CVE-2026-11721 ISC CVE debrief

Who should care

Organizations using 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 should assess their exposure and apply patches or mitigations. DNS administrators and security teams responsible for BIND 9 deployments should review the vulnerability details and take appropriate actions.

Technical summary

A vulnerability in BIND 9 allows an attacker's zone to respond with an RRSIG that has fewer labels than the zone containing the RRSIG, potentially leading to cache poisoning if the resolver has 'synth-from-dnssec yes;' configured, which is the default setting. 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. The attack requires the resolver to be configured with 'synth-from-dnssec yes;', which is the default configuration.

Defensive priority

High priority due to potential for cache poisoning and impact on DNS resolution.

Recommended defensive actions

• Inventory BIND 9 installations and verify versions.
• Apply patches or updates from the vendor.
• Review and adjust 'synth-from-dnssec' configuration if necessary.
• Monitor DNS resolution and cache poisoning attempts.
• Implement compensating controls such as DNSSEC validation.
• Review relevant monitoring, detection, and logs for exposed assets that need extra review.
• Track exceptions, retest remediated assets, and close the item only after evidence is documented.

Evidence notes

The CVE record and NVD entry provide details on the vulnerability and affected versions. Vendor references suggest patching or upgrading to mitigate the issue. To verify exposure, defenders should review BIND 9 installations, check for affected versions, and assess the 'synth-from-dnssec' configuration. Evidence is limited to public sources, and further verification may be necessary.

Official resources