Amazon Linux 2 : unbound, --advisory ALAS2UNBOUND-1.17-2026-009 (ALASUNBOUND-1.17-2026-009)

high Nessus Plugin ID 331996

Synopsis

The remote Amazon Linux 2 host is missing a security update.

Description

The version of unbound installed on the remote host is prior to 1.17.0-2. It is, therefore, affected by multiple vulnerabilities as referenced in the ALAS2UNBOUND-1.17-2026-009 advisory.

In NLnet Labs Unbound 1.7.0 up to and including 1.25.1, insufficient validation of the RRSIG.Labels field combined with premature cache writes during RFC 8198 aggressive NSEC processing leads to cache poisoning that permits a malicious actor controlling a single delegated zone to poison arbitrary sibling zones under NSEC-signed parent domains. A malicious actor with one registered domain under an NSEC-signed TLD can serve malicious insecure DNS responses for unrelated sibling domains (sharing the same parent zone).
Arbitrary delegations that do not exist under the parent domain and are covered by the parent's NSEC chain can be brought into insecure existence by fraudulent wildcard DS records (less labels than expected, unknown algorithm) from the malicious sibling domain. This allows the malicious actor to inject insecure wildcard records for those delegations. (CVE-2026-44690)

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. (CVE-2026-50251)

Tenable has extracted the preceding description block directly from the tested product security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

Solution

Run 'yum update unbound' or or 'yum update --advisory ALAS2UNBOUND-1.17-2026-009' to update your system.

Plugin Details

Severity: High

ID: 331996

File Name: al2_ALASUNBOUND-1_17-2026-009.nasl

Version: 1.1

Type: Local

Agent: unix

Family: Amazon Linux Local Security Checks

Published: 8/4/2026

Updated: 8/4/2026

Supported Sensors: Agentless Assessment, Continuous Assessment, Frictionless Assessment Agent, Frictionless Assessment AWS, Nessus Agent, Tenable Cloud Security, Tenable Self-Hosted Container Security, Nessus

Risk Information

VPR

Risk Factor: Low

Score: 3

Percentile: 23.72

CVSS v2

Risk Factor: High

Base Score: 7.8

Temporal Score: 5.8

Vector: CVSS2#AV:N/AC:L/Au:N/C:N/I:C/A:N

CVSS Score Source: CVE-2026-44690

CVSS v3

Risk Factor: High

Base Score: 7.5

Temporal Score: 6.5

Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Temporal Vector: CVSS:3.0/E:U/RL:O/RC:C

Vulnerability Information

CPE: cpe:/o:amazon:linux:2, p-cpe:/a:amazon:linux:python2-unbound, p-cpe:/a:amazon:linux:python3-unbound, p-cpe:/a:amazon:linux:unbound-anchor, p-cpe:/a:amazon:linux:unbound-debuginfo, p-cpe:/a:amazon:linux:unbound-devel, p-cpe:/a:amazon:linux:unbound-libs, p-cpe:/a:amazon:linux:unbound-utils, p-cpe:/a:amazon:linux:unbound

Required KB Items: Host/local_checks_enabled, Host/AmazonLinux/release, Host/AmazonLinux/rpm-list

Exploit Ease: No known exploits are available

Patch Publication Date: 8/4/2026

Vulnerability Publication Date: 7/22/2026

Reference Information

CVE: CVE-2026-44690, CVE-2026-50251

Detections

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