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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.

PUBLISHED Reserved 2026-04-09 | Published 2026-05-20 | Updated 2026-05-20 | Assigner isc




HIGH: 7.5CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Problem types

CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

CWE-416 Use After Free

Product status

Default status
unaffected

9.20.0 (custom)
affected

9.21.0 (custom)
affected

9.20.9-S1 (custom)
affected

9.18.28 (custom)
unaffected

9.18.28-S1 (custom)
unaffected

Credits

ISC would like to thank Naoki Wakamatsu for bringing this vulnerability to our attention.

References

kb.isc.org/docs/cve-2026-5947 (CVE-2026-5947) vendor-advisory

downloads.isc.org/isc/bind9/9.20.23 patch

downloads.isc.org/isc/bind9/9.21.22 patch

cve.org (CVE-2026-5947)

nvd.nist.gov (CVE-2026-5947)

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