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CVE Record

CVE-2026-33278: Possible arbitrary code execution during DNSSEC validation

NLnet Labs Unbound 1.19.1 up to and including version 1.25.0 has a vulnerability in the DNSSEC validator that enables denial of service and possible remote code execution as a result of deep copying a data structure and erroneously overwriting a destination pointer. An adversary can exploit the vulnerability by controlling a malicious signed zone and querying a vulnerable Unbound. When DS sub-queries need to suspend validation due to NSEC3 computational budget exhaustion (introduced in Unbound 1.19.1), Unbound deep-copies response messages to preserve them across memory region teardown. A struct-assignment bug overwrites the destination's pointer with the source's pointer. After the sub-query region is freed, the resumed validator dereferences this dangling pointer, triggering a crash or potentially enabling arbitrary code execution. Unbound 1.25.1 contains a patch with a fix to preserve the correct pointer when deep copying the data structure.

CriticalCVSS 9.1Not KEV-listedUpdated
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Glexia's TakeAutomated analysiscritical

Security readout for executives and security teams

Plain-English summary

This flaw affects NLnet Labs Unbound DNS resolvers during DNSSEC validation. A maliciously controlled signed DNS zone can make a vulnerable resolver crash and may allow remote code execution. The vendor states Unbound 1.25.1 fixes the issue. No source in the bundle establishes active exploitation.

Executive priority

Treat this as urgent for DNS infrastructure. A successful attack could disrupt name resolution and may compromise resolver systems. Patch affected Unbound deployments quickly, starting with production recursive resolvers and internet-reachable resolver paths.

Technical view

Unbound 1.19.1 through 1.25.0 can mishandle deep-copied validator response data when DS sub-queries suspend after NSEC3 computational budget exhaustion. A struct-assignment bug preserves a source pointer, then later dereferences it after the source region is freed, causing use-after-free and possible arbitrary code execution.

Likely exposure

Organizations running Unbound 1.19.1 through 1.25.0 as DNSSEC-validating resolvers are the relevant exposure group. Exposure is higher where resolvers process queries that can reach attacker-controlled signed zones. Downstream OS packages need vendor-specific status checks.

Exploitation context

The provided sources describe remote exploitation by controlling a malicious signed zone and causing a vulnerable Unbound resolver to query it. The CVE is not marked KEV, and the bundle provides no evidence of active exploitation or public weaponization.

Researcher notes

The key bug class is use-after-free from an incorrect pointer overwrite during deep copy. The trigger depends on DNSSEC validation state involving DS sub-query suspension after NSEC3 computational budget exhaustion. Evidence for arbitrary code execution is stated as possible, not proven exploited.

Mitigation direction

  • Upgrade NLnet Labs Unbound to 1.25.1 or a vendor-fixed build.
  • Apply applicable Red Hat errata for affected Red Hat deployments.
  • Inventory resolver hosts and prioritize DNSSEC-validating Unbound services.
  • Check NLnet Labs and OS vendor advisories for package-specific guidance.
  • Monitor resolver crash rates until remediation is complete.

Validation and detection

  • Identify Unbound installations and record exact package versions.
  • Compare versions against the affected 1.19.1 through 1.25.0 range.
  • Confirm whether deployed resolvers perform DNSSEC validation.
  • Verify patched systems report Unbound 1.25.1 or vendor-fixed package status.
  • Review DNS resolver logs for unexplained validation-related crashes.
Prepared
Confidence
high
Sources
9

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-416: Exact CWE lookup

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cwe · low confidence lookup

CWE-672: 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.

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description · low confidence lookup

Execution behavior lookup

The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.

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cve · low confidence lookup

CVE-2026-33278 mapping review

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

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Vulnerability profileCVE Program record
Severity
Critical
CVSS
9.1 (4.0)
Known Exploited
No
Published

Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:U/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
8Source links

SSVC decision data

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

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
9.1CVSS 4.0CriticalCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:U/U:Red——NLnet Labs
8.1CVSS 3.1HighCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H2.25.9redhat-SADP

Vulnerability scoring details

Base CVSS 4.0 score

9.1Critical
CVSS 4.0 vector shape for CVE-2026-33278Attack 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:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:U/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: Unbound DNSSEC Validator Use-After-Free via Deep Copy Pointer Overwrite Leading to DoS and Possible Remote Code Execution
other:Red Hat severity ratingcvssV3_1
  • 2026-05-19T11:46:56.937Z: Reported to Red Hat.
  • 2026-05-20T11:33:59.504Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
NLnet LabsUnbound1.19.1unaffected
Weakness

CWE details

CWE-416CWE-672

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