CVE-2026-50251 (GCVE-0-2026-50251)

Vulnerability from cvelistv5 – Published: 2026-07-22 13:08 – Updated: 2026-07-22 14:34
VLAI
Title
Attacker supplied '0.0.0.0'/'::' glue triggers defensive full-cache flush
Summary
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.
Severity
5.3 (Medium)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
SSVC
Exploitation: none Automatable: yes Technical Impact: partial
CISA Coordinator (v2.0.3)
CWE
  • CWE-184 - Incomplete List of Disallowed Inputs
Assigner
References
Impacted products
Vendor Product Version
NLnet Labs Unbound Affected: 0 , < 1.25.2 (semver)
Create a notification for this product.
Date Public
2026-07-22 00:00
Credits
Qifan Zhang (Palo Alto Networks)
Show details on NVD website
To clipboard 

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