# CVE-2026-5947: SIG(0) validation during query flood may lead to undefined behavior

CVE-2026-5947 is fixed in 6 products on 1 platform, first by bind9 1:9.20.18-1ubuntu2.1, released May 19, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-5947
- Priority: High
- CVSS: 7.5 (Important)
- EPSS: 0.0139
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: May 19, 2026
- Weakness: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-5947

## Description

Undefined behavior may result due to a race condition leading to a use-after-free violation.  If BIND receives an incoming DNS message signed with SIG(0), it begins work to validate that signature.  If, during that validation, the "recursive-clients" limit is reached (as would occur during a query flood), and that same DNS message is discarded per the limit, there is a brief window of time while the SIG(0) validation may attempt to read the now-discarded DNS message. This issue affects BIND 9 versions 9.20.0 through 9.20.22, 9.21.0 through 9.21.21, and 9.20.9-S1 through 9.20.22-S1. BIND 9 versions 9.18.28 through 9.18.49 and 9.18.28-S1 through 9.18.49-S1 are NOT affected.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | bind opensuse-leap 16.0 | 9.20.23-160000.1.1 | [OPENSUSE-SU-2026:21123-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-21123-1.md) | Jun 20, 2026 |
| Linux | bind sles 15.7 | 9.20.23-150700.3.25.1 | [SUSE-SU-2026:2673-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-2673-1.md) | Jul 2, 2026 |
| Linux | bind sles 16.0 | 9.20.23-160000.1.1 | [SUSE-SU-2026:22198-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-22198-1.md) | Jun 20, 2026 |
| Linux | bind9 Debian 13.5 (trixie) | 1:9.20.23-1~deb13u1 | [DEBIAN-CVE-2026-5947](https://patchblox.com/security/data/fix/linux/debian-cve-2026-5947.md) | May 27, 2026 |
| Linux | bind9 Debian 14 | 1:9.20.23-1 | [DEBIAN-CVE-2026-5947](https://patchblox.com/security/data/fix/linux/debian-cve-2026-5947.md) | May 27, 2026 |
| Linux | bind9 Ubuntu 25.10 (questing) | 1:9.20.11-1ubuntu2.4 | [bind9 1:9.20.11-1ubuntu2.4](https://patchblox.com/security/data/fix/linux/ubuntu-questing-bind9-1-9.20.11-1ubuntu2.4.md) | May 27, 2026 |
| Linux | bind9 Ubuntu 26.04.1 LTS (resolute) | 1:9.20.18-1ubuntu2.1 | [bind9 1:9.20.18-1ubuntu2.1](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-bind9-1-9.20.18-1ubuntu2.1.md) | May 19, 2026 |
| Linux | bind9-dnsutils Ubuntu 25.10 (questing) | 1:9.20.11-1ubuntu2.4 | [bind9-dnsutils 1:9.20.11-1ubuntu2.4](https://patchblox.com/security/data/fix/linux/ubuntu-questing-bind9-dnsutils-1-9.20.11-1ubuntu2.4.md) | May 27, 2026 |
| Linux | bind9-dnsutils Ubuntu 26.04.1 LTS (resolute) | 1:9.20.18-1ubuntu2.1 | [bind9-dnsutils 1:9.20.18-1ubuntu2.1](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-bind9-dnsutils-1-9.20.18-1ubuntu2.1.md) | May 19, 2026 |
| Linux | bind9-host Ubuntu 25.10 (questing) | 1:9.20.11-1ubuntu2.4 | [bind9-host 1:9.20.11-1ubuntu2.4](https://patchblox.com/security/data/fix/linux/ubuntu-questing-bind9-host-1-9.20.11-1ubuntu2.4.md) | May 27, 2026 |
| Linux | bind9-host Ubuntu 26.04.1 LTS (resolute) | 1:9.20.18-1ubuntu2.1 | [bind9-host 1:9.20.18-1ubuntu2.1](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-bind9-host-1-9.20.18-1ubuntu2.1.md) | May 19, 2026 |
| Linux | bind9-libs Ubuntu 25.10 (questing) | 1:9.20.11-1ubuntu2.4 | [bind9-libs 1:9.20.11-1ubuntu2.4](https://patchblox.com/security/data/fix/linux/ubuntu-questing-bind9-libs-1-9.20.11-1ubuntu2.4.md) | May 27, 2026 |
| Linux | bind9-libs Ubuntu 26.04.1 LTS (resolute) | 1:9.20.18-1ubuntu2.1 | [bind9-libs 1:9.20.18-1ubuntu2.1](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-bind9-libs-1-9.20.18-1ubuntu2.1.md) | May 19, 2026 |
| Linux | bind9-utils Ubuntu 25.10 (questing) | 1:9.20.11-1ubuntu2.4 | [bind9-utils 1:9.20.11-1ubuntu2.4](https://patchblox.com/security/data/fix/linux/ubuntu-questing-bind9-utils-1-9.20.11-1ubuntu2.4.md) | May 27, 2026 |
| Linux | bind9-utils Ubuntu 26.04.1 LTS (resolute) | 1:9.20.18-1ubuntu2.1 | [bind9-utils 1:9.20.18-1ubuntu2.1](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-bind9-utils-1-9.20.18-1ubuntu2.1.md) | May 19, 2026 |

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Data as of 2026-11-08T06:59:42Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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