Executive brief
BIND is a widely-used DNS server software that powers authoritative and recursive nameservers. This vulnerability allows an attacker to inject unauthorized DNS zone data into secondary DNS servers protected by TSIG (transaction signature), bypassing the intended security mechanism. An attacker without valid credentials could poison DNS records served to clients, leading to traffic redirection, phishing attacks, or service disruption.
Technical details
The vulnerability exists in BIND's handling of multi-message TCP incremental zone transfers (IXFR) when TSIG authentication is enabled. The `named` daemon begins serving zone data from an incoming transfer before validating the final TSIG signature that should authenticate the entire transfer. An attacker can initiate a multi-message IXFR without possessing a valid TSIG key and cause the secondary server to accept and serve the attacker's zone contents; even when the final TSIG signature never arrives, the server does not rollback to its previous state. The vulnerability requires network access to the secondary DNS server and the ability to initiate TCP zone transfers. Patches are available in BIND 9.20.29, 9.21.26, and respective security update branches.
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, 9.20.9-S1 through 9.20.27-S1
Timeline
- 2026-09-16: disclosed: Vulnerability published by ISC
- 2026-09-16: patched: Fixes released in BIND 9.20.29 and 9.21.26