to full cluster-admin rights. This effectively grants an attacker or malicious insider complete authority over all clusters managed by that ACM instance.

Technical Breakdown of the Vulnerability

The root cause of this security defect lies within the multicluster-operators-subscription component, which underpins ACM’s Application Subscription feature. Red Hat’s advisory notes that an attacker with basic edit privileges in a hub namespace can craft a malicious scenario. This involves creating a Channel resource that points to a Helm repository under their control, and then linking it to a Subscription resource that references this channel.

Crucially, the app-subscription controller processes this request using its own elevated service account authority. It fails to adequately verify whether the user initiating the subscription possesses the necessary “open-cluster-management:subscription-admin” role. Furthermore, the controller does not restrict the deployment of resources to the subscription’s designated namespace, creating a critical security loophole.

Exploitation Path and Impact

This lack of proper authorization allows an attacker to embed cluster-scoped objects directly into their malicious Helm chart. A primary example is a ClusterRoleBinding that links their own ServiceAccount to the built-in “cluster-admin” ClusterRole. When the controller applies this crafted chart, the malicious binding is created with full administrative privileges, instantly promoting the attacker to a cluster administrator.

Red Hat categorizes the underlying weakness as CWE-267, “Privilege Defined With Unsafe Actions,” and it is officially documented under Bugzilla entry 2483292. The vulnerability is particularly alarming because it directly contravenes ACM’s established security model, which mandates that users without “subscription-admin” privileges should only be able to deploy resources within their own subscription namespace.

Organizations that rely on ACM to enforce multi-tenant separation across their hub-managed clusters could inadvertently be exposing every managed cluster to takeover. Any user with mere edit-level access to a single namespace, a permission often broadly granted to development and application teams, could potentially exploit this flaw.

Given ACM’s widespread adoption in enterprise and government sectors for centrally governing fleets of OpenShift and Kubernetes clusters, a successful exploitation could have far-reaching consequences. Attackers could gain the ability to access sensitive secrets, modify critical workloads, or pivot into every cluster registered with the compromised hub.

As of now, Red Hat has not released a dedicated mitigation that meets its criteria for ease of deployment, broad applicability, or long-term stability. The affected package is identified as rhacm2/multicluster-operators-subscription-rhel9 under Red Hat Advanced Cluster Management for Kubernetes 2, and its status is listed as “Affected” with no errata yet issued.

What You Should Do

• Audit Permissions: Immediately review and restrict who holds namespace-scoped edit access on ACM hub namespaces, especially for non-administrative users.
• Monitor Resource Creation: Implement strict monitoring for the creation of Channel and Subscription resources, specifically looking for unauthorized references to external Helm repositories.
• Restrict Admin Privileges: Limit “subscription-admin” privileges strictly to essential, trusted operators following the principle of least privilege.
• Implement Admission Control: As an interim compensating control, enable admission control policies that specifically block the deployment of cluster-scoped resources via application subscriptions until an official fix is available.

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