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CVE Record

CVE-2026-42944: Heap overflow with multiple NSID, COOKIE, PADDING EDNS options

NLnet Labs Unbound 1.14.0 up to and including version 1.25.0 has a vulnerability that results in heap overflow when encoding multiple NSID and/or DNS Cookie EDNS and/or EDNS Padding options in the reply packet. The relevant options ('nsid', 'answer-cookie', 'pad-responses' (default)) need to be enabled for the vulnerability to be exploited. An adversary who can query Unbound can exploit the vulnerability by attaching multiple NSID and/or DNS Cookie EDNS and/or EDNS Padding options to the query. A flaw in the size calculation of the EDNS field truncates the correct value which allows the encoder to overflow the available space when writing. Those two combined lead to a heap overflow write of Unbound controlled data and eventually a crash. Unbound 1.25.1 contains a patch with a fix to de-duplicate the EDNS options and a fix to prevent truncation of the EDNS field size calculation.

HighCVSS 8.7Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

This affects NLnet Labs Unbound DNS resolvers. A remote unauthenticated requester can trigger a heap overflow when specific EDNS reply options are enabled, causing Unbound to crash. The main business risk is DNS resolver outage, not confirmed data theft or code execution.

Executive priority

Treat as high-priority availability risk for DNS infrastructure. Patch exposed or shared resolvers quickly because exploitation requires only query access when the affected options are enabled.

Technical view

Unbound 1.14.0 through 1.25.0 can miscalculate EDNS field size when multiple NSID, DNS Cookie, or EDNS Padding options are encoded. With relevant options enabled, crafted queries can cause a heap overflow write of Unbound-controlled data and eventual crash. Version 1.25.1 fixes de-duplication and truncation handling.

Likely exposure

Internet-facing or broadly reachable Unbound resolvers are most exposed, especially where nsid, answer-cookie, or pad-responses are enabled. pad-responses is described as default. Internal-only resolvers still matter if untrusted clients can query them.

Exploitation context

The source bundle says an adversary who can query Unbound can exploit this by sending multiple relevant EDNS options. KEV is false, and the supplied sources do not show active exploitation in the wild.

Researcher notes

Evidence supports denial of service via heap overflow and crash. The bundle does not support claiming active exploitation, arbitrary code execution, or data compromise. Product scope should stay limited to NLnet Labs Unbound and vendor-packaged Unbound builds referenced by advisories.

Mitigation direction

  • Upgrade Unbound to 1.25.1 or a vendor package containing the backported fix.
  • Check NLnet Labs and OS vendor advisories for platform-specific update guidance.
  • Restrict resolver query access to trusted networks where operationally possible.
  • Consider disabling unneeded affected EDNS options while awaiting patch guidance.
  • Prioritize highly reachable recursive resolvers and DNS infrastructure dependencies.

Validation and detection

  • Inventory Unbound versions and flag 1.14.0 through 1.25.0.
  • Confirm whether nsid, answer-cookie, or pad-responses are enabled.
  • Verify installed vendor packages against listed Red Hat or platform errata.
  • Review resolver monitoring for abnormal Unbound crashes or DNS availability drops.
  • Confirm updated instances report Unbound 1.25.1 or a fixed vendor build.
Prepared
Confidence
high
Sources
8

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-197: 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
cwe · low confidence lookup

CWE-787: 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-2026-42944 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-2026-42944Attack 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. Source timelineNLnet Labs

    Fixes released with version 1.25.1

  7. CVE publishedCVE Program

    The CVE record was published.

  8. ADP timelineredhat-SADP

    Made public.

  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: Heap overflow and crash with multiple nsid, cookie, padding EDNS options
other:Red Hat severity ratingcvssV3_1
  • 2026-05-19T09:59:55.126Z: Reported to Red Hat.
  • 2026-05-20T11:33:22.428Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
NLnet LabsUnbound1.14.0unaffected
Weakness

CWE details

CWE-197CWE-787

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

CWE-197 · source CWE mapping

Numeric Truncation Error

Numeric Truncation Error represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs
CWE-787 · source CWE mapping

Out-of-bounds Write

Out-of-bounds Write represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs