CVE-2026-85501

Retrap: Novel Vulnerabilities to launch Algorithmic Complexity Attacks on DNSSEC

Published Sep 16, 2026 CVSS 3.1 · 5.3 Medium EPSS < 1% KEV · Not listed
Description
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.

Remediation

Vendor Solution

This issue is fixed starting with version 1.26.1

Tracking

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Advisories

Source ID Title
Debian DSA Debian DSA DSA-6507-1 unbound security update
Ubuntu USN Ubuntu USN USN-8946-1 Unbound vulnerabilities
Ubuntu USN Ubuntu USN USN-8946-2 Unbound vulnerabilities
History
Type Values Removed Values Added
CPEs cpe:2.3:a:nlnetlabs:unbound:*:*:*:*:*:*:*:*
Type Values Removed Values Added
First Time appeared Nlnetlabs
Nlnetlabs unbound
Vendors & Products Nlnetlabs
Nlnetlabs unbound
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
Description 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.
Title Retrap: Novel Vulnerabilities to launch Algorithmic Complexity Attacks on DNSSEC
Weaknesses CWE-770
References
Metrics cvssV3_1

{'score': 5.3, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
NLnet Labs
Published
Updated
Sep 16, 2026, 2:12 PM
Reserved
Sep 7, 2026, 2:06 PM
Status
Analyzed
Published
Sep 16, 2026, 9:17 AM
Modified
Sep 23, 2026, 7:10 PM
Severity
Moderate
Public date
Sep 16, 2026, 8:32 AM