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NVD / CVE Program· CNA NLnet Labs

CVE-2026-55990: Packet of death for a DNSCrypt misconfigured Unbound

Noroxi assessment

Priority: Monitor (action score 23/100)

  • 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: 0.24% (FIRST EPSS).
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. Special conditions such as timing or configuration are required.
  • 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 NLnet Labs Unbound 1.7.0 up to and including 1.25.1, when the 'dnscrypt:' clause lists more 'dnscrypt-provider-cert:' files than there are matching 'dnscrypt-secret-key:' files, Unbound fills only the matched prefix and leaves the tail slots at the '0xdb' fill that libsodium's allocator writes into every allocation. Unbound would then iterate over the number of cert files, not the actual slots, so it walks into a slot with garbage data filled with '0xdb' bytes. Any unauthenticated client that sends one UDP datagram of ≥ 68 bytes whose first 8 bytes are '0xdb' to 'dnscrypt-port' will use that garbage entry which leads to a garbage dereference killing the server. This is a silent faulty configuration that goes unnoticed until triggered with the right client query. Unbound needs to be compiled with DNSCrypt support ('--enable-dnscrypt').

MediumCVSS 5.9 · v3.1—No exploit Fix available
Published
Jul 22, 2026
Updated
Oct 24, 2026
EPSS
0.2% · 16th percentile

Report tools

JSON

Action score

23

Monitor

Low priority for now.

CVSS
23 / 40 · 5.9 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.2%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · NLnet Labs
5.9
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 unbound1.7.0 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

    • 1.7.0 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
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

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-2026-55717Jul 22, 2026 · 'serve-expired-client-timeout' and 'response-ip' CNAME redirect could lead to a crash
    23Monitor
  • ← EarlierCVE-2026-55973Jul 22, 2026 · 'dns-error-reporting: yes' leads to stack buffer overflow
    30Monitor
  • Later →CVE-2026-55991Jul 22, 2026 · Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2
    23Monitor

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)
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-slfo.1.1_1.1 · SUSE:Linux Micro 6.1Package 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.
  • Special conditions such as timing or configuration are required.

If successful

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

Weakness class (CWE)

CWE-457 · Use of Uninitialized Variable

CVSS vector

CVSS:3.1/AV:N/AC:H/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.

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