CVE-2026-109125: nlnet labs unbound Observable Behavioral Discrepancy

Vulnerability summary

CVE-2026-109125 is a medium (CVSS 5.3) Observable Behavioral Discrepancy vulnerability in nlnet labs unbound. EPSS estimates a 0.13% chance of exploitation in the next 30 days, higher than 2.6% of scored vulnerabilities.

Timeline

1. 21-10-2026
   PublishedVulnerability first disclosed.

Description

In NLnet Labs Unbound up to and including version 1.26.1, an observable cache-level side-channel can be exposed with 'unwanted-reply-threshold > 0' configurations (not a default configuration). If a response-like UDP datagram reaches the upstream source port currently used by a recursive query, but its TXID does not match any pending query, Unbound counts the packet as an unwanted reply. When the accumulated count reaches the configured threshold, Unbound clears the worker's RRset cache and message cache. This cache clear action does not destroy the current upstream UDP pending query. Therefore, an external observer can distinguish whether a candidate port matches the current upstream source port by checking whether an existing cached item has been cleared. This is a cache side-channel vulnerability that can be used to probe the upstream source port. Although the attack is not feasible on default configurations and the suggested value (10 million) makes it an ureliable attack on a public resolver, in scenarios where the attacker can keep a concurrent query window alive, the attacker can first filter the source port and then perform birthday-style guessing against the TXID, making cache poisoning a feasible attack path.

• CWE-205Observable Behavioral DiscrepancyThe product's behaviors indicate important differences that may be observed by unauthorized actors in a way that reveals (1) its internal state or decision process, or (2) differences from other products with equivalent functionality.