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UnboundApplication · Nlnetlabs

CVE-2026-33278

CRITICAL · 9.8 CVSS v3.1 Published 2026-05-20
Fix available
A fix is available. Upgrade to 1.25.1 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Unbound DNS server versions 1.19.1 through 1.25.0 have a DNSSEC validation bug where deep-copying response messages during DS sub-query suspension (due to NSEC3 computational budget exhaustion) causes a struct-assignment to overwrite the destination pointer with the source pointer. After the sub-query memory region is freed, the validator dereferences this dangling pointer, causing denial of service or potential remote code execution.

MitigationUpgrade Unbound to version 1.25.1 or later to apply the patch that corrects the pointer preservation during deep copying. If immediate upgrade is not possible, consider disabling DNSSEC validation as a temporary workaround.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
UnboundApplication
Affected:>= 1.19.1, < 1.25.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check Unbound version
    Run 'unbound -V' or 'unbound-control status' to retrieve the installed version number
    Affected if Version is >= 1.19.1 and < 1.25.1
  2. Verify DNSSEC validation is enabled
    Check unbound.conf for 'dnssec: yes' or 'dnssec-check: yes' directive, or run 'unbound-control list_forwards' and look for DNSSEC-enabled zones
    Affected if DNSSEC validation is enabled in the configuration
  3. Confirm server processes DNSSEC-signed zones with NSEC3
    Review zone configurations or logs to determine if the server serves or validates zones using NSEC3 records (common in signed zones). Check logs for NSEC3-related activity or inspect zone configurations.
    Affected if Server handles DNSSEC-signed zones using NSEC3 records, which triggers DS sub-queries that can exhaust NSEC3 computational budget

Environment is affected if Unbound version is between 1.19.1 and 1.25.0, DNSSEC validation is enabled, and the server processes DNSSEC-signed zones using NSEC3 records.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 1.25.1 or later
Fixed in 1.25.1
Interim mitigation

Upgrade Unbound to version 1.25.1 or later to apply the patch that corrects the pointer preservation during deep copying. If immediate upgrade is not possible, consider disabling DNSSEC validation as a temporary workaround.

Recommended fix High confidence

Unbound 1.25.1 or later

  1. Check current Unbound version using 'unbound -V' or 'unbound-control version'
  2. Stop the Unbound service: 'systemctl stop unbound' or 'service unbound stop'
  3. Upgrade Unbound to version 1.25.1 or later using the package manager (e.g., 'yum update unbound' or 'apt-get install unbound=1.25.1')
  4. Alternatively, download and compile Unbound 1.25.1 or later from source: git clone https://github.com/NLnetLabs/unbound.git, cd unbound, git checkout release-1.25.1, ./configure, make, make install
  5. Verify the new version is installed: 'unbound -V'
  6. Restart the Unbound service: 'systemctl start unbound' or 'service unbound start'
  7. Test that Unbound is functioning correctly and resolving queries
Caveat Users should review configuration for compatibility with 1.25.x series; major version differences should be checked in release notes

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Unbound Scoped from the published advisory
  • Consultation2.0 h
  • Implementation3.0 h
  • Testing4.0 h
  • Review / QA2.0 h
11.0 hours of engineering $1,900
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-33278 in production — separate from our analysis above.

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What this is

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What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data