BIND9 Cache Poison via RRSIG Label Mismatch (9.11-9.21.23)
CVE-2026-11721 Published on July 22, 2026
Cache poisoning possible with label count discrepancy, RRSIG, and wildcards
It is possible for an attacker's zone to respond to a query with an RRSIG that has a smaller number of labels than the zone in which the RRSIG is contained. This causes `named` to produce a wildcard name for a zone that is shorter than the attacker's zone, which can result in cache poisoning. For this attack to have any effect, the resolver under attack must have set `synth-from-dnssec yes;` (which is the default).
This issue affects BIND 9 versions 9.11.0 through 9.18.50, 9.20.0 through 9.20.24, 9.21.0 through 9.21.23, 9.11.3-S1 through 9.18.50-S1, and 9.20.9-S1 through 9.20.24-S1.
Vulnerability Analysis
CVE-2026-11721 can be exploited with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality, a high impact on integrity, and no impact on availability.
Weakness Type
Improper Validation of Specified Quantity in Input
The product receives input that is expected to specify a quantity (such as size or length), but it does not validate or incorrectly validates that the quantity has the required properties.
Products Associated with CVE-2026-11721
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Affected Versions
ISC BIND 9:- Version 9.11.0, <= 9.18.50 is affected.
- Version 9.20.0, <= 9.20.24 is affected.
- Version 9.21.0, <= 9.21.23 is affected.
- Version 9.11.3-S1, <= 9.18.50-S1 is affected.
- Version 9.20.9-S1, <= 9.20.24-S1 is affected.