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NVD / CVE Program· CNA NLnet Labs

CVE-2026-42923: Degradation of service with unbounded NSEC3 hash calculations

Noroxi assessment

Priority: Monitor (action score 27/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.33% (FIRST EPSS).
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • According to CISA the attack is automatable: targets can be found by mass scanning.
  • Fixed version: NLnet Labs Unbound 1.25.1; azl3 unbound 1.19.1-5 on Azure Linux 3.0 1.25.1-1.

Low priority for now. Upgrade to NLnet Labs Unbound 1.25.1; azl3 unbound 1.19.1-5 on Azure Linux 3.0 1.25.1-1 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

MediumCVSS 6.9 · v4.0—No exploit Fix available
Published
May 20, 2026
Updated
Jul 24, 2026
EPSS
0.3% · 27th percentile

Report tools

JSON

Action score

27

Monitor

Low priority for now.

CVSS
27 / 40 · 6.9 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.3%

CISA SSVC decision

Exploitation
none
Automatable
yes
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · NLnet Labs
6.9
NVD has not scored this yet; the score shown is the CNA’s.

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/U:Amber

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2026-42923

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
nlnetlabsunbound

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • nlnetlabs unbound1.19.1 and later · before 1.25.1

Versions reported by the vendor

Affected version ranges reported by the assigning authority (NLnet Labs). Independent of NVD's CPE analysis and usually ahead of it.

  • NLnet Labs Unbound

    • before 1.25.1affected · semver

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Alpine:v3.22unboundfrom 1.19.1 · before 1.25.2-r01.25.2-r0
Alpine:v3.24unboundfrom 1.19.1 · before 1.25.1-r01.25.1-r0
Debian:12unboundfrom 0—
Debian:13unboundbefore 1.22.0-2+deb13u31.22.0-2+deb13u3
Debian:14unboundbefore 1.25.1-11.25.1-1
openSUSE:Leap 16.0unboundbefore 1.24.1-160000.2.11.24.1-160000.2.1
openSUSE:Tumbleweedunboundbefore 1.25.1-1.11.25.1-1.1
Red Hat:hummingbird:1python3-unboundbefore 0:1.25.1-1.hum10:1.25.1-1.hum1
Red Hat:hummingbird:1unboundbefore 0:1.25.1-1.hum10:1.25.1-1.hum1
Red Hat:hummingbird:1unbound-anchorbefore 0:1.25.1-1.hum10:1.25.1-1.hum1
Red Hat:hummingbird:1unbound-develbefore 0:1.25.1-1.hum10:1.25.1-1.hum1
Red Hat:hummingbird:1unbound-dracutbefore 0:1.25.1-1.hum10:1.25.1-1.hum1
Red Hat:hummingbird:1unbound-libsbefore 0:1.25.1-1.hum10:1.25.1-1.hum1
Red Hat:hummingbird:1unbound-muninbefore 0:1.25.1-1.hum10:1.25.1-1.hum1
Red Hat:hummingbird:1unbound-utilsbefore 0:1.25.1-1.hum10:1.25.1-1.hum1
SUSE:Linux Enterprise Micro 5.5unboundbefore 1.20.0-150100.10.25.11.20.0-150100.10.25.1
SUSE:Linux Enterprise Server 16.0unboundbefore 1.24.1-160000.2.11.24.1-160000.2.1

Same product

nlnetlabs: all records

Other highest-scoring records for the same primary product.

  • CVE-2019-25042Unbound before 1.9.5 allows an out-of-bounds write via a compressed name in rdata_copy.
    40Plan
  • CVE-2019-25039Unbound before 1.9.5 allows an integer overflow in a size calculation in respip/respip.c.
    40Plan
  • CVE-2019-25038Unbound before 1.9.5 allows an integer overflow in a size calculation in dnscrypt/dnscrypt.c.
    40Plan

Published before and after this one in the same product

  • ← EarlierCVE-2026-41292May 20, 2026 · Long list of incoming EDNS options degrades performance
    26Monitor
  • ← EarlierCVE-2026-42534May 20, 2026 · Jostle logic bypass degrades resolution performance
    27Monitor
  • Later →CVE-2026-42944May 20, 2026 · Heap overflow with multiple NSID, COOKIE, PADDING EDNS options
    34Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
NLnet Labs Unbound1.25.1Vendor (CNA)
azl3 unbound 1.19.1-5 on Azure Linux 3.01.25.1-1 · CBL-Mariner ReleasesMicrosoft (MSRC)
alpine:unbound1.25.2-r0 · Alpine:v3.22Package registry (OSV)
debian:unbound1.22.0-2+deb13u3 · Debian:13Package registry (OSV)
opensuse:unbound1.25.1-1.1 · openSUSE:TumbleweedPackage registry (OSV)
red hat:python3-unbound0:1.25.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound0:1.25.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-anchor0:1.25.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-devel0:1.25.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-dracut0:1.25.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-libs0:1.25.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-munin0:1.25.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-utils0:1.25.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
suse:unbound1.24.1-160000.2.1 · SUSE:Linux Enterprise Server 16.0Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

  • CVE-2026-443900 days apartUnbounded name compression in certain cases causes degradation of service
    27Monitor
  • CVE-2026-412920 days apartLong list of incoming EDNS options degrades performance
    26Monitor

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
Integrity
Availability
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
X

Weakness class (CWE)

CWE-407 · Inefficient Algorithmic Complexity

CVSS vector

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records