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CVE-2026-55990 NLnet Labs CVE debrief

A vulnerability was found in Unbound DNSCrypt. 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. This faulty configuration can lead to a garbage dereference when an unauthenticated client sends a UDP datagram of ≥ 68 bytes whose first 8 bytes are '0xdb' to 'dnscrypt-port', potentially causing a crash. Users of Unbound with DNSCrypt support enabled should verify their configuration to prevent potential crashes.

Vendor
NLnet Labs
Product
Unbound
CVSS
MEDIUM 5.9
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-22
Original CVE updated
2026-07-22
Advisory published
2026-07-22
Advisory updated
2026-07-22

Who should care

Users of Unbound with DNSCrypt support enabled should verify their configuration to prevent potential crashes. This includes ensuring matching 'dnscrypt-provider-cert:' and 'dnscrypt-secret-key:' files and restricting access to 'dnscrypt-port' to authenticated clients only. Security teams and operators managing Unbound deployments should review their configurations and monitor logs for potential crashes or errors.

Technical summary

The vulnerability occurs when Unbound is compiled with DNSCrypt support and the 'dnscrypt:' clause lists more 'dnscrypt-provider-cert:' files than there are matching 'dnscrypt-secret-key:' files. This leads to a garbage dereference when an unauthenticated client sends a UDP datagram of ≥ 68 bytes whose first 8 bytes are '0xdb' to 'dnscrypt-port'. The issue arises from Unbound iterating over the number of cert files, not the actual slots, causing it to access garbage data filled with '0xdb' bytes.

Defensive priority

Medium priority due to potential for crashes and data exposure.

Recommended defensive actions

  • Verify Unbound configuration to ensure matching 'dnscrypt-provider-cert:' and 'dnscrypt-secret-key:' files.
  • Restrict access to 'dnscrypt-port' to authenticated clients only.
  • Monitor Unbound logs for potential crashes or errors.
  • Review compensating controls for exposed systems while remediation is scheduled and verified.
  • Check relevant monitoring, detection, and logs for exposed assets that need extra review.
  • Track exceptions, retest remediated assets, and close the item only after evidence is documented.
  • Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.

Evidence notes

The vulnerability was reported by [email protected]. The CVE record was published on 2026-07-22T14:17:21.810Z and last modified on 2026-07-22T20:33:11.590Z. Evidence is limited to the supplied source corpus and CVE record details. Defenders should verify Unbound configuration and DNSCrypt support.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-55990 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-55990

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-55990 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-55990

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.