Executive brief
BIND is a widely-used DNS server responsible for translating domain names into IP addresses. A flaw in DNSSEC validation allows a forged NSEC3 record from an unrelated domain zone to be accepted as proof of insecurity, potentially allowing attackers to downgrade a secure delegation and serve forged DNS responses. This could redirect users to malicious websites or intercept network traffic.
Technical details
The vulnerability is a DNSSEC validation flaw in BIND's NSEC3 processing logic. An attacker can craft a validly signed NSEC3 record from an unrelated sibling zone and present it as proof of non-existence (an insecurity proof), causing the resolver to incorrectly downgrade a secure delegation to an insecure one. This allows the attacker to serve forged unsigned DNS answers that would normally be rejected by DNSSEC validation. The attack requires network access to the resolver and does not require authentication, though successful exploitation depends on proper DNSSEC signing in the attacker-controlled zone. Patches are available in BIND 9.20.29 and 9.21.26 (released 2026-09-16).
Affected products
- ISC BIND 9.11.0 through 9.18.50, 9.20.0 through 9.20.27, 9.21.0 through 9.21.25, 9.11.3-S1 through 9.18.50-S1, and 9.20.9-S1 through 9.20.27-S1
Timeline
- 2026-09-16: disclosed
- 2026-09-16: patched: Fixed in BIND 9.20.29 and 9.21.26