Vulnerability · Published Oct 21, 2026 · Assigned by NLnet Labs

CVE-2026-35501Bypass of a configured island trust anchor underneath an insecure parent from an atacker-controlled NSEC record.

Low 3.7

In plain English

A low-severity security flaw 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 0.1% (higher than 1% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)3.7Low

Exploit odds (30 days)0.1%top 98.7% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

When NLnet Labs Unbound 1.0.1 up to and including version 1.26.1 is configured with both a root trust anchor and an additional 'trust-anchor:' for an island-of-security zone whose parent is provably insecure under the root chain, a malicious actor with a forged NSEC reply with the SOA bit cleared can trick Unbound to treat the configured trust island as insecure. This happens because the DNSSEC validator in Unbound accepts and caches the NSEC insecure response bypassing the RRSIG validation that the validator was supposed to do. The acceptance of the NSEC record at the trust anchor apex was allowed due to and old fix for type DS and NSEC queries from clients when trust anchor for that name existed.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	unbound	no fix listed	DEBIAN-CVE-2026-35501
Debian:13	unbound	1.26.2-0+deb13u1	DEBIAN-CVE-2026-35501
Debian:14	unbound	1.26.2-1	DEBIAN-CVE-2026-35501
Ubuntu:22.04:LTS	unbound	no fix listed	UBUNTU-CVE-2026-35501
Ubuntu:24.04:LTS	unbound	no fix listed	UBUNTU-CVE-2026-35501
Ubuntu:26.04:LTS	unbound	no fix listed	UBUNTU-CVE-2026-35501
Ubuntu:26.10	unbound	no fix listed	UBUNTU-CVE-2026-35501

Affected products (as reported)

Vendor	Product	Versions
NLnet Labs	Unbound	1.0.1 – before 1.26.2

Weakness: CWE-354 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

References

https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-35501.txt vendor-advisory