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PatchSiren cyber security CVE debrief

CVE-2026-11721 ISC CVE debrief

AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record was published on 2026-07-22T15:16:51.837Z and has not been modified since then. This vulnerability affects BIND 9, specifically 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 issue involves an attacker's zone responding 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.

Vendor
ISC
Product
BIND 9
CVSS
HIGH 7.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-22
Original CVE updated
2026-07-22
Advisory published
2026-07-22
Advisory updated
2026-07-22

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.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-11721 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-11721

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-11721 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-11721

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.