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.

No CVSS v4.0

Attack Vector Network

Attack Complexity Low

Privileges Required None

Scope Unchanged

Confidentiality Impact None

Integrity Impact None

Availability Impact High

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

No EPSS score available.

Exploitation none

Automatable yes

Technical Impact partial

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions
ISC BIND 9 unaffected
Version Status Constraints
9.20.0 affected ≤ 9.20.22
9.21.0 affected ≤ 9.21.21
9.20.9-S1 affected ≤ 9.20.22-S1
9.18.28 unaffected ≤ 9.18.49
9.18.28-S1 unaffected ≤ 9.18.49-S1

No data.

No data.

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Project Subscriptions

Vendors Products
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Advisories

No advisories yet.

Fixes

Solution

Upgrade to the patched release most closely related to your current version of BIND 9: 9.20.23, 9.21.22, or 9.20.23-S1.

Workaround

No workarounds known.

References
Link Providers
https://downloads.isc.org/isc/bind9/9.20.23 cve-icon cve-icon
https://downloads.isc.org/isc/bind9/9.21.22 cve-icon cve-icon
https://kb.isc.org/docs/cve-2026-5947 cve-icon cve-icon
History

Wed, 20 May 2026 14:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Wed, 20 May 2026 13:15:00 +0000

Type Values Removed Values Added
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.
Title SIG(0) validation during query flood may lead to undefined behavior
First Time appeared Isc
Isc bind
Weaknesses CWE-362
CWE-416
CPEs cpe:2.3:a:isc:bind:*:*:*:*:*:*:*:*
Vendors & Products Isc
Isc bind
References
Metrics cvssV3_1

{'score': 7.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'}

Projects

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cve-icon MITRE

Status: PUBLISHED

Assigner: isc

Published:

Updated: 2026-05-20T13:39:38.654Z

Reserved: 2026-04-09T06:40:58.672Z

Link: CVE-2026-5947

cve-icon Vulnrichment

Updated: 2026-05-20T13:39:19.676Z

cve-icon NVD

Status : Awaiting Analysis

Published: 2026-05-20T13:16:40.303

Modified: 2026-05-20T14:04:57.320

Link: CVE-2026-5947

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-05-20T14:45:32Z

Weaknesses