ALAS2023-2026-3313

References: CVE-2026-109125  CVE-2026-35501  CVE-2026-50106  CVE-2026-96405 
FAQs regarding Amazon Linux ALAS/CVE Severity

Issue Overview:

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)

Issue Correction:
Run dnf update unbound --releasever 2023.12.20261026 or dnf update --advisory ALAS2023-2026-3313 --releasever 2023.12.20261026 to update your system.
More information on how to update your system can be found on this page: Amazon Linux 2023 documentation

New Packages:

aarch64:
    unbound-anchor-debuginfo-1.17.1-1.amzn2023.0.17.aarch64
    unbound-debuginfo-1.17.1-1.amzn2023.0.17.aarch64
    python3-unbound-debuginfo-1.17.1-1.amzn2023.0.17.aarch64
    unbound-libs-1.17.1-1.amzn2023.0.17.aarch64
    unbound-libs-debuginfo-1.17.1-1.amzn2023.0.17.aarch64
    unbound-devel-1.17.1-1.amzn2023.0.17.aarch64
    unbound-1.17.1-1.amzn2023.0.17.aarch64
    unbound-anchor-1.17.1-1.amzn2023.0.17.aarch64
    python3-unbound-1.17.1-1.amzn2023.0.17.aarch64
    unbound-utils-debuginfo-1.17.1-1.amzn2023.0.17.aarch64
    unbound-utils-1.17.1-1.amzn2023.0.17.aarch64
    unbound-debugsource-1.17.1-1.amzn2023.0.17.aarch64

src:
    unbound-1.17.1-1.amzn2023.0.17.src

x86_64:
    unbound-debuginfo-1.17.1-1.amzn2023.0.17.x86_64
    unbound-utils-debuginfo-1.17.1-1.amzn2023.0.17.x86_64
    unbound-libs-debuginfo-1.17.1-1.amzn2023.0.17.x86_64
    python3-unbound-1.17.1-1.amzn2023.0.17.x86_64
    unbound-debugsource-1.17.1-1.amzn2023.0.17.x86_64
    unbound-libs-1.17.1-1.amzn2023.0.17.x86_64
    unbound-1.17.1-1.amzn2023.0.17.x86_64
    unbound-anchor-debuginfo-1.17.1-1.amzn2023.0.17.x86_64
    python3-unbound-debuginfo-1.17.1-1.amzn2023.0.17.x86_64
    unbound-devel-1.17.1-1.amzn2023.0.17.x86_64
    unbound-anchor-1.17.1-1.amzn2023.0.17.x86_64
    unbound-utils-1.17.1-1.amzn2023.0.17.x86_64