CVE-2026-50251 Attacker supplied '0.0.0.0'/'::' glue triggers defensive full-cache flush
Attackers can trigger indefinite cache flushes in Unbound by exploiting glue record handling, causing denial of service when unwanted-reply-threshold is enabled.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-50251?
CVE-2026-50251 is a medium-severity denial of service vulnerability in Nlnetlabs Unbound, published on 22 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Weakness: CWE-184
Which versions are affected by CVE-2026-50251?
It affects 1 product.
Products include Nlnetlabs Unbound.
How do I fix CVE-2026-50251?
Apply the vendor's patch.
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-2026-50251 being exploited?
There is no known exploitation of CVE-2026-50251 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 ALPINE-CVE-2026-50251, AZL-92979, DEBIAN-CVE-2026-50251, ECHO-b852-f6a0-98dc, GHSA-rj42-r8f9-w75j, UBUNTU-CVE-2026-50251.