CVE-2026-44621: With NLnet Labs Unbound up to and including version 1.25.1, applications using libunbound and configured with 'unwanted-reply-threshold', could eventually be abruptly terminated if

mediumCVSS 5.9CVE-2026-44621

With NLnet Labs Unbound up to and including version 1.25.1, applications using libunbound and configured with 'unwanted-reply-threshold', could eventually be abruptly terminated if the threshold is reached and libunbound needs to call 'libworker_alloc_cleanup' since the function is absent from the function call allow list. When an application using libunbound sets 'unwanted-reply-threshold' to any non-zero value and the iterator queries an authoritative that replies with enough wrong-transaction-ID UDP datagrams to cross the threshold, the 'libworker_alloc_cleanup' will eventually be called. Since the function is absent from the function call allow list, this leads to a fatal exit of libunbound and eventual termination of the embedding application.Unbound itself is not affected since its relevant function 'worker_alloc_cleanup' is registed in the allow list and proceeds to perform the documented cache flush.

Details

Source: NVD Recent CVEs (US · database · site)
Severity: medium — CVSS 5.9
Published: 2026-07-22
Last updated: 2026-07-24
Exploitation: Not in CISA KEV at last sync

Original advisory: https://nvd.nist.gov/vuln/detail/CVE-2026-44621

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-446210.25% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 16% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-44621	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

Recent advisories for With NLnet Labs

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

mediumCVE-2026-56444: In NLnet Labs Unbound 1.20.0 up to and including 1.25.1, when Unbound is configured with 'serv…nvd · 2026-07-22
mediumCVE-2026-56416: In NLnet Labs Unbound up to and including version 1.25.1, when the validator builds the canoni…nvd · 2026-07-22
mediumCVE-2026-55991: In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, a remote unauthenticated client can t…nvd · 2026-07-22
mediumCVE-2026-55990: In NLnet Labs Unbound 1.7.0 up to and including 1.25.1, when the 'dnscrypt:' clause lists more…nvd · 2026-07-22
highCVE-2026-55973: In NLnet Labs Unbound 1.23.0 up to and including 1.25.1, when 'dns-error-reporting: yes' is se…nvd · 2026-07-22
mediumCVE-2026-55717: In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set toge…nvd · 2026-07-22