Medium [CVE-2026-50251] Denial of Service via crafted DNS glue records
This medium-severity Red Hat Linux advisory covers CVE-2026-50251 affecting Red Hat Hardened Images, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6.
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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. A flaw was found in Unbound.
A remote malicious actor can exploit a vulnerability by controlling a DNS delegation that returns specific glue records.
- 1.25.1
Official advisory · high-confidence parse· fetched 1 day ago·verify at source
- unbound-main-1.25.2-0.1.hum1
- RHSA-2026:43588
Official advisory · high-confidence parse· fetched 1 day ago·verify at source
Mitigation checklist
- To mitigate this issue, configure Unbound to disable the `unwanted-reply-threshold` option by setting it to `0` in the `unbound.conf` file. This prevents the cache clearing behavior triggered by malicious glue records. After modifying the configuration, restart the Unbound service for the changes to take effect.
Official advisory · high-confidence parse· fetched 1 day ago·verify at source
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