dbcve.org
UnboundApplication · Nlnetlabs

CVE-2026-42959

HIGH · 7.5 CVSS v3.1 Published 2026-05-20
Fix available
A fix is available. Upgrade to 1.25.1 or later.
See remediation →
84/100
Remediation priority · High
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 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.

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's DNSSEC validator uses an incorrect counter when calculating write offsets for ADDITIONAL section rrsets during chase-reply message construction. DNAME duplication inflates the ANSWER section count while authority filtering reduces the AUTHORITY section count, creating an uninitialized array slot. When the validator later dereferences this uninitialized pointer, it causes an immediate process crash.

MitigationUpgrade Unbound to version 1.25.1 or later, which contains the fix to use proper counters for calculating write offsets.

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:< 1.25.1

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
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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. Identify Unbound installation and version
    Run `unbound -V 2>&1` or `unbound-control status` to display the installed Unbound version number.
    Affected if Version shown is less than 1.25.1
  2. Confirm DNSSEC validation is enabled
    Inspect the Unbound configuration file (typically /etc/unbound/unbound.conf or /etc/unbound/conf.d/*.conf) for the `dnssec-validate` directive.
    Affected if Directive is set to `yes` or `auto` (meaning DNSSEC validation is active)
  3. Verify the server is actively running and processing queries
    Run `unbound-control status` and check the `num-queries` counter or review system logs for recent query activity.
    Affected if Server is running and actively processing DNS queries with DNSSEC validation enabled on a vulnerable version
  4. Check for crash indicators in logs
    Review system logs (journalctl -u unbound, /var/log/unbound.log, or /var/log/messages) for recent segfaults, crashes, or abrupt service restarts.
    Affected if Recent crashes are logged following DNSSEC-related query processing

User is affected if Unbound version is below 1.25.1 AND DNSSEC validation is enabled AND the server processes DNS queries that trigger the DNAME/authority section interaction.

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, which contains the fix to use proper counters for calculating write offsets.

Recommended fix High confidence

Unbound 1.25.1

  1. 1. Check the currently installed Unbound version (e.g., unbound -V or rpm -q unbound)
  2. 2. Upgrade Unbound to version 1.25.1 or later using your system's package manager (e.g., yum update unbound, apt-get install unbound, or download from www.nlnetlabs.nl)
  3. 3. Restart the Unbound service to load the fixed version
  4. 4. Verify the upgraded version is running (unbound -V)

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

Fix this in Unbound Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing6.0 h
  • Review / QA3.0 h
15.0 hours of engineering $2,560
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $4,096.

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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-2026-42959 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
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