CVE-2026-3104 ISC CVE debrief

Who should care

DNS and infrastructure teams running ISC BIND 9 resolver instances, especially administrators responsible for internet-facing or high-volume recursive DNS services on the affected 9.20.x or 9.21.x branches.

Technical summary

The vulnerability is caused by a memory management issue in the BIND resolver path. According to the CVE description, a specially crafted domain name can trigger a leak simply by being queried. NVD classifies the issue as CVSS 3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H and maps it to CWE-401 (memory leak) with ISC also referencing CWE-772. The practical impact is loss of availability as leaked memory accumulates in an affected resolver process.

Defensive priority

High. This is a remotely triggerable availability issue with no privileges or user interaction required, and ISC has published fixed releases for the affected branches.

Recommended defensive actions

• Upgrade ISC BIND 9 to a fixed release: 9.20.21 for the 9.20.x branch or 9.21.20 for the 9.21.x branch.
• If you are on BIND 9.18.x, verify the exact build against the vendor note; ISC states the 9.18.0 through 9.18.46 and 9.18.11-S1 through 9.18.46-S1 ranges are not affected.
• Inventory all resolver instances and confirm whether any systems are on affected 9.20.x, 9.21.x, or -S1 builds.
• Prioritize patching systems that handle untrusted DNS traffic or provide critical recursive resolution services.
• Monitor resolver memory usage and service stability until upgrades are completed, and plan maintenance windows if the resolver is sensitive to restart or reload operations.
• Review the ISC vendor advisory for any additional deployment guidance before and after applying the update.

Evidence notes

This debrief is based only on the supplied NVD record and ISC references. The core impact statement comes from the CVE description and the vendor advisory link. Version ranges and fixed releases are taken from the NVD CPE criteria and ISC patch references. No exploit steps or unsupported impact claims are included.

Official resources