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CVE-2026-85501· NVD / CVE Program· CNA NLnet Labs

Retrap: Novel Vulnerabilities to launch Algorithmic Complexity Attacks on DNSSEC

Novel vulnerabilities to launch algorithmic complexity attacks on DNSSEC have been researched under the term 'ReTrap'. These result in degradation of service when malicious zones are used to serve the algorithmic complexity vulnerabilities. NLnet Labs Unbound up to and including 1.26.0 is vulnerable to some of them. TagTrap, where the triple(Zone, Algo, KeyTag) matching mechanism introduces a significant attack vector when resolvers handle malicious responses containing numerous mismatched DNSKEY, RRSIG, and DS record. DelegationTrap, where constructing the chain-of-trust requires iterative validation of DNSKEY and DS records from the root zone downward. For deeply nested domains, this results in significant computational overhead. NsecTrap, where responses with excessive invalid NSEC records compel the resolver to validate each one. AdditionalTrap, where Unbound by default would try to DNSSEC validate the ADDITIONAL section as well. This can be exploited to waste validation resources by malicious users.

HighCVSS 7.5 · v3.1—No exploit Fix available
Published
Sep 16, 2026
Updated
Sep 23, 2026
EPSS
0.5% · 39th percentile
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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 · 0.5%

CISA SSVC decision

Exploitation
none
Automatable
yes
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
7.5
CNA · NLnet Labs
5.3
2.2 point gap

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

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 1.26.1

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 1.26.1affected · semver

Package-level exposure

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

EcosystemPackageAffected rangeFix
Debian:12unboundall versions—
Debian:13unboundbefore 1.26.1-0+deb13u11.26.1-0+deb13u1
Debian:14unboundbefore 1.26.1-11.26.1-1
Red Hat:hummingbird:1python3-unboundbefore 0:1.26.1-1.hum10:1.26.1-1.hum1
Red Hat:hummingbird:1unboundbefore 0:1.26.1-1.hum10:1.26.1-1.hum1
Red Hat:hummingbird:1unbound-anchorbefore 0:1.26.1-1.hum10:1.26.1-1.hum1
Red Hat:hummingbird:1unbound-develbefore 0:1.26.1-1.hum10:1.26.1-1.hum1
Red Hat:hummingbird:1unbound-dracutbefore 0:1.26.1-1.hum10:1.26.1-1.hum1
Red Hat:hummingbird:1unbound-libsbefore 0:1.26.1-1.hum10:1.26.1-1.hum1
Red Hat:hummingbird:1unbound-muninbefore 0:1.26.1-1.hum10:1.26.1-1.hum1
Red Hat:hummingbird:1unbound-utilsbefore 0:1.26.1-1.hum10:1.26.1-1.hum1

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 Unbound1.26.1Vendor (CNA)
azl3 unbound 1.26.0-1 on Azure Linux 3.01.26.1-1Microsoft (MSRC)
debian:unbound1.26.1-0+deb13u1 · Debian:13Package registry (OSV)
red hat:python3-unbound0:1.26.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound0:1.26.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-anchor0:1.26.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-devel0:1.26.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-dracut0:1.26.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-libs0:1.26.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-munin0:1.26.1-1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-utils0:1.26.1-1.hum1 · Red Hat:hummingbird: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.

  • CVE-2026-802250 days apartPossible degradation of service from continuous queries on the same TCP/DoT connection
    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-770 · Allocation of Resources Without Limits or Throttling

CVSS vector

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

nvd-primary

Attack context

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

Noroxi analysis

No Noroxi analysis for this record yet

We don't hand-write analysis for all 385,000+ vulnerabilities; that wouldn't be honest. For notable, high-impact vulnerabilities our team writes the mechanism, detection and remediation steps.

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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