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UnboundApplication · Nlnetlabs

CVE-2026-56444

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
In NLnet Labs Unbound 1.20.0 up to and including 1.25.1, when Unbound is configured with 'serve-expired: yes' and 'serve-expired-client-timeout > discard-timeout > 0' (contrary to the suggested values), the discard-timeout branch during the serve expired logic drops an aged client reply without performing the correct accounting for the number of reply addresses for the query. Other identical branches outside of serve expired perform the correct decrement. Since the counter is never decremented in such scenario, it can reach the maximum limit and new clients for duplicate in-flight queries are silently dropped resulting in degradation of resolution service. A malicious actor can exploit the vulnerability by querying the resolver for a client-controlled slow-on-demand authoritative zone that can drive the counter past the threshold. Shipped defaults for 'serve-expired-client-timeout: 1800' and 'discard-timeout: 1900' make the branch unreachable.

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:>= 1.20.0, < 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 version by running 'unbound -V' or 'unbound-control status'
  2. 2. Back up the current Unbound configuration files (typically in /etc/unbound/ or /usr/local/etc/unbound/)
  3. 3. Stop the Unbound service using the appropriate command for your system (e.g., 'systemctl stop unbound' or 'service unbound stop')
  4. 4. Upgrade Unbound to version 1.25.2 or later using your system's package manager or by compiling from source
  5. 5. Verify the installed version with 'unbound -V' to confirm the upgrade was successful
  6. 6. Restart the Unbound service (e.g., 'systemctl start unbound' or 'service unbound start')
  7. 7. Verify normal operation by testing DNS resolution with 'dig @localhost example.com'
  8. 8. If using the vulnerable configuration ('serve-expired: yes' with 'serve-expired-client-timeout > discard-timeout > 0'), review and adjust the configuration to align with recommended defaults

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-2026-56444 in production — separate from our analysis above.

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What this is

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