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Vulnerabilities

Vulnerability · CVE-2026-42923

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…

Published May 20, 2026 · catalog updated Jul 24, 2026 · OpenBSD OpenBSD

Check if you run it

No confirmed exploitation is on record for CVE-2026-42923. EPSS is 0.40%. Act only if a listed version of OpenBSD OpenBSD is installed.

Gaps on this record

  • No version range is stored. A customer cannot check an installed version against this record.
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
Medium6.9
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.40%
Remediation
Vendor / distro source
Affected products
cpe nlnetlabs/unbound

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.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • cpe nlnetlabs/unboundversion unspecified

How to fix CVE-2026-42923

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-42923. EPSS is 0.40%. Act only if a listed version of OpenBSD OpenBSD is installed.

Which versions are affected?

Affected version ranges are not on this record. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-407. EPSS 0.40%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. May 20, 2026 · NIST NVD

    Detected

    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

  2. Jul 24, 2026 · NIST NVD

    Updated

    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

Sources

  • CVE.orgVulnerability recordCVE-2026-42923

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-42923

    30-day exploitation probability 0.4%

    first.org
  • FreeBSD SecurityVendor advisoryCVE-2026-42923

    CVE-2026-42923 is named by FreeBSD Security.

    freebsd.org
  • NIST NVDVulnerability recordCVE-2026-42923

    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 se

    nvd.nist.gov
  • OpenBSD ErrataVendor advisoryCVE-2026-42923

    CVE-2026-42923 is named on the public OpenBSD Errata page.

    openbsd.org

References

Freshness

Last verified in this catalog Oct 23, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.