# CVE-2026-5950: ISC BIND 9 unbounded resend loop in resolver state machine

Source: https://junglewise.ai/threats/cve-2026-5950-isc-bind-9-unbounded-resend-loop-in-resolver-state-machine  
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-2026-5950 · Severity: medium · CVSS 5.3 · Published 2026-05-20

A vulnerability in the BIND 9 DNS resolver can be exploited to cause severe resource exhaustion on the server. BIND 9 is widely used software that translates human-readable domain names into IP addresses; an exploit could lead to a denial-of-service, making websites and internal services unreachable. This issue allows an unauthenticated remote attacker to overwhelm the system by sending specially crafted DNS queries.

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

## Quick answers

**Is CVE-2026-5950 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-5950 being exploited in the wild.

**How severe is CVE-2026-5950?** Medium severity, with a CVSS score of 5.3, published 20 May 2026.

**Which products and versions are affected?** Isc BIND 9 9.18.36 through 9.18.48, 9.20.8 through 9.20.22, 9.21.7 through 9.21.21, 9.18.36-S1 through 9.18.48-S1, 9.20.9-S1 through 9.20.22-S1.

**Is there a fix for CVE-2026-5950?** A patch was published on 2026-05-20: Patched versions 9.18.49, 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-2026-5950 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-5950?** 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

An unbounded resend loop vulnerability (CWE-606) exists in the BIND 9 resolver state machine during the handling of 'bad' upstream servers. The flaw is triggered when specific retry conditions are met, causing the resolver to enter an infinite or excessive loop of resending queries. A remote, unauthenticated attacker can exploit this by sending DNS queries that force the resolver into this state, leading to CPU and memory exhaustion. The vulnerability specifically affects recursive resolvers; authoritative-only servers are generally not impacted. Patches have been released in versions 9.18.49, 9.20.23, and 9.21.22.

## Affected products

- ISC BIND 9 9.18.36 through 9.18.48, 9.20.8 through 9.20.22, 9.21.7 through 9.21.21, 9.18.36-S1 through 9.18.48-S1, 9.20.9-S1 through 9.20.22-S1

## Timeline

- 2026-05-13: other: Early notification provided to partners
- 2026-05-20: disclosed: Public disclosure of the vulnerability
- 2026-05-20: patched: Patched versions 9.18.49, 9.20.23, and 9.21.22 released

## References

- [https://downloads.isc.org/isc/bind9/9.18.49](https://downloads.isc.org/isc/bind9/9.18.49)
- [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-2026-5950](https://kb.isc.org/docs/cve-2026-5950)

## 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)