Amazon Linux 2023 : python3-unbound, unbound, unbound-anchor (ALAS2023-2026-3313)

medium Nessus Plugin ID 371837

Synopsis

The remote Amazon Linux 2023 host is missing a security update.

Description

It is, therefore, affected by multiple vulnerabilities as referenced in the ALAS2023-2026-3313 advisory.

In NLnet Labs Unbound up to and including version 1.26.1, an observable cache-level side-channel can be exposed with 'unwanted-reply-threshold > 0' configurations (not a default configuration). If a response- like UDP datagram reaches the upstream source port currently used by a recursive query, but its TXID does not match any pending query, Unbound counts the packet as an unwanted reply. When the accumulated count reaches the configured threshold, Unbound clears the worker's RRset cache and message cache. This cache clear action does not destroy the current upstream UDP pending query. Therefore, an external observer can distinguish whether a candidate port matches the current upstream source port by checking whether an existing cached item has been cleared. This is a cache side-channel vulnerability that can be used to probe the upstream source port. Although the attack is not feasible on default configurations and the suggested value (10 million) makes it an ureliable attack on a public resolver, in scenarios where the attacker can keep a concurrent query window alive, the attacker can first filter the source port and then perform birthday-style guessing against the TXID, making cache poisoning a feasible attack path.
(CVE-2026-109125)

When NLnet Labs Unbound 1.0.1 up to and including version 1.26.1 is configured with both a root trust anchor and an additional 'trust-anchor:' for an island-of-security zone whose parent is provably insecure under the root chain, a malicious actor with a forged NSEC reply with the SOA bit cleared can trick Unbound to treat the configured trust island as insecure. This happens because the DNSSEC validator in Unbound accepts and caches the NSEC insecure response bypassing the RRSIG validation that the validator was supposed to do. The acceptance of the NSEC record at the trust anchor apex was allowed due to and old fix for type DS and NSEC queries from clients when trust anchor for that name existed. (CVE-2026-35501)

When NLnet Labs Unbound up to and including version 1.26.1 is configured with both a root trust anchor and an additional 'trust-anchor:' for an island-of-security zone whose parent is provably insecure under the root chain, a malicious actor who can deliver one forged NXDOMAIN response for a name inside the island, with the NSEC/SOA RRSIG 'SignerName' set to the insecure parent, tricks Unbound's validator to use the root trust anchor instead, and consecutively the insecure parent, thus causing that record to be treated as insecure and denying existence for records in the trust island. (CVE-2026-50106)

NLnet Labs Unbound up to and including version 1.26.1 limits compression-pointer traversal for each individual domain-name operation but has no aggregate inbound decompression-work budget. Many chained compression pointers in a message results in a lot of CPU usage following them. A malicious actor with recursive-query access and control of an ordinarily delegated authority that serves specially crafted replies can exploit the vulnerability and degrade Unbound's general performance. (CVE-2026-96405)

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 'dnf update unbound --releasever 2023.12.20261026' or or 'dnf update --advisory ALAS2023-2026-3313 --releasever 2023.12.20261026' to update your system.

Plugin Details

Severity: Medium

ID: 371837

File Name: al2023_ALAS2023-2026-3313.nasl

Version: 1.1

Type: Local

Agent: unix

Family: Amazon Linux Local Security Checks

Published: 10/25/2026

Updated: 10/25/2026

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: 3.5

Percentile: 51.44

CVSS v2

Risk Factor: Medium

Base Score: 6.1

Temporal Score: 4.5

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

CVSS Score Source: CVE-2026-50106

CVSS v3

Risk Factor: Medium

Base Score: 6.5

Temporal Score: 5.7

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

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

Vulnerability Information

CPE: cpe:/o:amazon:linux:2023, p-cpe:/a:amazon:linux:python3-unbound-debuginfo, p-cpe:/a:amazon:linux:python3-unbound, p-cpe:/a:amazon:linux:unbound-anchor-debuginfo, p-cpe:/a:amazon:linux:unbound-anchor, p-cpe:/a:amazon:linux:unbound-debuginfo, p-cpe:/a:amazon:linux:unbound-debugsource, p-cpe:/a:amazon:linux:unbound-devel, p-cpe:/a:amazon:linux:unbound-libs-debuginfo, p-cpe:/a:amazon:linux:unbound-libs, p-cpe:/a:amazon:linux:unbound-utils-debuginfo, 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: 10/24/2026

Vulnerability Publication Date: 10/21/2026

Reference Information

CVE: CVE-2026-109125, CVE-2026-35501, CVE-2026-50106, CVE-2026-96405

IAVA: 2026-A-1194

Detections

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