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Vulnerability · Published Sep 16, 2026 · Assigned by NLnet Labs

CVE-2026-85501Retrap: Novel Vulnerabilities to launch Algorithmic Complexity Attacks on DNSSEC

Medium 5.3

In plain English

A medium-severity resource allocation without limits in Unbound from NLnet Labs. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 42% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)5.3Medium
Exploit odds (30 days)0.5%top 58.3% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official 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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Debian:12unboundno fix listedDEBIAN-CVE-2026-85501
Debian:13unbound1.26.1-0+deb13u1DEBIAN-CVE-2026-85501
Debian:14unbound1.26.1-1DEBIAN-CVE-2026-85501
Ubuntu:22.04:LTSunbound1.13.1-1ubuntu5.17UBUNTU-CVE-2026-85501
Ubuntu:22.04:LTSunbound1.13.1-1ubuntu5.17USN-8946-1
Ubuntu:24.04:LTSunbound1.19.2-1ubuntu3.12USN-8946-1
Ubuntu:24.04:LTSunbound1.19.2-1ubuntu3.12UBUNTU-CVE-2026-85501
Ubuntu:26.04:LTSunbound1.24.2-1ubuntu2.4USN-8946-1
Ubuntu:26.04:LTSunbound1.24.2-1ubuntu2.4UBUNTU-CVE-2026-85501
Ubuntu:26.10unboundno fix listedUBUNTU-CVE-2026-85501

Affected products (as reported)

VendorProductVersions
NLnet Labs Unbound 0 – before 1.26.1

Weakness: CWE-770 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

References