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UnboundApplication · Nlnetlabs

CVE-2026-44621

MEDIUM · 5.9 CVSS v3.1 Developing Published 2026-07-22
Fix available
A fix is available. Upgrade to 1.25.2 or later.
See remediation →
68/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click 4 weeks old

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · low confidence
Developing — this CVE was published recently and its authoritative NVD entry is still being established, so we're holding a technical summary until the source data settles rather than publish something unreliable. Re-checking after 2026-08-05.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
UnboundApplication
Affected:< 1.25.2

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
High
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 1.25.2 or later
Fixed in 1.25.2
Recommended fix High confidence

Unbound >= 1.25.2

  1. 1. Identify the current Unbound installation version using 'unbound -V' or 'unbound-control status'
  2. 2. For package managers (e.g., apt, yum, dnf), update the package repository and upgrade Unbound: 'apt-get update && apt-get install unbound' or 'yum update unbound'
  3. 3. If building from source, download Unbound 1.25.2 or later from nlnetlabs.nl and rebuild
  4. 4. After upgrade, verify the version is >= 1.25.2 with 'unbound -V'
  5. 5. If using libunbound in a custom application, rebuild the application against the updated libunbound library
  6. 6. Test the 'unwanted-reply-threshold' configuration to confirm the fix works correctly
  7. 7. Restart any services or applications using libunbound

Generated from the published advisory — verify against the referenced sources before acting.

We can perform the upgrade in your staging environment and verify nothing breaks — typical engagement from $1,600. Get the upgrade done

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-44621 in production — separate from our analysis above.

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