Overdoing NSEC3 Hurts#

Petr Špaček, Internet Systems Consortium

• Why: A crisp security nugget: NSEC3 iteration counts that operators set for "extra security" are actually a DoS amplifier and enable a poorly documented downgrade attack.
• What: Measured cost of extra iterations under random-subdomain attacks: ~⅓ performance loss at 5 iterations, ~⅔ at 25, across NSD, Unbound, and BIND. Worse, when iterations exceed a resolver's limit, the answer may be treated as insecure — and an attacker who controls any over-limit NSEC3 zone can craft a fake delegation that makes a delegation validate as insecure.
• How: Walks through the iteration-limit collapse over time (2500 in RFC 5155 → 150 in implementations → 0 per RFC 9276 → 50 in BIND 9.19.18 → ~20 next), then the downgrade mechanics using real records.
• Where applicable: Every DNSSEC-signed zone operator. Advice: use NSEC if you can, otherwise follow RFC 9276 exactly (0 iterations, no salt, no opt-out).