CVE-2026-56444Network attack vector

Resolution degradation via missing reply-counter decrement

Published Jul 22, 2026 · Updated Jul 22, 2026

Denial of service in NLnet Labs Unbound 1.20.0 through 1.25.1 allows remote attackers to degrade DNS resolution via crafted queries. The serve-expired discard-timeout branch drops an aged client reply but fails to decrement the query's reply-address counter, eventually rejecting duplicate in-flight queries. Exploitation requires serve-expired: yes, a positive discard-timeout lower than serve-expired-client-timeout, and a slow attacker-controlled authoritative zone; shipped defaults prevent the branch.

CVSS severity5.9
Medium
EPSS probability0.26%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in NLnet Labs Unbound 1.20.0 through 1.25.1 allows remote attackers to degrade DNS resolution via crafted queries. The serve-expired discard-timeout branch drops an aged client reply but fails to decrement the query's reply-address counter, eventually rejecting duplicate in-flight queries. Exploitation requires serve-expired: yes, a positive discard-timeout lower than serve-expired-client-timeout, and a slow attacker-controlled authoritative zone; shipped defaults prevent the branch.

The record

CVE
CVE-2026-56444
Published
Jul 22, 2026
Updated
Jul 22, 2026
Vendor
NLnet Labs
Product
Unbound
Classifications
CWE-772, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 22, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 22, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=1.20.0 <1.25.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

NLnet Labs · Unboundversion=1.20.0 <1.25.2

Published CVSS scores

5.9NLnet LabsCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-772Missing Release of Resource after Effective Lifetime
T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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