LiveActive security incident?Get immediate response
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
Discuss exposureBack to CVEs
Glexia's TakeAutomated analysiscritical

Security readout for executives and security teams

Plain-English summary

A remote attacker who controls a signed DNS zone may crash a vulnerable Unbound resolver and could potentially execute code. No authentication or user interaction is required. The confirmed affected range is Unbound 1.19.1 through 1.25.0; version 1.25.1 fixes the defect.

Executive priority

Treat this as an emergency remediation for affected resolvers because compromise could disrupt DNS services or potentially enable code execution. Patch internet-accessible and high-dependency resolvers first. The absence of confirmed active exploitation reduces certainty about immediate attacks, but not the severity of successful exploitation.

Technical view

During DNSSEC validation, suspended DS sub-queries can trigger an incorrect deep copy after NSEC3 budget exhaustion. A struct assignment replaces the destination pointer with a source-region pointer. When that region is freed, resumed validation dereferences a dangling pointer, causing a use-after-free, resolver crash, or possible arbitrary code execution.

Likely exposure

Organizations running Unbound 1.19.1 through 1.25.0 are potentially exposed when untrusted parties can cause the resolver to query attacker-controlled signed zones. Exposure includes directly reachable resolvers and restricted resolvers accepting queries from potentially compromised clients. The supplied evidence does not quantify deployment prevalence.

Exploitation context

The attack is remote, unauthenticated, and requires control of a malicious signed DNS zone plus a query reaching the vulnerable resolver. The source bundle reports possible code execution and reliable denial-of-service impact. CISA KEV status is false, and no supplied source establishes active exploitation; exploit maturity is recorded as unknown.

Researcher notes

This is a DNSSEC validator use-after-free corresponding to CWE-416 and CWE-672. The vulnerable path was introduced with NSEC3 computational-budget handling in 1.19.1. The patch preserves the correct destination pointer during deep copying. The supplied evidence establishes potential, not demonstrated, arbitrary code execution; researchers should avoid equating a crash with confirmed code execution.

Mitigation direction

  • Upgrade Unbound to 1.25.1 or a vendor-provided package containing the fix.
  • Review applicable operating-system vendor advisories and install their corrected packages.
  • Until patched, limit untrusted query access to affected resolvers where operationally feasible.
  • Prioritize externally reachable and business-critical recursive resolvers.

Validation and detection

  • Inventory Unbound versions across appliances, servers, containers, and embedded deployments.
  • Confirm no deployed instance remains between versions 1.19.1 and 1.25.0 inclusive.
  • Verify distribution packages against applicable vendor errata, not upstream version strings alone.
  • Review resolver crash and restart records, while recognizing absence of crashes does not prove safety.
  • Confirm resolver health and DNSSEC validation behavior after remediation.
Prepared
Confidence
high
Sources
5

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

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-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.

Open ATT&CK lookup
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.

Open ATT&CK lookup
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.

Open ATT&CK lookup
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.

CWE-416 · source CWE mapping

Use After Free

Use After Free represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs
CWE-672 · source CWE mapping

Operation on a Resource after Expiration or Release

Operation on a Resource after Expiration or Release represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs