CVE-2026-85501 Retrap: Novel Vulnerabilities to launch Algorithmic Complexity Attacks on DNSSEC
Attackers can cause denial of service in Unbound DNS resolver by exploiting algorithmic complexity flaws in DNSSEC validation.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-85501?
CVE-2026-85501 is a medium-severity denial of service vulnerability in Nlnetlabs Unbound, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Weakness: CWE-770
Which versions are affected by CVE-2026-85501?
It affects 1 product.
Products include Nlnetlabs Unbound.
How do I fix CVE-2026-85501?
Upgrade to a fixed version.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-85501 being exploited?
There is no known exploitation of CVE-2026-85501 in the wild and no public exploit code.
Other vulnerabilities in Nlnetlabs Unbound
- CVE-2026-81642 Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY
- CVE-2026-33278 Possible arbitrary code execution during DNSSEC validation
- CVE-2026-42944 Heap overflow with multiple NSID, COOKIE, PADDING EDNS options
- CVE-2026-42959 Crash during DNSSEC validation of malicious content
- CVE-2026-82717 CNAME synthesis could lead to heap corruption
Sources
Also known as AZL-101706, CLSA-2026-1791278576, DEBIAN-CVE-2026-85501, ECHO-d0ad-8b4a-fcc9, GHSA-qwgf-hj58-8c2w, UBUNTU-CVE-2026-85501.