Bug 2535057 (CVE-2026-85501)

Summary: CVE-2026-85501 unbound: Unbound: Denial of Service via algorithmic complexity attacks on DNSSEC
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: akhatavk, aos-team-art-private, asdas, dpaolell, jdelft, jupierce, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, sghai, sidsharm, suppawar, vlaad, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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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.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Bug Depends On: 2537353    
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Description OSIDB Bzimport 2026-09-16 09:06:52 UTC 
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.