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CVE-2026-42534· NVD / CVE Program· CNA NLnet Labs

Jostle logic bypass degrades resolution performance

NLnet Labs Unbound up to and including version 1.25.0 has a vulnerability in the jostle logic that could defeat its purpose and degrade resolution performance. Retransmits of the same query could renew the age of slow running queries and not allow the jostle logic to see them as aged and potential targets for replacement with new queries. An adversary who can query a vulnerable Unbound and who can control a domain name server that replies slowly and/or maliciously to Unbound's queries can exploit the vulnerability and degrade the resolution performance of Unbound. When Unbound's 'num-queries-per-thread' reaches its limit, the jostle logic kicks in. When a new query comes in, half of the available queries that are also slow to resolve are candidates for replacement. The vulnerability then happens because duplicate queries that need resolution would skew the aging result by using the timestamp of the latest duplicate query instead of the original one that started the resolution effort. Cache and local data response performance remains unaffected. Coordinated attacks could raise this to a denial of resolution service. Unbound 1.25.1 contains a patch with a fix to attach an initial, non-updatable start time for incoming queries that allow the jostle logic to work as intended.

MediumCVSS 6.9 · v4.0—No exploit Fix available
Published
May 20, 2026
Updated
Sep 1, 2026
EPSS
0.7% · 52th percentile
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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 · 0.7%

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.

Affected systems

VendorProduct
nlnetlabsunbound

Affected versions

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

  • nlnetlabs unboundbefore 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
AlmaLinux:10python3-unboundbefore 1.24.2-7.el10_2.21.24.2-7.el10_2.2
AlmaLinux:10unboundbefore 1.24.2-7.el10_2.21.24.2-7.el10_2.2
AlmaLinux:10unbound-anchorbefore 1.24.2-7.el10_2.21.24.2-7.el10_2.2
AlmaLinux:10unbound-develbefore 1.24.2-7.el10_2.21.24.2-7.el10_2.2
AlmaLinux:10unbound-dracutbefore 1.24.2-7.el10_2.21.24.2-7.el10_2.2
AlmaLinux:10unbound-libsbefore 1.24.2-7.el10_2.21.24.2-7.el10_2.2
AlmaLinux:10unbound-utilsbefore 1.24.2-7.el10_2.21.24.2-7.el10_2.2
AlmaLinux:8python3-unboundbefore 1.16.2-5.12.el8_101.16.2-5.12.el8_10
AlmaLinux:8unboundbefore 1.16.2-5.12.el8_101.16.2-5.12.el8_10
AlmaLinux:8unbound-develbefore 1.16.2-5.12.el8_101.16.2-5.12.el8_10
AlmaLinux:8unbound-libsbefore 1.16.2-5.12.el8_101.16.2-5.12.el8_10
AlmaLinux:9python3-unboundbefore 1.24.2-3.el9_8.21.24.2-3.el9_8.2
AlmaLinux:9unboundbefore 1.24.2-3.el9_8.21.24.2-3.el9_8.2
AlmaLinux:9unbound-develbefore 1.24.2-3.el9_8.21.24.2-3.el9_8.2
AlmaLinux:9unbound-dracutbefore 1.24.2-3.el9_8.21.24.2-3.el9_8.2
AlmaLinux:9unbound-libsbefore 1.24.2-3.el9_8.21.24.2-3.el9_8.2
Alpine:v3.22unboundbefore 1.25.2-r01.25.2-r0
Alpine:v3.24unboundbefore 1.25.1-r01.25.1-r0
Debian:12unboundall versions—
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:enterprise_linux:10.2python3-unboundbefore 0:1.24.2-7.el10_2.20:1.24.2-7.el10_2.2

+37

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
  • CVE-2019-25035Unbound before 1.9.5 allows an out-of-bounds write in sldns_bget_token_par.
    40Plan
  • CVE-2019-25034Unbound before 1.9.5 allows an integer overflow in sldns_str2wire_dname_buf_origin, leading to an out-of-bounds write.
    40Plan
  • CVE-2019-25033Unbound before 1.9.5 allows an integer overflow in the regional allocator via the ALIGN_UP macro.
    40Plan

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)
almalinux:python3-unbound1.24.2-7.el10_2.2 · AlmaLinux:10Package registry (OSV)
almalinux:unbound1.24.2-7.el10_2.2 · AlmaLinux:10Package registry (OSV)
almalinux:unbound-anchor1.24.2-7.el10_2.2 · AlmaLinux:10Package registry (OSV)
almalinux:unbound-devel1.24.2-7.el10_2.2 · AlmaLinux:10Package registry (OSV)
almalinux:unbound-dracut1.24.2-7.el10_2.2 · AlmaLinux:10Package registry (OSV)
almalinux:unbound-libs1.24.2-7.el10_2.2 · AlmaLinux:10Package registry (OSV)
almalinux:unbound-utils1.24.2-7.el10_2.2 · AlmaLinux:10Package registry (OSV)
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-unbound0:1.25.1-2.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:python3-unbound-debuginfo0:1.24.2-7.el10_2.2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound0:1.25.1-2.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-anchor0:1.24.2-7.el10_2.2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-anchor-debuginfo0:1.24.2-7.el10_2.2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-debuginfo0:1.24.2-7.el10_2.2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-debugsource0:1.24.2-7.el10_2.2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-dracut0:1.24.2-7.el10_2.2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-libs0:1.24.2-7.el10_2.2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-libs-debuginfo0:1.24.2-7.el10_2.2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-munin0:1.25.1-2.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-utils0:1.24.2-7.el10_2.2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
rocky linux:unbound0:1.24.2-7.el10_2.2 · Rocky Linux:10Package 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.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

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.

No nightly-computed relations.

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-440 · Expected Behavior Violation

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.

Noroxi analysis

No Noroxi analysis for this record yet

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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