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Vulnerability · Published Jul 22, 2026 · Assigned by NLnet Labs

CVE-2026-44687Off-by-one error in 'harden-below-nxdomain' logic can shadow a stub/forward zone by a legitimate parent's NXDOMAIN

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 0.2% (higher than 12% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)3.7Low
Exploit odds (30 days)0.2%top 87.7% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official description

In NLnet Labs Unbound 1.13.2 up to and including 1.25.1, stub or forward zones where the name is below an intermediate labed below a DNSSEC signed zone could be shadowed by the intermediate label's secure NXDOMAIN answer from the parent. This is caused by an off-by-one error in 'harden-below-nxdomain' logic; enabled by default. It effectively bypasses the configuration and the configured stub/forward zone is never contacted. 'harden-below-nxdomain' does an upward DNS cache walk together with a delegation point guard that does not allow NXDOMAIN synthesis above stub/forward zones. The guard tests the domain name but before stripping a label. This results in an iteration where the domain name equals the configured stub/forward zone apex that passes the guard, strips one more label, and probes the cache at the apex's immediate public parent. If that parent has a cached DNSSEC-secure NXDOMAIN, which it will for any private namespace nested two or more labels under a signed public name, the walk returns it and the configured stub/forward upstream is never contacted. This can only be triggered by the query for the intermediate label (between the stub/forward apex and the DNSSEC parent zone).

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Alpine:v3.22unbound1.25.2-r0ALPINE-CVE-2026-44687
Alpine:v3.23unbound1.25.2-r0ALPINE-CVE-2026-44687
Alpine:v3.24unbound1.25.2-r0ALPINE-CVE-2026-44687
Debian:12unboundno fix listedDEBIAN-CVE-2026-44687
Debian:13unbound1.26.1-0+deb13u1DEBIAN-CVE-2026-44687
Debian:14unbound1.25.2-1DEBIAN-CVE-2026-44687
Ubuntu:22.04:LTSunboundno fix listedUBUNTU-CVE-2026-44687
Ubuntu:24.04:LTSunboundno fix listedUBUNTU-CVE-2026-44687
Ubuntu:26.04:LTSunboundno fix listedUBUNTU-CVE-2026-44687
Ubuntu:26.10unboundno fix listedUBUNTU-CVE-2026-44687

Affected products (as reported)

VendorProductVersions
NLnet Labs Unbound 1.13.2 – before 1.25.2

Weakness: CWE-193 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

References