dbcve.org
UnboundApplication · Nlnetlabs

CVE-2026-42923

MEDIUM · 5.3 CVSS v3.1 Published 2026-05-20
Fix available
A fix is available. Upgrade to 1.25.1 or later.
See remediation →
62/100
Remediation priority · Elevated
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 vulnerability in the DNSSEC validator where the code path to consult the negative cache for DS records does not take into account the limit on NSEC3 hash calculations introduced in 1.19.1. This leads to degradation of service during the attack. An adversary that controls a DNSSEC signed zone can exploit this by signing NSEC3 records with acceptably high iterations for child delegations and querying a vulnerable Unbound. Unbound will keep performing the allowed hash calculations on the NSEC3 records and will not limit the work by the mitigation introduced in 1.19.1. As a side effect, a global lock for the negative cache will be held for the duration of the hashing, blocking other threads that need to consult the negative cache. Coordinated attacks could raise the vulnerability to denial of service. Unbound 1.25.1 contains a patch with a fix to bound the vulnerable code path with the existing limit for NSEC3 hash calculations.

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 up to 1.25.0 has a DNSSEC validation vulnerability where the negative cache lookup path for DS records ignores the NSEC3 hash calculation limit introduced in version 1.19.1. An attacker controlling a DNSSEC-signed zone can craft NSEC3 records with high iterations; when Unbound processes these, it performs unbounded hash calculations while holding a global negative cache lock, blocking other threads and causing denial of service.

MitigationUpgrade Unbound to version 1.25.1 or later, which applies the existing NSEC3 hash calculation limit to the vulnerable code path.

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
None
Integrity
None
Availability
Low

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

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 installed Unbound version
    Run `unbound -V` or `unbound-version` to retrieve the version number
    Affected if Version is >= 1.19.1 and < 1.25.1
  2. Locate Unbound configuration file
    Check common paths such as /etc/unbound/unbound.conf or files in /etc/unbound/unbound.conf.d/
    Affected if Configuration file exists and DNSSEC validation is enabled
  3. Verify DNSSEC validation is enabled
    Search configuration for `dnssec` directive set to `yes` or `validate`
    Affected if DNSSEC validation is enabled in the configuration
  4. Confirm negative caching behavior
    Negative caching for DS records is enabled by default when DNSSEC is active; no specific configuration toggle required
    Affected if DNSSEC is enabled (negative cache for DS records is active by default)

You are affected if Unbound version is 1.19.1 through 1.25.0 and DNSSEC validation is enabled in the configuration.

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 applies the existing NSEC3 hash calculation limit to the vulnerable code path.

Recommended fix High confidence

Unbound 1.25.1

  1. 1. Identify the current Unbound version installed in your environment using 'unbound -V' or checking your package manager.
  2. 2. Download Unbound version 1.25.1 from the official NLnet Labs repository or your system's package manager.
  3. 3. Upgrade Unbound to version 1.25.1 using your system's package management tools (e.g., apt-get, yum, dnf) or by compiling from source.
  4. 4. Verify the upgrade was successful by running 'unbound -V' and confirming the version shows 1.25.1.
  5. 5. Restart the Unbound service to load the updated version using 'systemctl restart unbound' or the appropriate service command for your system.
  6. 6. Monitor the service to ensure it is running correctly and DNS resolution is functioning as expected.

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

Fix this in Unbound Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,240
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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-2026-42923 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

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