CVE-2026-109125 Possible cache poisoning vulnerability via a cache side-channel exposed by 'unwanted-reply-threshold'
- Exploited in the wild
- Not known
- Fix
- Available
What is CVE-2026-109125?
CVE-2026-109125 is a medium-severity vulnerability in NLnet Labs Unbound, published on 21 Oct 2026.
Official 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.
Weakness: CWE-205
Which versions are affected by CVE-2026-109125?
How do I fix CVE-2026-109125?
Upgrade to a fixed version.
Is CVE-2026-109125 being exploited?
There is no known exploitation of CVE-2026-109125 in the wild and no public exploit code.
Sources
Also known as AZL-110528, DEBIAN-CVE-2026-109125, ECHO-b3bc-a96b-2f61, GHSA-66rc-377g-m4m4, UBUNTU-CVE-2026-109125.