What is CVE-2026-3591?

CVE-2026-3591 is a medium-severity vulnerability in Isc Bind, classified under Authentication Bypass by Primary Weakness. CVSS score: 5.4/10. Published 2026-03-25.

How severe is CVE-2026-3591?

Medium severity. CVSS v3 base score is 5.4 out of 10.

Vulnerability in Isc Bind

CVE-2026-3591

A use-after-return vulnerability exists in the `named` server when handling DNS queries signed with SIG(0). Using a specially-crafted DNS request, an attacker may be able to cause an ACL to improperly (mis)match an IP address. In a default-allow ACL (denying only specific IP addresses), this may lead to unauthorized access. Default-deny ACLs should fail-secure. This issue affects BIND 9 versions 9.20.0 through 9.20.20, 9.21.0 through 9.21.19, and 9.20.9-S1 through 9.20.20-S1. BIND 9 versions 9.18.0 through 9.18.46 and 9.18.11-S1 through 9.18.46-S1 are NOT affected.

Published 2026-03-25 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.003 (23.9th percentile), scored 2026-10-19.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 24th percentile — 23.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.004 · newest: 0.003 · change: -0.000

Affected products

Isc Bind
Isc Bind 9 — versions 9.20.0, 9.21.0, 9.20.9-S1

Weakness classification (CWE)

References

kb.isc.org/docs/cve-2026-3591 (vendor-advisory, Vendor Advisory)
downloads.isc.org/isc/bind9/9.20.21 (Patch, patch)
downloads.isc.org/isc/bind9/9.21.20 (Patch, patch)

Frequently asked questions

What is CVE-2026-3591?: CVE-2026-3591 is a medium-severity vulnerability in Isc Bind, classified under Authentication Bypass by Primary Weakness. CVSS score: 5.4/10. Published 2026-03-25.
How severe is CVE-2026-3591?: Medium severity. CVSS v3 base score is 5.4 out of 10.