Amazon Linux 2 Security Advisory: ALAS2UNBOUND-1.17-2026-009
Advisory Released Date: 2026-08-04
Advisory Updated Date: 2026-08-04
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)
Affected Packages:
unbound
Note:
This advisory is applicable to Amazon Linux 2 - Unbound-1.17 Extra. Visit this page to learn more about Amazon Linux 2 (AL2) Extras and this FAQ section for the difference between AL2 Core and AL2 Extras advisories.
Issue Correction:
Run yum update unbound or yum update --advisory ALAS2UNBOUND-1.17-2026-009 to update your system.
aarch64:
unbound-1.17.0-2.amzn2.0.12.aarch64
unbound-devel-1.17.0-2.amzn2.0.12.aarch64
unbound-libs-1.17.0-2.amzn2.0.12.aarch64
unbound-anchor-1.17.0-2.amzn2.0.12.aarch64
unbound-utils-1.17.0-2.amzn2.0.12.aarch64
python2-unbound-1.17.0-2.amzn2.0.12.aarch64
python3-unbound-1.17.0-2.amzn2.0.12.aarch64
unbound-debuginfo-1.17.0-2.amzn2.0.12.aarch64
src:
unbound-1.17.0-2.amzn2.0.12.src
x86_64:
unbound-1.17.0-2.amzn2.0.12.x86_64
unbound-devel-1.17.0-2.amzn2.0.12.x86_64
unbound-libs-1.17.0-2.amzn2.0.12.x86_64
unbound-anchor-1.17.0-2.amzn2.0.12.x86_64
unbound-utils-1.17.0-2.amzn2.0.12.x86_64
python2-unbound-1.17.0-2.amzn2.0.12.x86_64
python3-unbound-1.17.0-2.amzn2.0.12.x86_64
unbound-debuginfo-1.17.0-2.amzn2.0.12.x86_64