External risk intelligence

Apache Storm Nimbus Group Restriction Bypass

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-82431

The vulnerability affects cluster-level access control configuration within Apache Storm's Nimbus service. While the service is network-accessible, it is typically deployed within internal, trusted cluster management environments and protected by network access controls, making public internet exposure uncommon in standard real-world deployments.

Halo Surface Signal: 2 out of 5 — less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

This vulnerability in an access control component could allow any authenticated user to perform unauthorized operations on a cluster. This occurs because the system incorrectly assumes restrictions are in place when they are not, potentially leading to unintended access to sensitive cluster functions. The main concern is confirming if this specific technology is in use and, if so, understanding the potential exposure.

Unrestricted access if groups, not users, set restrictions.
Leadership should remember: configuration may not provide security.
Confirm if this system is deployed and assess relevance.

Attack Path

How an attacker could exploit the issue

An attacker with authentication could leverage a misconfiguration where group-based access controls are not properly evaluated. This occurs when a specific user list is left empty, bypassing intended restrictions and allowing any authenticated user to perform privileged operations like submitting tasks or accessing sensitive cluster configuration.

Authenticated access to the system.
Exploiting empty user list bypass.
Unrestricted privileged operations.

Live Threat

Current exploitation, exposure, and threat context

When `SimpleACLAuthorizer` is configured to restrict cluster access solely by group, but the `nimbus.users` list remains unset, access controls are effectively bypassed. This allows any authenticated principal to perform user-level operations such as submitting topologies, initiating file uploads, and retrieving Nimbus configurations, even when the operator believes restrictions are in place.

User-level operations.
Group-based access control failure.
Unauthorized topology submission.

Operational Fix

Recommended remediation, mitigation, and detection steps

In real-world scenarios, the platform or infrastructure team responsible for managing Apache Storm deployments, along with security operations, should lead the response to this critical access control vulnerability. The immediate practical move is to identify all deployed instances of Apache Storm, determine their network reachability and business criticality, and confirm the specific owner accountable for each instance. This information will inform a risk-based remediation plan, potentially involving coordination with vendor management if a managed service is in use.

Ownership: Platform and security operations teams.
Verify first: Confirm vulnerable Nimbus instances and reachability.
Action: Plan and coordinate remediation or mitigation.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is Apache Storm and the Nimbus service?

Apache Storm is a distributed real-time computation system used for processing large streams of data. The Nimbus service acts as the master node for the cluster, responsible for distributing code, assigning tasks to worker nodes, and monitoring for failures. It manages the operational state of the environment, making it a central control point for cluster configurations and job management.

What is the vulnerability in CVE-2026-82431?

This CVE involves a weakness categorized as CWE-863, which relates to incorrect authorization. The `SimpleACLAuthorizer` component fails to enforce security when only group-based restrictions are configured. If the user-level access list is empty, the system incorrectly skips the group-level check entirely. Consequently, the software grants full user-level permissions to any authenticated user, even when the administrator explicitly intended to restrict access via group membership.

How can an attacker trigger this bypass?

An attacker needs to be an authenticated principal within the environment to exploit this oversight. If a cluster is configured with the `nimbus.groups` setting but leaves `nimbus.users` unset, the logic flaw triggers automatically. Crucially, if you define at least one entry in the `nimbus.users` list, the bypass does not occur because the system then correctly proceeds to evaluate the group restrictions.

Is my cluster at risk per Halo Surface Signal?

Halo Surface Signal indicates that while the Nimbus service is network-accessible, it is typically deployed within internal, trusted cluster management environments. Because these clusters are usually protected by network access controls, direct public internet exposure is considered uncommon. However, you should still evaluate if your specific deployment is accessible beyond trusted internal networks.

How do I secure my Apache Storm deployment?

The primary fix is to upgrade to version 3.1.0, which ensures group lists are evaluated regardless of user list settings. If you cannot upgrade immediately, you can mitigate the risk by populating the `nimbus.users` list with authorized principals, which forces the system to perform the necessary group checks. Additionally, audit your Nimbus access logs to check for any activity from unauthorized principals.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.