# CVE-2026-50251: Attacker supplied '0.0.0.0'/'::' glue triggers defensive full-cache flush CVE-2026-50251 is fixed in 12 products on 1 platform, first by DEBIAN-CVE-2026-50251, released July 28, 2026. It is not reported as exploited. Patch Radar priority: Routine. - Web page: https://patchblox.com/security/cve/CVE-2026-50251 - Priority: Routine - CVSS: 5.3 (Moderate) - EPSS: 0.0025 - CISA KEV: not listed - Exploited (vendor report): no - Publicly disclosed: no - Published: Jul 22, 2026 - Weakness: Incomplete List of Disallowed Inputs - Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-50251 ## 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. ## Where it's fixed | Platform | Product | Fixed in | Fix | Released | |---|---|---|---|---| | Linux | python3-unbound amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 | | Linux | python3-unbound-debuginfo amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 | | Linux | unbound Debian 13.5 (trixie) | 1.26.1-0+deb13u1 | [DEBIAN-CVE-2026-50251](https://patchblox.com/security/data/fix/linux/debian-cve-2026-50251.md) | Jul 28, 2026 | | Linux | unbound Debian 14 | 1.25.2-1 | [DEBIAN-CVE-2026-50251](https://patchblox.com/security/data/fix/linux/debian-cve-2026-50251.md) | Jul 28, 2026 | | Linux | unbound amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 | | Linux | unbound opensuse-leap 16.0 | 1.25.2-160000.1.1 | [OPENSUSE-SU-2026:21550-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-21550-1.md) | Aug 10, 2026 | | Linux | unbound sles 15.6 | 1.25.2-150600.23.19.1 | [SUSE-SU-2026:3885-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-3885-1.md) | Aug 31, 2026 | | Linux | unbound sles 15.7 | 1.25.2-150600.23.19.1 | [SUSE-SU-2026:3885-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-3885-1.md) | Aug 31, 2026 | | Linux | unbound sles 16.0 | 1.25.2-160000.1.1 | [SUSE-SU-2026:23226-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-23226-1.md) | Aug 10, 2026 | | Linux | unbound-anchor amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 | | Linux | unbound-anchor-debuginfo amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 | | Linux | unbound-debuginfo amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 | | Linux | unbound-debugsource amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 | | Linux | unbound-devel amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 | | Linux | unbound-libs amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 | | Linux | unbound-libs-debuginfo amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 | | Linux | unbound-utils amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 | | Linux | unbound-utils-debuginfo amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 | --- Data as of 2026-11-09T01:18:40Z UTC. 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