NLnet Labs Unbound CVE-2026-55990

Two independent conditions must both be satisfied. First, the Unbound binary must have been compiled with --enable-dnscrypt; this is not the default and must be explicitly chosen at build time, so stock distribution packages without this flag are entirely unaffected. Second, the active unbound.conf must contain a dnscrypt: block where the count of dnscrypt-provider-cert: directives exceeds the count of dnscrypt-secret-key: directives - a silent misconfiguration that produces no startup error. Given both conditions, any unauthenticated attacker with UDP reachability to the configured dnscrypt-port can crash the server with a single packet whose first 8 bytes equal 0xdb. Limiting factors: non-default build requirement significantly reduces the exposed population; the cert/key count mismatch is an operator error that a correctly deployed system would not exhibit; the dnscrypt-port is often firewalled to specific client ranges. CVSS PR:N confirms no authentication is required once preconditions exist.

The NVD CVSS 3.1 score of 5.9 (AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H) accurately captures the risk profile: the network vector and zero-privilege requirement are genuinely concerning, but AC:H is warranted because two independent preconditions must both hold - the binary must be compiled with --enable-dnscrypt (a non-default build flag) and the configuration must contain a cert/key count mismatch. In practice, a properly configured DNSCrypt deployment matches these counts exactly, so the vulnerable state is a silent misconfiguration that most operators will not knowingly sustain. The impact is limited to availability (server crash, no data exfiltration or integrity violation). EPSS data was not provided in the input and cannot be estimated; no CISA KEV listing and no public POC code are noted. SSVC signals: exploitation status is 'none publicly known'; the attack is automatable once preconditions are met (single UDP packet); technical impact is 'partial' (DoS of DNS resolver, potentially significant for infrastructure but recoverable by restart). Real-world priority is moderate: administrators running DNSCrypt-enabled Unbound should audit their cert-to-key ratios immediately, but the double precondition significantly limits the exposed population versus the raw score.

An attacker who has identified that a target Unbound instance listens on a non-standard dnscrypt-port (discoverable via port scan or banner) and suspects a cert/key misconfiguration sends a single 68-byte UDP datagram to that port with the first 8 bytes set to 0xdb (the predictable libsodium allocator fill pattern). Unbound resolves the garbage slot as a valid DNSCrypt provider entry, dereferences an invalid pointer, and crashes, taking down DNS resolution for all clients relying on that resolver. No public exploit code exists at time of analysis, but the trigger is mechanically simple and requires no authentication.