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Vulnerabilities

Vulnerability · CVE-2026-109125

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 co

Published Oct 21, 2026 · catalog updated Oct 22, 2026 · In NLnet Labs Unbound

Last checked Nov 6, 2026. A connected source was read within the last 7 days.

Check if you run it

No confirmed exploitation is on record for CVE-2026-109125. EPSS is 0.17%. Act only if a listed version of In NLnet Labs Unbound is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (In NLnet Labs Unbound).
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
Medium5.3
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.17%
Remediation
Advisory link
Affected products
Debian:12 unbound · Debian:13 unbound · Debian:14 unbound

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.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • Debian:12 unbound1.17.1-2, 1.17.1-2+deb12u1, 1.17.1-2+deb12u2
  • Debian:13 unbound1.22.0-2, 1.22.0-2+deb13u1, 1.22.0-2+deb13u2
  • Debian:14 unbound1.22.0-2, 1.23.1-1, 1.24.0-1

How to fix CVE-2026-109125

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-109125. EPSS is 0.17%. Act only if a listed version of In NLnet Labs Unbound is installed.

Which versions are affected?

Compare what you have installed with 1.17.1-2, 1.17.1-2+deb12u1, 1.17.1-2+deb12u2, 1.22.0-2, 1.22.0-2+deb13u1, 1.22.0-2+deb13u2, 1.22.0-2, 1.23.1-1, 1.24.0-1. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-205. EPSS 0.17%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Technology hubs

Debian security

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. Oct 21, 2026 · NIST NVD

    Detected

    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

  2. Oct 22, 2026 · NIST NVD

    Updated

    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

Sources

  • CVE.orgVulnerability recordCVE-2026-109125

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-109125

    30-day exploitation probability 0.17%

    first.org
  • NIST NVDVulnerability recordCVE-2026-109125

    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

    nvd.nist.gov
  • Trivy Vulnerability DBPackage advisoryDEBIAN-CVE-2026-109125

    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 r

    osv.dev

References

Freshness

Last verified in this catalog Nov 6, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.

  • FIRST EPSSNov 6, 2026
  • Trivy Vulnerability DBOct 23, 2026
  • NIST NVDOct 22, 2026
  • CVE.orgOct 21, 2026