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Fencer Intel
medium 5.3

CVE-2026-109125 json

Possible cache poisoning vulnerability via a cache side-channel exposed by 'unwanted-reply-threshold'

published 19d ago modified 11d ago also EUVD-2026-107150

cvss
5.3
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 6%
public exploits
0
none known

Summary

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.

Patch / fix

  • NLnet Labs Unbound 1.26.2
  • unbound Debian 13 1.26.2-0+deb13u1
  • unbound Debian 14 1.26.2-1

debian advisories DEBIAN-CVE-2026-109125

  • unbound openSUSE Tumbleweed 1.26.2-1.1

opensuse advisories openSUSE-SU-2026:12257-1

  • unbound Azure Linux 3.0 1.26.2-1

azure linux advisories AZL-110528

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

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CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
11d ago

Exploit prediction

0.17% chance of exploitation in the next 30 days, higher than 6% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by version score vector
CNA 3.1 5.3 CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

Affected products

  • NLnet Labs Unbound

Classification

Weakness
  • CWE-205 Observable Behavioral Discrepancy

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.