HighCVE-2026-42959

    Crash during DNSSEC validation of malicious content (CVE-2026-42959)

    Published Last modified Sources: Ubuntu, Debian, CVE.org, Msrc Update Guide, EUVD, BSI

    What is affected, what to do

    NLnet Labs Unbound up to and including version 1.25.0 has a denial of service vulnerability in the DNSSEC validator that can lead to a crash given malicious upstream replies. When Unbound constructs chase-reply messages for validation, the code uses the wrong counter to calculate write offsets for ADDITIONAL section rrsets.

    Update NLnet Labs Unbound to version 1.25.1.

    Affected
    • NLnet Labs Unbound lt *..1.25.1
    Fixed in
    • NLnet Labs Unbound 1.25.1

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    8.7

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v4

    7.5

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: Ubuntu, Debian, CVE.org, Msrc Update Guide, EUVD, BSI

    0.8 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 56.6

    Show all metrics
    Severity
    High
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    86.89 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    no public PoC known
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    8.7 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    7.5
    CVSS v4

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    8.7
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    0.8 %
    Low — model estimates < 1% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    56.6
    High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    —
    Published
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-824

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    NLnet Labs Unbound up to and including version 1.25.0 has a denial of service vulnerability in the DNSSEC validator that can lead to a crash given malicious upstream replies. When Unbound constructs chase-reply messages for validation, the code uses the wrong counter to calculate write offsets for ADDITIONAL section rrsets. DNAME duplication could increase the ANSWER section count and authority filtering could decrease the AUTHORITY section count and create an uninitialized array slot. Combining these two, the validator later dereferences this uninitialized pointer, causing an immediate process crash. An adversary controlling a DNSSEC-signed domain can trigger this bug with a single query by configuring a DNAME chain with unsigned CNAMEs and a response containing unsigned AUTHORITY records alongside signed ADDITIONAL glue records. Unbound 1.25.1 contains a patch with a fix to use the proper counters to calculate the write offsets.

    Source: Ubuntu Security Notices, Debian Security Advisories, cvelistv5, Microsoft Security Update Guide, NVD (NIST), CVE.org (MITRE), ENISA EUVD, BSI WID Portal, BSI CSAF

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    NLnet LabsUnbound
    • < 1.25.1
    • 1.25.1
    nlnetlabsunbound
    • < 1.25.1
    • 1.25.1
    OracleSolaris
    • <11.3 ESU 36.36
    • <11.4 SRU 94
    • —
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxdebianunboundtrixie

    Weakness classes (CWE)

    • CWE-824Access of Uninitialized Pointer

      Base

      The product accesses or uses a pointer that has not been initialized.

      Entry at MITRE CWE-824

    References & sources

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Record modifiednlnetlabs.nl

      Show 3 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-42959.txt
      Referenceremoved
      https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-42959.txt
      —
      Reference typeremoved
      https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-42959.txt Types: Mitigation, Vendor Advisory
      —

      Source identifier: sep@nlnetlabs.nl

    2. Record modifiedRed Hat ADP

      Show 15 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Affected versions (publisher)added
      —
      Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/42xxx/CVE-2026-42959.json">CVE-2026-42959</a>
      Referenceadded
      —
      https://access.redhat.com/errata/RHSA-2026:19752
      Referenceadded
      —
      https://access.redhat.com/errata/RHSA-2026:23231
      Referenceadded
      —
      https://access.redhat.com/errata/RHSA-2026:24365
      Referenceadded
      —
      https://access.redhat.com/errata/RHSA-2026:24369
      Referenceadded
      —
      https://access.redhat.com/security/cve/CVE-2026-42959
      Referenceadded
      —
      https://bugzilla.redhat.com/show_bug.cgi?id=2479806
      Referenceadded
      —
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42959.json
      Referenceremoved
      https://access.redhat.com/errata/RHSA-2026:19752
      —
      Referenceremoved
      https://access.redhat.com/errata/RHSA-2026:23231
      —
      Referenceremoved
      https://access.redhat.com/errata/RHSA-2026:24365
      —
      Referenceremoved
      https://access.redhat.com/errata/RHSA-2026:24369
      —
      Referenceremoved
      https://access.redhat.com/security/cve/CVE-2026-42959
      —
      Referenceremoved
      https://bugzilla.redhat.com/show_bug.cgi?id=2479806
      —
      Referenceremoved
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42959.json
      —

      Source identifier: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c

    Linked advisories

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