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

CVE-2026-50251Attacker supplied '0.0.0.0'/'::' glue triggers defensive full-cache flush

Medium 5.3

In plain English

A medium-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 0.3% (higher than 17% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)5.3Medium
Exploit odds (30 days)0.3%top 83.0% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official description

In NLnet Labs Unbound up to and including version 1.25.1, when 'unwanted-reply-threshold' is enabled (set to any value greater than zero), glue records of 0.0.0.0/::0 can short-circuit Unbound, on systems that can direct such traffic, by issuing DNS queries and receiving seemingly unwanted replies since the remote IP does not match the original source IP of 0.0.0.0/::0. This behavior keeps on looping for the glue records and pushing the counter to the configured 'unwanted-reply-threshold' that triggers a defensive cache clear. A malicious actor who controls a delegation that returns in-bailiwick glue of 0.0.0.0/::0 can drive the counter to the limit of 'unwanted-reply-threshold' to the threshold and trigger a cache clean of the message and rrset caches; at will, indefinitely, without sending a single spoofed packet. The iterator uses the 0.0.0.0/::0 glue, and a system that can route this (e.g., Linux kernel routes the datagram over loopback), Unbound's own listener answers from 127.0.0.1. Because of the mismatch of 0.0.0.0 and 127.0.0.1, in this example, Unbound accounts the reply as an unwanted (probably spoofed) answer. The counter resets to zero on every cache flush, so the attack loops forever.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Alpine:v3.22unbound1.25.2-r0ALPINE-CVE-2026-50251
Alpine:v3.23unbound1.25.2-r0ALPINE-CVE-2026-50251
Alpine:v3.24unbound1.25.2-r0ALPINE-CVE-2026-50251
Debian:12unboundno fix listedDEBIAN-CVE-2026-50251
Debian:13unbound1.26.1-0+deb13u1DEBIAN-CVE-2026-50251
Debian:14unbound1.25.2-1DEBIAN-CVE-2026-50251
Ubuntu:22.04:LTSunboundno fix listedUBUNTU-CVE-2026-50251
Ubuntu:24.04:LTSunboundno fix listedUBUNTU-CVE-2026-50251
Ubuntu:26.04:LTSunboundno fix listedUBUNTU-CVE-2026-50251
Ubuntu:26.10unboundno fix listedUBUNTU-CVE-2026-50251

Affected products (as reported)

VendorProductVersions
NLnet Labs Unbound 0 – before 1.25.2

Weakness: CWE-184 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

References