--- title: "CVE-2026-5946: Invalid handling of CLASS != IN" slug: "cve-2026-5946" description: "Multiple flaws have been identified in named related to the handling of DNS messages whose CLASS is not Internet (IN) — for example, CHAOS or HESIOD, or DNS messages that specify meta-classes (ANY or NONE) in the question section." updated: 2026-05-20T11:06:54Z published: 2026-05-20T11:06:54Z --- > ## Documentation Index > Fetch the complete documentation index at: https://kb.isc.org/llms.txt > Use this file to discover all available pages before exploring further. # CVE-2026-5946: Invalid handling of CLASS != IN **CVE:** [CVE-2026-5946](https://www.cve.org/CVERecord?id=CVE-2026-5946) **Title:** Invalid handling of CLASS != IN **Document version:** 2.0 **Posting date:** 20 May 2026 **Program impacted:** [BIND 9](https://www.isc.org/bind/) **Versions affected:** BIND - 9.11.0 -> 9.16.50 - 9.18.0 -> 9.18.48 - 9.20.0 -> 9.20.22 - 9.21.0 -> 9.21.21 BIND Supported Preview Edition - 9.11.3-S1 -> 9.16.50-S1 - 9.18.11-S1 -> 9.18.48-S1 - 9.20.9-S1 -> 9.20.22-S1 **Severity:** High **Exploitable:** Remotely **Description:** Multiple flaws have been identified in `named` related to the handling of DNS messages whose CLASS is not Internet (`IN`) — for example, `CHAOS` or `HESIOD`, or DNS messages that specify meta-classes (`ANY` or `NONE`) in the question section. Specially crafted requests reaching the affected code paths — recursion, dynamic updates (`UPDATE`), zone change notifications (`NOTIFY`), or processing of `IN`-specific record types in non-`IN` data — can cause assertion failures in `named`. **Impact:** An attacker able to send specially crafted DNS messages to an affected `named` instance can cause it to terminate unexpectedly, resulting in a denial of service. - Authoritative servers are affected by this vulnerability. - Resolvers are affected by this vulnerability. **CVSS Score:** 7.5 **CVSS Vector:** CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H For more information on the Common Vulnerability Scoring System and to obtain your specific environmental score please visit: [https://nvd.nist.gov/vuln-metrics/cvss/v3-calculator?vector=AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H&version=3.1](https://nvd.nist.gov/vuln-metrics/cvss/v3-calculator?vector=AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H&version=3.1). **Workarounds:** Don't configure zones other than Internet (`IN`) class. Furthermore, do not expose the server that allows DNS Dynamic Update to the general Internet. **Active exploits:** We are not aware of any active exploits. **Solution:** Upgrade to the patched release most closely related to your current version of BIND 9: - 9.18.49 - 9.20.23 - 9.21.22 BIND Supported Preview Edition is a special feature preview branch of BIND provided to eligible ISC support customers. - 9.18.49-S1 - 9.20.23-S1 **Acknowledgments:** ISC would like to thank Mcsky23 for bringing this vulnerability to our attention. **Document revision history:** - 1.0 Early Notification, 13 May 2026 - 2.0 Public disclosure, 20 May 2026 **Related documents:** See our [BIND 9 Security Vulnerability Matrix](https://kb.isc.org/docs/aa-00913) for a complete listing of security vulnerabilities and versions affected. **Do you still have questions?** Questions regarding this advisory should be mailed to [bind-security@isc.org](mailto:bind-security@isc.org) or posted as confidential GitLab issues at [https://gitlab.isc.org/isc-projects/bind9/-/issues/new?issue[confidential]=true](https://gitlab.isc.org/isc-projects/bind9/-/issues/new?issue%5Bconfidential%5D=true). **Note:** ISC patches only currently supported versions. When possible we indicate EOL versions affected. For current information on which versions are actively supported, please see [https://www.isc.org/download/](https://www.isc.org/download/). **ISC Security Vulnerability Disclosure Policy:** Details of our current security advisory policy and practice can be found in the ISC Software Defect and Security Vulnerability Disclosure Policy at [https://kb.isc.org/docs/aa-00861](https://kb.isc.org/docs/aa-00861). The Knowledgebase article [https://kb.isc.org/docs/cve-2026-5946](https://kb.isc.org/docs/cve-2026-5946) is the complete and official security advisory document. **Legal Disclaimer:** Internet Systems Consortium (ISC) is providing this notice on an "AS IS" basis. No warranty or guarantee of any kind is expressed in this notice and none should be implied. 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