# CVE-2026-5947: ISC BIND 9 use-after-free in SIG(0) validation during query flood

Source: https://junglewise.ai/threats/cve-2026-5947-isc-bind-9-use-after-free-in-sig-0-validation-during-query-flood  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-20. Updated: 2026-05-20  
License: CC BY 4.0 with a link back

CVE-2026-5947 · Severity: high · CVSS 7.5 · Published 2026-05-20

ISC BIND 9, a widely used DNS server suite, is vulnerable to a race condition that can cause the service to crash. An attacker can trigger this by sending specific DNS messages during a high-traffic query flood, potentially leading to a denial-of-service (DoS) condition. This impact can disrupt internet connectivity or internal network name resolution for organizations relying on affected versions.

Technologies: [Isc BIND 9](https://junglewise.ai/threats/technologies/bind-9). Vendors: [Isc](https://junglewise.ai/threats/vendors/isc).

## Quick answers

**Is CVE-2026-5947 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-5947 being exploited in the wild.

**How severe is CVE-2026-5947?** High severity, with a CVSS score of 7.5, published 20 May 2026.

**Which products and versions are affected?** Isc BIND 9 9.20.0 through 9.20.22, 9.21.0 through 9.21.21, 9.20.9-S1 through 9.20.22-S1.

**Is there a fix for CVE-2026-5947?** A patch was published on 2026-05-20: Fixed versions 9.20.23 and 9.21.22 released Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-5947 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-5947?** Check whether you run Isc BIND 9 at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A race condition exists in BIND 9's handling of SIG(0) signed DNS messages. When BIND receives such a message, it initiates signature validation; however, if the 'recursive-clients' limit is reached simultaneously (e.g., during a query flood), the message may be discarded while the validation process still attempts to read it. This results in a use-after-free (UAF) violation. An unauthenticated remote attacker can exploit this to cause the BIND process to abort with a segmentation violation, resulting in a denial-of-service. The vulnerability affects both authoritative servers and resolvers. Patches are available in versions 9.20.23 and 9.21.22.

## Affected products

- ISC BIND 9 9.20.0 through 9.20.22, 9.21.0 through 9.21.21, 9.20.9-S1 through 9.20.22-S1

## Timeline

- 2026-05-13: other: Early Notification
- 2026-05-20: advisory: Public disclosure by ISC
- 2026-05-20: patched: Fixed versions 9.20.23 and 9.21.22 released

## References

- [https://downloads.isc.org/isc/bind9/9.20.23](https://downloads.isc.org/isc/bind9/9.20.23)
- [https://downloads.isc.org/isc/bind9/9.21.22](https://downloads.isc.org/isc/bind9/9.21.22)
- [https://kb.isc.org/docs/cve-2026-5947](https://kb.isc.org/docs/cve-2026-5947)

## Related threats

- [ISC BIND recursive server assert failure denial of service](https://junglewise.ai/threats/cve-2026-97313-a-recursive-bind-server-that-receives-a-query-with-recursion)
- [ISC BIND DNSSEC validation bypass in NSEC records](https://junglewise.ai/threats/cve-2026-13321-isc-bind-dnssec-validation-bypass-in-nsec-records)
- [ISC BIND 9 denial of service via NSEC and NSEC3 assertion failure](https://junglewise.ai/threats/cve-2026-13204-isc-bind-9-denial-of-service-via-nsec-and-nsec3-assertion-failure)
- [ISC BIND 9 denial of service via CNAME or DNAME record ordering](https://junglewise.ai/threats/cve-2026-12617-isc-bind-9-denial-of-service-via-cname-or-dname-record-ordering)
- [ISC BIND 9 cache poisoning via RRSIG label count discrepancy](https://junglewise.ai/threats/cve-2026-11721-isc-bind-9-cache-poisoning-via-rrsig-label-count-discrepancy)