CVE-2026-109125 NLnet Labs CVE debrief

Who should care

Defenders of systems using Unbound with 'unwanted-reply-threshold' greater than 0 should assess exposure and verify version 1.26.2 or later is deployed. Operators, platform administrators, vulnerability management teams, and security teams should review Unbound configurations and monitor for potential cache poisoning attempts. They should also assess the feasibility of the attack in specific deployment contexts and plan vendor-supported updates or mitigat

Why it matters

Defenders should assess exposure if Unbound is used with 'unwanted-reply-threshold' greater than 0 and verify version 1.26.2 or later is deployed to prevent potential cache poisoning via cache side-channel.

• Verify Unbound version and configuration to prevent potential cache poisoning
• Monitor for potential cache poisoning attempts to detect possible exploitation
• Assess the feasibility of the attack in specific deployment contexts

Technical summary

The vulnerability exists in NLnet Labs Unbound up to version 1.26.1 when 'unwanted-reply-threshold' is set greater than 0. An external observer can probe the upstream source port by distinguishing whether a cached item has been cleared. The attack requires a concurrent query window to be alive and is not feasible on default configurations. Affected systems using Unbound with 'unwanted-reply-threshold' greater than 0 should assess exposure and verify version 1.26.2 or later is deployed to prevent potential cache poisoning via cache side-channel.

Defensive priority

Assess exposure if Unbound is used with 'unwanted-reply-threshold' greater than 0; verify version 1.26.2 or later is deployed.

Recommended defensive actions

• Assess if Unbound is used with 'unwanted-reply-threshold' greater than 0
• Verify version 1.26.2 or later is deployed
• Monitor for potential cache poisoning attempts
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record and source item provide details on the vulnerability and its conditions. Vendor advisory is available. Affected systems using Unbound with 'unwanted-reply-threshold' greater than 0 should be assessed for exposure. Defenders should verify version 1.26.2 or later is deployed. The attack requires a concurrent query window to be alive and is not feasible on default configurations. Evidence limits suggest verifying Unbound configurations and monitoring for potential cache poisoning attempts.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.