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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
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

An attacker controlling a DNSSEC-signed domain can cause a vulnerable Unbound resolver to crash when it processes a malicious DNS reply. This can interrupt DNS resolution for dependent users and services. The supplied CVSS data indicates availability impact, with no confidentiality or integrity impact identified.

Executive priority

Patch promptly, prioritizing shared and business-critical DNS resolvers. Exploitation could cause material service disruption with one triggering query, but the supplied evidence does not establish active exploitation. Maintain redundancy and monitor restarts until remediation is complete.

Technical view

Unbound through 1.25.0 can miscalculate ADDITIONAL-section write offsets while constructing DNSSEC chase replies. Interactions between DNAME duplication and AUTHORITY filtering may leave an array slot uninitialized. Later pointer dereferencing crashes the process. Unbound 1.25.1 corrects the counter usage.

Likely exposure

Exposure applies to Unbound installations running version 1.25.0 or earlier that perform DNSSEC validation and process attacker-controlled upstream replies. A single query may trigger the condition. Confirm downstream package backports because distribution package versions may not directly match upstream numbering.

Exploitation context

The source describes remote, unauthenticated, low-complexity denial of service by an adversary controlling a DNSSEC-signed domain. No user interaction is required. The bundle states KEV is false and provides no evidence of active exploitation, public exploit code, or attacks in the wild.

Researcher notes

CWE-824 reflects access through an uninitialized pointer. The vulnerable state depends on incorrect section counters during chase-reply construction. The bundle’s structured affected entry lists version “0” and default status “unaffected,” conflicting with the narrative range through 1.25.0; validate exact downstream applicability with vendor advisories.

Mitigation direction

  • Upgrade upstream Unbound to version 1.25.1 or later.
  • For packaged deployments, apply the applicable vendor security update containing the backported fix.
  • Prioritize resolvers supporting critical authentication, networking, and production services.
  • Use service supervision and redundancy to reduce outage impact while updates are deployed.

Validation and detection

  • Inventory Unbound versions and identify systems performing DNSSEC validation.
  • Confirm each installation is version 1.25.1 or has a documented vendor backport.
  • Review resolver crash, restart, and availability telemetry for unexplained incidents.
  • Verify updated resolver services operate normally and DNSSEC validation remains functional.
Prepared
Confidence
medium
Sources
6

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

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ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-824: Exact CWE lookup

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cve · low confidence lookup

CVE-2026-42959 mapping review

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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.

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