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UnboundApplication · Nlnetlabs

CVE-2026-55990

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 1.7.0 up to and including 1.25.1, when the 'dnscrypt:' clause lists more 'dnscrypt-provider-cert:' files than there are matching 'dnscrypt-secret-key:' files, Unbound fills only the matched prefix and leaves the tail slots at the '0xdb' fill that libsodium's allocator writes into every allocation. Unbound would then iterate over the number of cert files, not the actual slots, so it walks into a slot with garbage data filled with '0xdb' bytes. Any unauthenticated client that sends one UDP datagram of ≥ 68 bytes whose first 8 bytes are '0xdb' to 'dnscrypt-port' will use that garbage entry which leads to a garbage dereference killing the server. This is a silent faulty configuration that goes unnoticed until triggered with the right client query. Unbound needs to be compiled with DNSCrypt support ('--enable-dnscrypt').

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:>= 1.7.0, < 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 all Unbound instances currently running with DNSCrypt support enabled (compiled with '--enable-dnscrypt').
  2. 2. For each affected instance, backup the current configuration files (typically /etc/unbound/unbound.conf and related files).
  3. 3. Upgrade Unbound to version 1.25.2 or later. On Debian/Ubuntu: 'sudo apt-get update && sudo apt-get install unbound'. On RHEL/CentOS: 'sudo yum update unbound' or 'sudo dnf update unbound'.
  4. 4. Verify the installed version matches or exceeds 1.25.2 using 'unbound -V' or 'unbound-checkconf'.
  5. 5. Review the dnscrypt configuration to ensure the number of 'dnscrypt-provider-cert:' files matches the number of 'dnscrypt-secret-key:' files.
  6. 6. Restart the Unbound service: 'sudo systemctl restart unbound' (or appropriate service manager).
  7. 7. Test that the DNS service responds correctly and monitor for any crashes.
Caveat Upgrading to a new minor version may introduce minor configuration changes; review release notes for any non-default setting changes. Ensure any custom DNSCrypt configurations are still valid after upgrade.

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-2026-55990 in production — separate from our analysis above.

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