LiveActive security incident?Get immediate response
CVE Record

CVE-2026-42959: Crash during DNSSEC validation of malicious content

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.

HighCVSS 8.7Not KEV-listedUpdated
Discuss exposureBack to CVEs
Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

Unbound DNS resolvers through 1.25.0 can crash while validating specially crafted DNSSEC-related replies. A successful attack can interrupt DNS resolution for systems depending on that resolver, causing service disruption without data theft or code execution indicated in the sources.

Executive priority

Treat this as urgent availability risk for DNS infrastructure. It does not indicate data compromise, but resolver crashes can interrupt applications, authentication, browsing, and service discovery that rely on affected Unbound instances.

Technical view

The DNSSEC validator can dereference an uninitialized pointer when chase-reply construction miscalculates ADDITIONAL section rrset offsets. DNAME-related ANSWER growth and AUTHORITY filtering can combine to leave an uninitialized array slot, causing an immediate process crash.

Likely exposure

Organizations running NLnet Labs Unbound up to and including 1.25.0, especially recursive resolvers performing DNSSEC validation, should consider themselves exposed. The provided affected metadata is sparse, so distribution package status should be checked directly.

Exploitation context

The bundle states an adversary controlling a DNSSEC-signed domain can trigger the crash with a single query and malicious upstream replies. KEV is false, and no supplied source states active exploitation in the wild.

Researcher notes

The root issue is CWE-824 uninitialized pointer use in validator reply construction. The vendor-described trigger requires malicious DNSSEC validation content involving DNAME, unsigned CNAMEs, AUTHORITY records, and signed ADDITIONAL glue. Avoid assuming broader product impact beyond supplied sources.

Mitigation direction

  • Upgrade NLnet Labs Unbound to version 1.25.1 or later.
  • Apply applicable Red Hat errata for packaged Unbound deployments.
  • Check vendor or distribution advisories for backported fixes.
  • Prioritize resolvers that support critical business services.
  • Monitor resolver crashes and unexpected DNS availability drops.

Validation and detection

  • Inventory all Unbound resolver instances and package versions.
  • Confirm whether DNSSEC validation is enabled on those resolvers.
  • Verify Unbound is upgraded to 1.25.1 or a fixed vendor package.
  • Review resolver logs for recent crashes or abnormal restarts.
  • Check Red Hat CSAF or errata status for managed systems.
Prepared
Confidence
high
Sources
10

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-824: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cve · low confidence lookup

CVE-2026-42959 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

Open ATT&CK lookup
Vulnerability profileCVE Program record
Severity
High
CVSS
8.7 (4.0)
Known Exploited
No
Published

Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/U:Red

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

2CVSS vectors
9Timeline events
2ADP providers
9Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: yesTechnical Impact: partial

CVSS vector scores

2 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
8.7CVSS 4.0HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/U:Red——NLnet Labs
7.5CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6redhat-SADP

Vulnerability scoring details

Base CVSS 4.0 score

8.7High
CVSS 4.0 vector shape for CVE-2026-42959Attack VectorAttack ComplexityAttack RequirementsPrivileges RequiredUser InteractionVS ConfidentialityVS IntegrityVS AvailabilitySS ConfidentialitySS IntegritySS Availability

Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/U:Red

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Attack Requirements
NonePresent
Privileges Required
NoneLowHigh
User Interaction
NonePassiveActive
VS Confidentiality
HighLowNone
VS Integrity
HighLowNone
VS Availability
HighLowNone
SS Confidentiality
HighLowNone
SS Integrity
HighLowNone
SS Availability
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. Source timelineNLnet Labs

    Issue reported by Qifan Zhang

  2. Source timelineNLnet Labs

    NLnet Labs shares patch

  3. Source timelineNLnet Labs

    Qifan Zhang verifies patch

  4. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  5. ADP timelineredhat-SADP

    Reported to Red Hat.

  6. ADP timelineredhat-SADP

    Made public.

  7. Source timelineNLnet Labs

    Fixes released with version 1.25.1

  8. CVE publishedCVE Program

    The CVE record was published.

  9. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
redhat-SADPunbound: Unbound DNSSEC Validator Denial of Service via Incorrect Write Offset Counter in Chase-Reply Messages
other:Red Hat severity ratingcvssV3_1
  • 2026-05-19T11:23:24.234Z: Reported to Red Hat.
  • 2026-05-20T00:00:00.000Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
NLnet LabsUnbound0unaffected
Weakness

CWE details

CWE-824

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-824 · source CWE mapping

Access of Uninitialized Pointer

Access of Uninitialized Pointer represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs