Copyright © 2012 Red Hat, Inc. All rights reserved. Moderate 2026-09-16T08:32:25 unbound: Unbound: Denial of Service via algorithmic complexity attacks on DNSSEC 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L CWE-770

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.

A flaw was found in Unbound. This vulnerability, termed 'ReTrap', allows a remote attacker to launch algorithmic complexity attacks on the Domain Name System Security Extensions (DNSSEC) validation process. By serving malicious zones or responses, an attacker can exploit various mechanisms, such as mismatched DNSKEY records, deeply nested domains, or excessive invalid NSEC records. Successful exploitation leads to a degradation of service, effectively causing a Denial of Service (DoS) for affected Unbound resolvers.

This Moderate flaw in Unbound can lead to a denial of service due to algorithmic complexity attacks on DNSSEC. An attacker can exploit this by serving malicious DNS zones, causing Unbound to consume excessive resources during DNSSEC validation, thereby degrading service availability. This affects Red Hat products utilizing Unbound for DNSSEC resolution, where the default configuration validates the additional section of DNS responses. To mitigate implement hard caps on iterative processing loops and resource allocations per request. Apply rate limiting and set strict connection/processing timeouts to prevent a single complex operation from tying up worker threads. For this specific flaw, administrators can also configure system-level resource constraints (like CPU cgroups) for the service. Red Hat Hardened Images 2026-09-17T00:00:00Z RHSA-2026:68590 unbound-main-1.26.1-1.hum1 Red Hat Enterprise Linux 10 Fix deferred unbound Red Hat Enterprise Linux 6 Out of support scope unbound Red Hat Enterprise Linux 7 Fix deferred unbound Red Hat Enterprise Linux 8 Fix deferred unbound Red Hat Enterprise Linux 9 Fix deferred unbound Red Hat OpenShift Container Platform 4 Fix deferred openshift/ose-rhel-coreos-8 Red Hat OpenShift Container Platform 4 Fix deferred openshift/ose-rhel-coreos-9 https://www.cve.org/CVERecord?id=CVE-2026-85501 https://nvd.nist.gov/vuln/detail/CVE-2026-85501 https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-85501.txt