CVE-2026-82431: Apache Storm Client: Authorization Bypass When nimbus.groups Is Configured Without nimbus.users
Description
SimpleACLAuthorizer evaluated the user-level command set by returning early when nimbus.users was empty, before nimbus.groups was considered. An operator who restricted cluster access by group alone, leaving nimbus.users unset, therefore received no restriction at all: every authenticated principal was permitted every user-level operation, including submitTopology, beginFileUpload and getNimbusConf.
docs/SECURITY.md presents nimbus.groups as a supported way to lock down a cluster, so a deployment following the documentation could believe it was restricted while it was not. The failure is silent; nothing in the logs or the configuration indicates that the group list is being ignored.
Both lists left empty continues to mean that no restriction is configured, which is the shipped default and is unchanged.
Mitigation
Upgrade to 3.1.0, where nimbus.groups is evaluated whether or not nimbus.users is set.
Users who cannot upgrade immediately should additionally populate nimbus.users with the intended principals, since a non-empty user list causes the group list to be evaluated on affected versions. Operators should review Nimbus access logs for operations by principals outside the intended groups.
Note that after upgrading, a cluster configured with nimbus.groups alone becomes restrictive for the first time. This includes NimbusClient, which calls getLeader on every connection, so clients outside the configured groups will begin to be refused.
Credit
The ASF -- found using Claude agents to study the security of open-source projects, validated and reported by Apache Storm.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Apache Stormto a version that resolves this vulnerability.Fixed in 3.1.0 - Configuration
For clusters that cannot be upgraded immediately, populate `nimbus.users` with the intended user principals so that user-level authorization is enforced and `nimbus.groups` is not ignored on affected versions.
Apache Storm Nimbus authorization nimbus.users = (populate with intended users) - Compensating control
After upgrading, review Nimbus access logs for operations by principals outside the intended `nimbus.groups`, since clusters configured with `nimbus.groups` alone become restrictive only after upgrade and the failure is silent.
Event History
Frequently Asked Questions
Which deployments are exposed to this authorization bypass?
Deployments using SimpleACLAuthorizer that configured nimbus.groups while leaving nimbus.users empty are exposed. This includes operators who relied on group-only restrictions described in the security documentation.
Does the default configuration become more permissive because of this issue?
No. Leaving both nimbus.users and nimbus.groups empty means no restriction is configured, which is the shipped default and is unchanged. The issue affects the specific configuration where groups are set but users are unset.
What access does an attacker need to exploit this?
The attacker needs to be an authenticated principal. Under the affected group-only configuration, every authenticated principal can perform user-level operations, including submitTopology, beginFileUpload, and getNimbusConf.
What can be done before upgrading?
Populate nimbus.users with the intended principals in addition to the group configuration. On affected versions, a non-empty user list causes nimbus.groups to be evaluated.
How can operators investigate possible prior misuse?
Review Nimbus access logs for user-level operations, including submitTopology, beginFileUpload, and getNimbusConf, performed by principals that should not have been authorized.