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NVD / CVE Program· CNA NLnet Labs

CVE-2026-40691: Packet of death for DNSCrypt over TCP

Noroxi assessment

Priority: Monitor (action score 30/100)

  • 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: 0.29% (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 1.25.2; azl3 unbound 1.25.1-1 on Azure Linux 3.0 1.25.2-1.

Low priority for now. Upgrade to NLnet Labs Unbound 1.25.2; azl3 unbound 1.25.1-1 on Azure Linux 3.0 1.25.2-1 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

In Unbound 1.9.0 up to and including 1.25.1, when a DNSCrypt query is received over TCP, the routine that encrypts the reply in place fails to bound the reply length against the destination buffer size. The size clamp that protects the UDP path is not applied on the TCP path, so a reply larger than 65504 bytes is shifted forward by 48 bytes inside a buffer of capacity equal to 'msg-buffer-size', writing past the end of the heap allocation. A single malicious encrypted query crashes the resolver and lead to denial of service. This vulnerability needs Unbound to be compiled with DNSCrypt support ('--enable-dnscrypt') and the 'dnscrypt:' clause to be configured and enabled for the listening interfaces.

HighCVSS 7.5 · v3.1—No exploit Fix available
Published
Jul 22, 2026
Updated
Oct 24, 2026
EPSS
0.2% · 22th percentile

Report tools

JSON

Action score

30

Monitor

Low priority for now.

CVSS
30 / 40 · 7.5 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.2%

CISA SSVC decision

Exploitation
none
Automatable
yes
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · NLnet Labs
7.5
NVD has not scored this yet; the score shown is the CNA’s.

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.

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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 unbound1.9.0 and later · before 1.25.2

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

    • 1.9.0 and later · before 1.25.2affected · semver

Package-level exposure

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

EcosystemPackageAffected rangeFix
Debian:12unboundfrom 0—
Debian:13unboundbefore 1.26.1-0+deb13u11.26.1-0+deb13u1
Debian:14unboundbefore 1.25.2-11.25.2-1
openSUSE:Leap 16.0unboundbefore 1.25.2-160000.1.11.25.2-160000.1.1
openSUSE:Tumbleweedunboundbefore 1.25.2-1.11.25.2-1.1
Red Hat:hummingbird:1python3-unboundbefore 0:1.25.2-0.1.hum10:1.25.2-0.1.hum1
Red Hat:hummingbird:1unboundbefore 0:1.25.2-0.1.hum10:1.25.2-0.1.hum1
Red Hat:hummingbird:1unbound-anchorbefore 0:1.25.2-0.1.hum10:1.25.2-0.1.hum1
Red Hat:hummingbird:1unbound-develbefore 0:1.25.2-0.1.hum10:1.25.2-0.1.hum1
Red Hat:hummingbird:1unbound-dracutbefore 0:1.25.2-0.1.hum10:1.25.2-0.1.hum1
Red Hat:hummingbird:1unbound-libsbefore 0:1.25.2-0.1.hum10:1.25.2-0.1.hum1
Red Hat:hummingbird:1unbound-muninbefore 0:1.25.2-0.1.hum10:1.25.2-0.1.hum1
Red Hat:hummingbird:1unbound-utilsbefore 0:1.25.2-0.1.hum10:1.25.2-0.1.hum1
SUSE:Linux Enterprise Micro 5.5unboundbefore 1.25.2-150100.10.28.11.25.2-150100.10.28.1
SUSE:Linux Enterprise Module for Basesystem 15 SP7unboundbefore 1.25.2-150600.23.19.11.25.2-150600.23.19.1
SUSE:Linux Micro 6.0unboundbefore 1.25.2-1.11.25.2-1.1
SUSE:Linux Micro 6.1unboundbefore 1.25.2-slfo.1.1_1.11.25.2-slfo.1.1_1.1
SUSE:Linux Micro 6.2unboundbefore 1.25.2-160000.1.11.25.2-160000.1.1

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-2026-44608May 20, 2026 · Use after free and crash under special conditions in RPZ code
    18Monitor
  • ← EarlierCVE-2026-32665Jul 22, 2026 · Remote DNS-over-QUIC denial of service due to `quic-size` budget bypass
    30Monitor
  • Later →CVE-2026-41637Jul 22, 2026 · Degradation of resolution service from improperly accounted client-terminated DNS-over-QUIC queries
    14Monitor

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 Unbound1.25.2Vendor (CNA)
azl3 unbound 1.25.1-1 on Azure Linux 3.01.25.2-1 · CBL-Mariner ReleasesMicrosoft (MSRC)
debian:unbound1.26.1-0+deb13u1 · Debian:13Package registry (OSV)
opensuse:unbound1.25.2-1.1 · openSUSE:TumbleweedPackage registry (OSV)
red hat:python3-unbound0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-anchor0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-devel0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-dracut0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-libs0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-munin0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-utils0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
suse:unbound1.25.2-160000.1.1 · SUSE:Linux Micro 6.2Package 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.

  • CVE-2026-8164256 days apartHeap buffer overflow and possible Remote Code Execution when digesting DNSKEY
    36Monitor
  • CVE-2026-8271756 days apartCNAME synthesis could lead to heap corruption
    33Monitor
  • CVE-2026-8163456 days apartPossible heap buffer overflow during DNSSEC canonicalization
    30Monitor

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
none
Integrity
none
Availability
high · the service can be disrupted
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality impact
None
Integrity impact
None
Availability impact
High

Weakness class (CWE)

CWE-122 · Heap-based Buffer Overflow

CVSS vector

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

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.

ATT&CK techniques

—

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