# CVE-2026-3591: ISC BIND 9 stack use-after-return in SIG(0) handling

Source: https://junglewise.ai/threats/cve-2026-3591-isc-bind-9-stack-use-after-return-in-sig-0-handling  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-03-25. Updated: 2026-05-21  
License: CC BY 4.0 with a link back

CVE-2026-3591 · Severity: medium · CVSS 5.4 · Published 2026-03-25

A vulnerability in BIND 9, a widely used software for translating domain names into IP addresses, could allow attackers to bypass security access controls. By sending a specially crafted DNS request, an attacker may trick the server into misidentifying their IP address, potentially gaining unauthorized access to restricted network resources. This issue primarily impacts systems configured with 'default-allow' security rules.

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

## Quick answers

**Is CVE-2026-3591 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-3591 being exploited in the wild.

**How severe is CVE-2026-3591?** Medium severity, with a CVSS score of 5.4, published 25 March 2026.

**Which products and versions are affected?** Isc BIND 9 9.20.0 to 9.20.20, 9.21.0 to 9.21.19, 9.20.9-S1 to 9.20.20-S1.

**Is there a fix for CVE-2026-3591?** A patch was published on 2026-03-25: Fixed versions 9.20.21 and 9.21.20 released Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-3591 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-3591?** 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 stack use-after-return vulnerability (CWE-562) exists in the 'named' server component of BIND 9 when processing DNS queries signed with SIG(0). The flaw occurs during the handling of these signed requests, where a stack variable address is returned and subsequently used, potentially causing an Access Control List (ACL) to incorrectly match or mismatch an IP address. An attacker with the ability to send DNS requests can exploit this to bypass IP-based restrictions. The impact is most severe in 'default-allow' configurations where specific IPs are denied; 'default-deny' configurations are expected to fail-secure. Patches are available in versions 9.20.21 and 9.21.20.

## Affected products

- ISC BIND 9 9.20.0 to 9.20.20, 9.21.0 to 9.21.19, 9.20.9-S1 to 9.20.20-S1

## Timeline

- 2026-03-18: other: Early notification provided
- 2026-03-25: advisory: Public disclosure by ISC
- 2026-03-25: patched: Fixed versions 9.20.21 and 9.21.20 released

## References

- [https://downloads.isc.org/isc/bind9/9.20.21](https://downloads.isc.org/isc/bind9/9.20.21)
- [https://downloads.isc.org/isc/bind9/9.21.20](https://downloads.isc.org/isc/bind9/9.21.20)
- [https://kb.isc.org/docs/cve-2026-3591](https://kb.isc.org/docs/cve-2026-3591)

## 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)