Vulnerability in Nlnet Labs Unbound
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 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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.9 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.002 (15.1th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 15th percentile — 15.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.002 · change: 0.000
Affected products
- Nlnet Labs Unbound — versions 0
- Nlnetlabs Unbound
Weakness classification (CWE)
References
- www.nlnetlabs.nl/downloads/unbound/CVE-2026-44621.txt (vendor-advisory, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-44621?
- CVE-2026-44621 is a medium-severity vulnerability in Nlnet Labs Unbound, classified under Improper Check for Unusual or Exceptional Conditions. CVSS score: 5.9/10. Published 2026-07-22.
- How severe is CVE-2026-44621?
- Medium severity. CVSS v3 base score is 5.9 out of 10.