CVE-2026-109125
Received
Received - Intake
Cache Side-Channel in Unbound DNS Resolver
Vulnerability report for CVE-2026-109125, including description, CVSS score, EPSS score, affected products, exploitability, helpful resources, and attack-flow context.
Publication date: 2026-10-21
Last updated on: 2026-10-21
Assigner: NLnet Labs
Description
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.
CVSS Scores
EPSS Scores
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Meta Information
Affected Vendors & Products
| Vendor | Product | Version / Range |
|---|---|---|
| NLnet | Labs | Unbound 0 |
Helpful Resources
Exploitability
CWE
KEV
| CWE ID | Description |
|---|---|
| CWE-205 | The 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. |
