DENIC .de DNSSEC outage, faulty key rollover; 3.5 h disruption for German government and public-sector .de domains

DENIC published its formal technical post-mortem on 2026-05-08 (DENIC analysis blog (German), 2026-05-08 · heise online, 2026-05-08).

Confirmed root cause: a code defect in DENIC's third-generation custom signing infrastructure (deployed April 2026 atop Knot DNS). During a routine Zone-Signing-Key rotation the code generated three private key pairs all assigned the same Key Tag (33834) rather than a unique tag per key, and only one corresponding public DNSKEY record was published to the zone. The RRSIG records signed by the two unpublished keys were therefore unvalidatable; DNSSEC-validating resolvers marked all .de delegations as "Bogus", which through the bogus NSEC3 trust path also took down resolution for non-DNSSEC-signed .de domains.

The outage ran 2026-05-05 21:43 UTC → 2026-05-06 ~01:15 UTC (~3.5 h). Critically, DENIC notes the monitoring pipeline detected anomalous resolver behaviour but the alerting layer did not correctly forward the alerts, the SIEM-rule equivalent of a fire-but-don't-page failure. Knot DNS itself is not implicated; the bug was in DENIC's automation layer atop Knot.

Defender takeaway: DNSSEC registry-side errors are indistinguishable from attacker-induced trust failures from a resolver's perspective. Validating-resolver operators in DACH and EU public-sector environments should keep RFC 7646 Negative Trust Anchor capability live for continuity during registry incidents and ensure runbooks separate "registry KSK/ZSK rollover defect" from "zone-level attack on a downstream domain".