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

• Severity: medium (CVSS 5.9 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-05-20 · updated 2026-08-24
• Weakness: CWE-362

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.

Affected products

• ISC BIND: from 9.20.0 before 9.20.23; from 9.21.0 before 9.21.22 — fixed in 9.20.23, 9.21.22

How to fix

Upgrade to a fixed version: ISC BIND 9.20.23, 9.21.22. Check the vendor advisory for mitigations.

References

• https://downloads.isc.org/isc/bind9/9.20.23
• https://downloads.isc.org/isc/bind9/9.21.22
• https://kb.isc.org/docs/cve-2026-5947
• https://access.redhat.com/errata/RHSA-2026:7412
• https://access.redhat.com/security/cve/CVE-2026-5947
• https://bugzilla.redhat.com/show_bug.cgi?id=2479772
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-5947.json

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